Auditory Acclimatization in New Adult Hearing Aid Users: A Registered Systematic Review of Magnitude, Key Variables, and Clinical Relevance

Abstract

Purpose:

Auditory acclimatization refers to changes in auditory performance over time due to hearing aid modifications, extending beyond task-specific or training effects. This preregistered systematic review expands on previous ones by examining a broader range of outcomes, including auditory (e.g., speech recognition, electrophysiological responses) and selected nonauditory (e.g., self-reported outcomes) measures. It aimed to assess acclimatization's presence, magnitude, and influencing factors, focusing on controlled trials comparing postfitting aided outcomes with a control group. This is the first review to comprehensively report self-reported outcomes, advancing the field.

Method:

A systematic literature search was conducted in CINAHL, PubMed, and Web of Science in March 2024. Eligible studies followed the Population, Intervention, Comparison, Outcome, Study Design, and Timeline framework, including new adult hearing aid users with sensorineural hearing loss using air-conduction hearing aids. Studies were required to report outcomes, with a comparator and at least two data points in the same condition. Exclusions applied to studies involving children, advanced feature devices, surgical implants, non–peer-reviewed work, or uncontrolled studies. The review followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines and was registered on PROSPERO. A planned meta-analysis was excluded due to missing data.

Results:

The review included 25 controlled studies on auditory acclimatization. Of these, 18 examined speech recognition, with 10 reporting acclimatization, one mixed, and seven no acclimatization. Among eight studies with self-reported outcomes, three supported acclimatization, three showed mixed results, and two found no evidence. For electrophysiological outcomes, four of seven studies reported acclimatization, and three did not. Consistent hearing aid use and hearing loss severity influenced acclimatization, while cognitive abilities and age had no significant impact. Of the 25 studies, 16% were rated good quality, 80% were rated fair, and 4% were rated poor, with common issues including lack of randomization, blinding, and insufficient sample size reporting.

Conclusions:

This review highlighted the complexity of auditory acclimatization, influenced by various factors. Evidence suggested acclimatization occurred in some users and outcomes, though improvements were modest and variable. The most consistent gains were in speech recognition in noise and self-reported measures (e.g., Abbreviated Profile of Hearing Aid Benefit, Hearing Handicap Inventory for the Elderly, Glasgow Hearing Aid Benefit Profile), though changes were generally modest. Future studies should include essential statistical data, prioritize randomized controlled trials, and ensure early baseline and key interval measurements to better isolate and quantify acclimatization effects.

Supplemental Material:

https://doi.org/10.23641/asha.29242493

Auditory acclimatization is conceptualized as the improvement in auditory performance that occurs after the provision of amplified acoustic information, during which individuals gradually adapt over time to the changes in acoustic information provided by their hearing aids (Arlinger et al., 1996). This performance improvement cannot be solely attributed to task, procedural, or training effects (Arlinger et al., 1996). To measure auditory acclimatization, there is a need to (a) assess baseline performance, (b) track changes over time, and (c) compare with a control group. Functional changes in hearing following hearing aid use may stem from either improvement in the aided ear (i.e., an acclimatization effect) or worsening performance in the unaided ear (i.e., an effect of auditory deprivation; Silman et al., 1984).

Over the past three decades, numerous studies have been performed to investigate the presence and potential impact of auditory acclimatization. Study findings on acclimatization have, however, been inconsistent. Some studies have reported evidence suggesting an acclimatization effect (Karawani et al., 2022; Munro & Lutman, 2003), while others have not (Choi et al., 2011; Dawes et al., 2013; Saunders & Cienkowski, 1997).

With respect to speech recognition outcomes, accuracy of aided speech recognition is typically assessed in terms of percentage correct (Horwitz & Turner, 1997), either in quiet or in noise or as signal-to-noise ratio (SNR; in decibels) for a given criterion performance, such as 50% correct (Habicht et al., 2018). For example, in a study conducted by Gatehouse (1992), who introduced the concept of “acclimatization,” four participants received hearing aids in one ear. Speech recognition was measured in both the aided and unaided listening conditions of the fitted ear at the outset and after a 12-week period. The investigation found that the ear with the hearing aid experienced an improvement in speech recognition over the 12 weeks in the aided listening condition, whereas there was a decline in speech recognition in the fitted ear in the unaided listening condition. There were no changes or only slight declines in the performance in the nonfitted ear. Gatehouse (1992) interpreted this pattern of findings to support the presence of an acclimatization effect for aided listening. Similarly, other studies have also noted improvements in aided listening for new hearing aid users (Munro & Lutman, 2003). Wright and Gagné (2021) observed a 2-dB improvement in SNR for new users over 4 weeks, whereas experienced users showed no change. However, several studies (Amorim & Almeida, 2007; Dawes et al., 2014a; Saunders & Cienkowski, 1997) did not find improvements in speech recognition.

Self-reported measures have been used to assess subjective changes in aided listening among new hearing aid users in relation to auditory acclimatization. These measures typically include utilization, perceived benefits, satisfaction, and the impact of hearing aids on quality of life (e.g., the Hearing Handicap Inventory for the Elderly [HHIE], the Glasgow Hearing Aid Benefit Profile [GHABP], and the Abbreviated Profile of Hearing Aid Benefit [APHAB]). However, a potential limitation of self-report measures is the bias that can arise from factors such as social desirability, recall bias, expectations, memory constraints, emotional state, and individual differences in interpretation (Rosenman et al., 2011). Given that self-reports are subjective, it can be challenging to determine whether perceived improvements are due to auditory acclimatization, general adjustment to hearing aids, or other psychological factors.

Some studies have shown positive changes in self-reported outcomes among new hearing aid users over time (Chang et al., 2016; Dawes et al., 2014a; Vestergaard, 2006). For instance, in a study by Dawes et al. (2014a), participants completed the Speech, Spatial and Qualities of Hearing Scale–Difference version (SSQ-D; Gatehouse & Noble, 2004) and reported positive changes at 12 weeks compared to baseline. There were improvements in both new bilateral and unilateral hearing aid user groups, while there were no changes among the experienced hearing aid user group. However, other studies found no change in self-reported outcomes consistent with an acclimatization effect (Horwitz & Turner, 1997; Humes et al., 2002; Yund et al., 2006).

Researchers have also examined auditory event-related potentials (ERPs) in relation to hearing aid acclimatization. ERPs include both cortical (Habicht et al., 2018; Karawani et al., 2018a, 2022) and subcortical (Dawes et al., 2013; Karawani et al., 2018b) measures. Results have been mixed. Dawes et al. (2013) reported no changes in auditory brainstem response (ABR) Wave V latency or amplitude in new unilateral and bilateral users and a control group of experienced users after 3 months. Karawani et al. (2022) observed increased N1 amplitudes to speech syllables presented in quiet after 2 weeks among new hearing aid users. P2 amplitudes in quiet also increased but only after 6 weeks. In contrast, the control group exhibited no changes in the amplitudes and latencies of P1, N1, and P2 peaks between the baseline session and 24 weeks.

Various factors could explain inconsistent results across studies, including acclimatization effects being small on average and hard to detect without large study samples; insufficient experimental design to control the effects of repeated testing; degree of hearing loss; personal traits (e.g., personality, motivation, expectations); acoustic environments; and differences in acclimatization related to age, cognitive status, and hearing aid use (Dawes et al., 2014a; Horwitz & Turner, 1997; Palmer et al., 1998). Another challenge in existing studies is the definition of “new user.” Authors often differ on whether this term refers to individuals with no prior hearing aid experience or to those with less than 3, 6, or 12 months of experience. This distinction is important, as the first 6–12 weeks of hearing aid use may represent a period during which small but significant acclimatization effects are most likely to occur. In this review, we focus on studies involving individuals with no prior hearing aid experience, who are considered “new users” at the outset of their hearing aid use.

Previous reviews have examined auditory acclimatization, observing changes in adult performance over time. Arlinger et al. (1996) reported from the Eriksholm Workshop, which provided standardized definitions for auditory deprivation and acclimatization, summarized existing knowledge, and identified research gaps. They found that linear hearing aids were associated with acclimatization and deprivation effects, with studies showing a 0%–10% improvement in speech identification, although acclimatization could take months. They highlighted the need for research on participant expectations and reliable outcome measures due to inconsistencies in assessments. Byrne (1996) focused on nonspeech abilities like intensity discrimination, binaural masking, and sound localization, finding acclimatization or deprivation effects in these areas. They suggested these effects should be considered in research and clinical practice, particularly in hearing aid fitting.

Turner et al. (1996) reviewed 12 studies on hearing aid benefit over time, analyzing both objective (e.g., speech recognition) and subjective (e.g., questionnaires) measures. While some studies showed increased benefit, others showed no change, with significant individual variation. Palmer et al. (1998) reviewed 19 studies on functional and physiological auditory changes, noting variability due to factors like initial hearing loss and inconsistent control groups. They found learning continued up to 18 weeks postfitting, with some relearning lasting up to 2 years. Mueller and Powers (2001) noted that hearing aid users often adjust to changes in amplification, influenced by loudness levels and environmental sounds. They recommended incorporating acclimatization into patient counseling and fitting procedures. Munro (2008) reviewed 12 studies and found that hearing aids can lead to both perceptual and physiological changes, including improvements in speech perception and intensity discrimination, though the rate and significance of acclimatization vary. More recently, Lavie et al. (2022) found amplification-induced plasticity in older adults, improving speech perception, though overall gains were small and study quality moderate.

Overall, previous reviews highlight that auditory acclimatization typically results in perceptual improvements, such as better speech recognition and intensity discrimination, but the extent and timeline of these changes can vary widely among individuals (Arlinger et al., 1996; Byrne, 1996; Munro, 2008). The reviews underscore the need for consistent research methodologies and long-term studies to better understand and predict acclimatization effects and enhance hearing aid fitting practices (Mueller & Powers, 2001; Palmer et al., 1998). The inconsistency of research findings prompts consideration of the clinical relevance of acclimatization. If notable effects were present, they would likely be consistently observed across studies (Turner et al., 1996). There remains ongoing uncertainty about the presence and significance of auditory acclimatization, emphasizing the need for this updated and comprehensive review. Unlike the previous review by Lavie et al. (2022), which focused exclusively on speech perception outcomes, this review examined a broader range of outcome domains. These included (a) behavioral measures, such as the accuracy of aided speech recognition; (b) self-reported changes, including hearing aid benefit and satisfaction; and (c) electrophysiological measures. Notably, this was the first review to report comprehensively on self-reported measures, offering a novel and significant contribution to the field.

The aim of this review was to systematically evaluate auditory acclimatization in new hearing aid users in an up-to-date synthesis of research. This review builds on the foundational work of Arlinger et al. (1996) and Gatehouse (1992) by broadening the scope of auditory acclimatization to include both auditory and selected nonauditory factors, such as self-reported satisfaction and quality of life, as measuring potential changes over time. We incorporate patient-reported outcome measures, alongside speech recognition and electrophysiological measures, to capture a more comprehensive view of acclimatization than previous reviews. Our analysis excludes broader, indirect benefits of hearing aids, such as changes in depression, physical activity, listening effort, or social engagement, which fall outside the definition of auditory acclimatization.

Many previous studies on acclimatization lacked control groups (Humes et al., 2002; Philibert et al., 2005; Yund et al., 2006), making it difficult to distinguish acclimatization effects from those arising from repeated testing. Without a control group, attributing observed improvements solely to acclimatization is challenging, as factors such as familiarity with the testing environment or practice effects could provide a more parsimonious explanation for any improvements. To address this, our review explicitly focused on findings from controlled studies, which offer stronger evidence by enabling clearer differentiation between acclimatization effects and other potential influences.

Controlled studies were defined as those including a comparison group (e.g., experienced hearing aid users or non–hearing aid users) to isolate acclimatization effects from confounding factors. This criterion ensured our findings were drawn from robust evidence, allowing us to examine the magnitude, time course, clinical relevance, and influencing factors of acclimatization.

The specific research questions were as follows:

1. Is there evidence of systematic improvement in speech recognition, self-reported, and electrophysiological measures consistent with acclimatization following hearing aid use?

2. If changes were reported, what is their magnitude and are they likely to be clinically relevant?

3. If changes were reported, what is the time course to reach asymptote?

4. Are factors such as the duration and severity of hearing loss, or the length of hearing aid use, associated with systematic changes in these outcomes?

Method

The review protocol was registered on the International Prospective Register of Systematic Reviews (PROSPERO) website (Protocol ID: CRD42021258723). The review was performed and reported using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Page et al., 2021).

Information Sources and Search Strategy

A systematic literature search was conducted across three databases: CINAHL, PubMed, and Web of Science. The selection of these databases was based on their comprehensive coverage of relevant literature in the field of audiology. The databases were searched using their respective platforms: CINAHL via EBSCOhost, PubMed via National Center for Biotechnology Information, and Web of Science via Clarivate Analytics. The initial search was performed on February 8, 2023, and was updated on March 16, 2024, to capture any newly published articles. The search strategy was collaboratively developed by the primary author (C.W.) and co-authors (V.M., D.W.S., F.M.-A.) and executed by C.W., who performed the search. The search strategy utilized a combination of text words and controlled terms, focusing on two primary domains. All search terms used included: [“hearing aid” OR “hearing device” OR “amplification”] AND [“acclimat*” OR “acclimatization” OR “acclimatization period” OR “acclimatization effect” OR “perceptual learning” OR “plasticity” OR “benefit over time” OR “adaptation” OR “longitudinal change”].

Eligibility Criteria

The Population, Intervention, Comparison, Outcome, Study Design, and Timeline (PICOST) framework was utilized to determine the eligibility criteria for this study. Note that the PICOST eligibility criteria table included additional items, such as condition and language (see Table 1).

Table 1.

Eligibility criteria.

Criterion	Inclusion	Exclusion
Population	New hearing aid users aged 18 years and above; studies exclusively involving human subjects	Children (≤ 17 years old), animal research
Intervention	Digital air-conduction hearing aids (unilateral & bilateral)	Advanced feature hearing aids (e.g., frequency lowering), noncommercial hearing aids, surgical implants (e.g., cochlear implant, bone-anchored hearing aid), and when training was received (auditory training & aural rehabilitation)
Condition	Individuals with sensorineural hearing loss with new hearing aids	No hearing impairment, conductive hearing loss, and experienced users as the experimental group
Comparator	Any comparator to new hearing aid users (e.g., those with hearing loss with no amplification as a control group; experienced hearing aid users control group; normal-hearing individuals as control group, aided ear vs. unaided ear as the control in unilateral fittings)	Uncontrolled studies (where there was no comparator)
Outcomes	Behavioral outcomes (i.e., speech recognition in quiet and/or noisy environments) Self-reported outcomes (e.g., hearing disability, hearing aid benefit and/or satisfaction) Electrophysiological outcomes, including subcortical measures such as auditory brainstem response testing and frequency following response, and cortical measures such as cortical auditory evoked potentials	No reported outcomes, acoustic reflex tests, discrimination–limen measures, uncomfortable loudness level tests, and functional magnetic resonance imaging tests, depression, social engagement, and listening effort tests
Study design	Peer-reviewed journal publications	Unpublished studies, non–peer-reviewed publications, thesis/dissertations, systematic reviews, and case reports
Timing	At least two data point measures taken within the same condition to observe changes over time, such as baseline vs. 3 weeks. Studies were included from 1992 onward.	No post-intervention follow-up period
Language	English	Non-English

Selection Process

All articles retrieved from electronic searches were first identified and extracted by the primary author (C.W.) and then cross-checked by the second reviewer (F.M.-A.). Both reviewers independently screened the articles to ensure thoroughness and accuracy. Duplicates were identified and removed using Rayyan software (https://www.rayyan.ai/). The remaining articles were managed in an Excel spreadsheet. The selection process involved an initial review of titles. Articles that appeared relevant based on their titles were then assessed by reading their abstracts. Full-text articles were subsequently reviewed to determine eligibility for inclusion in the data synthesis. Additional articles were manually identified through reference lists and related articles that met the eligibility criteria. Both reviewers reached consensus on the included studies. The interreviewer discrepancy rate was 7%, primarily related to special hearing aid features and factors such as rehabilitation and training. Discrepancies were resolved through discussions with additional team members (V.M. and D.W.S.). The PRISMA protocol was followed (see Figure 1).

Figure 1.

Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram.

Data Extraction

Following the completion of the selection process, data extraction was meticulously carried out to ensure both accuracy and comprehensiveness by the primary author (C.W.). Data were extracted from each included study, encompassing several key variables. These included reference, country, population, sample size, study design, mean age, gender ratios, description of hearing loss, comparators, and length between the first and last measurement. Additionally, data were collected on the effects of hearing aid acclimatization on speech recognition, self-reported, and electrophysiological measures in adults with hearing loss.

Quality (Risk of Bias) Assessment and Determination of Level of Evidence

The quality of the included studies was assessed using the National Institutes of Health (NIH, 2021) Quality Assessment Tools, which provides a structured framework for evaluating key aspects of research, including study methodology, risk of bias, and reporting quality. Each study was evaluated using specific criteria tailored to its study design, with responses categorized as “Yes” (scored as 1 point) or “No,” “Cannot Determine,” “Not Reported,” and “Not Applicable” (each scored as 0 points). The total score, representing the number of affirmative responses, determined the study's overall quality rating.

For cohort and controlled interventional studies, scores between 11 and 14 were classified as good quality, scores between 6 and 10 as fair quality, and scores between 0 and 5 as poor quality. For before–after (pre–post) studies without a control group, scores of 9–12 were considered good quality, scores of 5–8 were categorized as fair, and scores of 0–4 were rated as poor. This scoring approach aligns with the system used by Oliaei et al. (2021).

In addition to quality assessment, the level of evidence for each of the included studies was determined using the Oxford Centre for Evidence-Based Medicine (OCEBM) Levels of Evidence (OCEBM Levels of Evidence Working Group, 2011), offering a hierarchical method for ranking evidence based on quality and relevance to clinical practice. It classifies evidence into five levels, with Level 1 representing the highest quality, typically derived from systematic reviews of randomized controlled trials (RCTs), and Level 5 representing the lowest, often based on expert opinion or case reports. Levels 2 through 4 cover progressively lower quality evidence, including individual RCTs, cohort studies, and case–control studies. This system helps contextualize study findings within the broader landscape of evidence-based practice. All quality and evidence assessments were conducted independently by the primary reviewer (C.W.), with any discrepancies resolved through discussion with a second reviewer (F.M.-A.).

Data Synthesis

Narrative synthesis. A narrative synthesis was conducted following the framework outlined by Campbell et al. (2020) and Popay et al. (2006), given the heterogeneity of study designs, outcome measures, and statistical reporting. The synthesis aimed to systematically integrate findings from controlled studies on auditory acclimatization by identifying patterns, evaluating consistency across studies, and assessing the strength of the evidence.

Studies were categorized based on key variables, including speech recognition, self-reported outcomes, and electrophysiological measures. A thematic approach was used to identify commonalities and discrepancies in findings, taking into account methodological differences such as participant selection, intervention protocols, and measurement timelines. To enhance transparency, findings were interpreted with consideration of potential biases, such as lack of randomization, inconsistent blinding, and variability in control group designs.

Given the absence of sufficient statistical data for meta-analysis, the narrative synthesis emphasized qualitative comparisons and structured reporting of trends rather than pooled effect sizes. The certainty of findings was evaluated based on study quality, risk of bias, and alignment with previous research on auditory acclimatization. Where applicable, study limitations were discussed to contextualize discrepancies and highlight areas for future research. This synthesis approach allowed for a comprehensive interpretation of the evidence while acknowledging variability in study methodologies. The findings provide insights into the potential mechanisms underlying auditory acclimatization, offering a structured foundation for future meta-analyses and more rigorous experimental designs.

Quantitative synthesis. A meta-analysis was attempted using Comprehensive Meta-Analysis software Version 4, employing an inverse variance weighting approach and a random-effects model (Borenstein et al., 2005), to assess the impact on three outcome domains by comparing the standardized mean difference over time between new hearing aid users and a control group. This analysis extracted means and standard deviations from 11 studies. A quantitative analysis was not performed. The challenge of calculating standard deviations for each group over time—due to the interdependence of data points across time points—hindered further analysis. The Borenstein et al. (2021) method, which adjusts for correlations between a study's intragroup effect sizes, was considered. However, this method required individual participant data that were unavailable. Without these critical correlation data, the composite effect size and its standard deviation could not be accurately computed, preventing the completion of the meta-analysis despite initial efforts.

Results

Search and Study Selection

A database search conducted in February 2023 and repeated in March 2024 yielded 949 records. After removing duplicates, 792 records were screened by title, resulting in the exclusion of 579 based on titles and 123 based on abstracts. The full texts of 90 articles were reviewed, and 32 met the inclusion criteria. An additional 11 relevant studies were identified through reference list review, bringing the total to 43 articles on acclimatization. Of these, 18 were uncontrolled studies and were excluded to focus on the 25 controlled studies, as shown in Figure 1. This review incorporates a larger number of studies, including 10 additional studies not included in previous reviews, reflecting expanded inclusion criteria and a broader scope of acclimatization research.

Study Characteristics

Table 2 provides an overview of the key characteristics of the 25 included studies. Among these studies, 18 studies focused on speech recognition, eight examined self-reported outcomes, and seven assessed electrophysiological measures. Notably, seven studies investigated multiple outcomes, spanning two or more of these domains. These 25 studies were examined for outcome analysis in Tables 3–5, and 11 of them were included in the attempted meta-analysis. The 14 studies that were not considered for meta-analysis were excluded due to insufficient reporting on means and standard deviations. Almost half of the studies were conducted in North America (44%), while slightly more than a third were conducted in Europe (36%). All studies used a longitudinal design, assessing scores at multiple time points post–hearing aid intervention. The study designs included true experimental (n = 4) and quasi-experimental (n = 17) studies. Additionally, there were pre-experimental studies, which consisted of within-subject designs (n = 4), where participants with unilateral fittings acted as their own control. Control groups included experienced hearing aid users, normal-hearing individuals, matched participants with hearing loss, and the nonfitted ear in unilateral fittings. Study durations ranged from 4 weeks to 1 year, with the most common duration being 12 weeks, followed by 6 months. Sample sizes ranged from four to over 200 participants. Among the studies, 20% involved unilateral hearing aid users, 36% involved bilateral hearing aid users, and 44% included both types.

Table 2.

Key characteristics of included studies (n = 25).

Authors (year)	Country	Study design	Participants & sample size (n)	Control	Examined components (longitudinally)	Description of HL	Hearing aid fitting type		Outcome domain
							Uni	Bil	B	SR	AEP
Gatehouse (1992)	UK	Pre-experimental (within-subject design)	New unilateral HA users (n = 4)	Yes (nonfitted ear)	Free field vs. headphones to fitted ear vs. headphones to nonfitted ear (control) & in both listening conditions	Bilateral Moderate SNHL	✓		✓
Silman et al. (1993)	US	Quasi-experimental	New unilateral HA users (n = 19) New bilateral HA users (n = 28) Control–normal (n = 19)	Yes (nonfitted ear & normal non–HA users)	Fitted vs. nonfitted ear & new users vs. normal participants (control) (do not say which listening condition tested in)	Bilateral symmetrical SNHL	✓	✓	✓
Bentler et al. (1993a)	US	Quasi-experimental	Sample (n = 65): new (n = 26), experienced (n = 39) Unilateral (n = 55) Bilateral (n = 10)	Yes (experienced HA users)	New vs. experienced, degree of HL, configuration, quiet vs. noise, circuit type, & wear time (all measures taken in aided condition)	Mild-to-moderate SNHL	✓	✓	✓
Bentler et al. (1993b)	US	Quasi-experimental	Sample (n = 65): new (n = 26), experienced (n = 39) Unilateral (n = 55) Bilateral (n = 10)	Yes (experienced HA users)	New vs. experienced, degree of HL, configuration, circuit type, & wear time	Mild-to-moderate SNHL	✓	✓		✓
Cox et al. (1996)	US	Quasi-experimental	New unilateral HA users (n = 22) Experienced unilateral HA users (n = 5)	Yes (experienced HA users)	New vs. experienced, aided vs. unaided listening conditions, & different HAs	Bilateral sloping mild-to-moderate or moderately severe	✓		✓
Horwitz & Turner (1997)	US	Quasi-experimental	New unilateral HA users (n = 13) Experienced unilateral HA users (n = 13)	Yes (experienced HA users)	New vs. experienced, aided vs. unaided listening condition, gain: fixed initial vs. daily adjusted	Moderately-to-severe SNHL sloping in high frequency	✓		✓	✓
Saunders & Cienkowski (1997)	US	Quasi-experimental	New bilateral HA users (n = 24) Experienced bilateral HA users (n = 24)	Yes (experienced HA users)	New vs. experienced, aided vs. unaided listening condition, different combinations of frequency response, & method of output limiting	Mild-to-moderate symmetrical SNHL		✓	✓
Munro & Lutman (2003)	UK	Pre-experimental (within-subject design)	*New unilateral HA users (n = 16)	Yes (nonfitted ear)	Fitted ear vs. nonfitted ear & aided vs. unaided listening condition	Bilateral, symmetrical, mild-to-moderate, sloping, high-frequency SNHL.	✓		✓
Vestergaard (2006)	Denmark	Quasi-experimental	New bilateral HA users (n = 20) Experienced bilateral HA users (n = 5)	Yes (experienced HA users)	New vs. experienced, wear time	Bilateral steeply sloping HL		✓		✓
Amorim & Almeida (2007)	Brazil	Pre-experimental (within-subject design)	New bilateral HA users (n = 8) New unilateral HA users (n = 8)	Yes (nonfitted ear)	Fitted ear vs. nonfitted ear, aided vs. unaided listening condition, & LE vs. RE	Bilateral symmetric moderate-to-severe mixed or SNHL	✓	✓	✓
Metselaar et al. (2009)	Netherlands	True experimental (double blind RCT)	Sample size in total (n = 254) New HA users (n = 113) Experienced HA users (n = 196) Prescriptive method (n = 190) Comparative (n = 119)	Yes (experienced HA users)	New vs. experienced, comparative vs. prescriptive fittings Three strata of maximum speech intelligibility	SNHL and mixed HL	✓	✓		✓
Choi et al. (2011)	Korea	Quasi-experimental	New bilateral HA users (n = 18) Nonusers (control) (n = 11)	Yes (non–HA users)	New HA users vs. non–HA users, cognitive aspects	Bilateral SNHL		✓	✓
Song et al. (2011)	US	Pre-experimental (within-subject design)	New unilateral HA users (n = 66)	Yes (nonfitted ear)	Fitted ear vs. nonfitted ear (All measures taken in unaided listening condition)	Bilateral HL	✓		✓
Dawes et al. (2013)	UK	Quasi-experimental	*New unilateral (n = 8) New bilateral (n = 10), experienced unilateral (n = 3), experienced bilateral (n = 3)	Yes (nonfitted ear & experienced HA users)	Fitted ear vs. nonfitted ear, new vs. experienced HA users	Bilateral symmetrical, mild-to-moderate, sloping high-frequency SNHL	✓	✓			✓
Dawes et al. (2014a)	UK	Quasi-experimental	*New unilateral (n = 16) New bilateral (n = 16) Experienced unilateral (n = 9) Experienced bilateral (n = 8)	Yes (nonfitted ear & experienced HA users)	Fitted vs. nonfitted ear, new vs. experienced, aided vs. unaided listening conditions, unilateral vs. bilateral fittings, & LE vs. RE	Bilateral, symmetrical, mild-to-moderate, sloping high-frequency SNHL	✓	✓	✓	✓
Dawes et al. (2014b)	UK	Quasi-experimental	*New unilateral (n = 11) New bilateral (n = 13) Experienced unilateral (n = 6) Experienced bilateral (n = 7)	Yes (nonfitted ear & experienced HA users)	Fitted vs. nonfitted ear, new vs. experienced, aided vs. unaided listening conditions, unilateral vs. bilateral fittings, & LE vs. RE	Bilateral, mild-to-moderate, sloping high-frequency SNHL	✓	✓	✓		✓
Lavie et al. (2015)	Israel	Quasi-experimental	New HA users: bilateral & unilateral (n = 36), Nonusers bilateral (n = 11)	Yes (non–HA users)	New HA users vs. non–HA users, dominant vs. nondominant ear, & unilateral vs. bilateral (All measures taken in unaided listening condition)	Bilateral, flat, or moderately sloping audiograms	✓	✓	✓
Chang et al. (2016)	Korea	Quasi-experimental	New unilateral (n = 145) New bilateral (n = 11) Experienced unilateral (n = 27) Experienced bilateral (n = 25)	No (WRS) Yes (self-reported: experienced HA users)	New vs. experienced, unilateral vs. bilateral, & HA type	Conductive:2 Sensorineural: 175 Mixed: 31	✓	✓		✓
Dawes & Munro (2017)	UK	Quasi-experimental	*New bilateral HA users (n = 35) Experienced bilateral HA users (n = 20)	Yes (experienced HA users)	New vs. experienced, specific degree of HL and HA use, & cognition (all measures taken in the aided condition)	Bilateral, symmetrical, mild-to-moderate		✓	✓	✓
Habicht et al. (2018)	Germany	Quasi-experimental	New bilateral HA user (n = 16) Experienced bilateral HA users (n = 14)	Yes (experienced HA users)	New vs. experienced, low vs. high linguistic complexity, & processing time.	Bilateral HL		✓	✓		✓
Karawani et al. (2018a)	US	True experimental (RCT)	*HA users (n = 18) Non-HA users (control; n = 14)	Yes (non–HA users)	New HA users vs. non–HA users, aided vs. unaided listening conditions, quiet vs. noise stimulus	Bilateral symmetrical mild-to-severe SNHL		✓			✓
Karawani et al. (2018b)	US	True experimental (RCT)	*New HA users (n = 20) Non-HA users (n = 15)	Yes (non–HA users)	New HA users vs. nonusers. sided vs. unaided listening conditions	Moderate ARHL		✓	✓	✓	✓
Megha & Maruthy (2020)	India	Quasi-experimental	New bilateral HA users (n = 30) Normal non–HA users (n = 17)	Yes (non–HA users; normal)	New HA users vs. normal-hearing individuals	Bilateral, mild-to-moderate SNHL	✓	✓	✓		✓
Wright & Gagné (2021)	Canada	True experimental (double blind RCT)	*New bilateral HA users (n = 32) Experienced bilateral HA users (n = 15)	Yes (experienced HA users)	New vs. experienced, NRA on vs. off, HA use, age, the severity of HL, gender, speed of processing perceived handicap, working memory, & wear time (all measures taken in aided condition)	Bilateral, mild to moderately severe SNHL		✓	✓
Karawani et al. (2022)	US	Quasi-experimental	New bilateral HA users (n = 17) Non-HA users (n = 14)	Yes (non–HA users)	New HA users vs. non–HA users, cortical changes: quiet vs. noise, wear time	Bilateral symmetrical SNHL		✓			✓

Note. HL = hearing loss; Uni = unilateral; Bil = bilateral; B = behavioral; SR = self-reported; AEP = auditory electrophysiological potential; UK = United Kingdom; HA = hearing aids; SNHL = sensorineural hearing loss; US = United States; LE = left ear; RE = right ear; WRS = Word Recognition Score; RCT = randomized controlled trial; NRA = noise reduction algorithm; ARHL = age-related hearing loss.

^*The asterisks in the participants and sample size column indicate which studies conducted a sample size estimation or power analysis.

Table 3.

Summary of controlled studies on speech recognition outcomes (n = 18).

Study	Duration of study	Time points measured	HA use (M)	How HA use was measured	Assessment measures	Speech in quiet /noise	HA acclimatization	Outcomes
Gatehouse (1992)	12 weeks	Baseline, 1, 2, 3, 4, 5, 6, 8, 10, and 12 weeks	Not reported	—	WRS & FAAF	WRS: Q FAAF: N	Yes	In the initial 3 weeks postfitting, benefits between the fitted and nonfitted ears were similar. After approximately 4–6 weeks, a distinct pattern emerged. Over 12 weeks, the fitted ear showed significant speech recognition improvement in aided FAAF conditions, with a decline in unaided conditions. In contrast, the nonfitted ear showed no significant changes in either unaided or aided conditions.
Silman et al. (1993)	±1 year	(1) Between 6 & 12 weeks postfitting (2) 1 year following the initial test	At least 4 hr/day	Self-reported (subjects indicated usage)	SRT, W-22, SIN, & NST	SRT, NST, W-22: Q SIN: N	Yes	Monaurally aided group: The W-22, NST, and SPIN tests showed enhanced speech performance in the fitted ear and a slight decline in the nonfitted ear. Binaurally aided group: A trend toward improvement in most ear and speech measures. Control group: No differences were found over time.
Bentler et al. (1993a)	1 year	Baseline, 1, 3, 6, and 12 months postfitting	Part time: < 4 hr/day Full time: 9.3 hr/day	Self-reported (questionnaire)	NST & SPIN	NST: Q/N SPIN: N	No	No time effect observed for high-predictability items in the experimental group. For low-predictability items, a quadratic trend emerged among part-time users. In the NST, a linear trend was noted for new users in quiet backgrounds. Conversely, noise conditions suggested a more complex third-degree polynomial fit across various factors, possibly influenced by experience and configuration, yet no significant interactions were observed. Score changes stayed below 5%, with greater variances in challenging conditions indicating stable performance.
Cox et al. (1996)	12 weeks	Baseline, 3, 6, 9, 12 weeks postfitting	Week 1: 5 hr/day Weeks 3–12: 8 hr/day or more	Self-reported	CST & SPAC	N	Yes	CST: Study group. A Student–Newman–Keuls post hoc test (α = .05) revealed no difference between the 0- and 12-week unaided scores. However, the 12-week aided scores were higher than the 0-week aided scores. While statistically significant, its absolute magnitude was only about 4%–5% on average. Mean benefit in noise rose from approximately 4% at fitting to about 8% after 12 weeks of regular use. Control group: No observed improvement in scores over time. SPAC: Study group. There were no significant differences across sessions for unaided listening. However, there was a significant change across sessions for aided listening, F(3, 60) = 5.57, p < .01. A Student–Newman–Keuls post hoc test (α = .05) revealed that scores at 9 weeks postfitting were better than those obtained at 0, 3, and 6 weeks. The time course of improved performance was between 6- and 9-week postfitting. Control group: Both aided and unaided listening conditions showed no significant differences in overall scores across the four tests.
Horwitz & Turner (1997)	18 weeks	Baseline, 3, 6, 10, 14, and 18 weeks postfitting	Not reported	—	NST	N	Yes	Fixed initial gain NST: For new HA users, benefit increased from 7 rau at fitting to 14 rau over five follow-up visits. In contrast, longstanding users maintained a stable benefit, averaging 10 rau initially and 9 rau after 18 weeks. Statistical analysis showed a significant time effect on benefit for new users, F(5, 120) = 4.187, p < .01, but no significant change for longstanding users over the 18 weeks, F(5, 120) = 0.592, p > .05. Daily adjusted gain NST: Average benefit for new HA users started at almost 8 rau and increased to 12 rau by Week 18. Longstanding users also experienced a slight increase, from 10 to 13 rau. The effect of time on mean benefit was significant for new users, F(5, 120) = 2.978, p < .025, but not for longstanding users, F(5, 120) = 1.465, p > .05.
Saunders & Cienkowski (1997)	3 months	Day 0; 1, 2, & 3 months postfitting	Not reported	—	SRT-Q, PSRT-N, and SSRT-N	SRT-Q PSRT-N SSRT-N	No	ANOVA showed no significant differences across visits for unaided listening on all measures (SRT-Q, PSRT-N, SSRT-N), with no interaction between volume setting and performance across sessions. However, a main effect of volume was found on SRT-Q (p = .02), as well as a session effect on SRT-Q (p = .018), but not PSRT-N or SSRT-N. Aided advantages were similar for new and experienced users across all tests, with no significant interactions between user status and session. HA configuration only affected performance on PSRT-N (p = .03) and had a main effect on SRT-Q (p = .02).
Munro & Lutman (2003)	12 weeks	Baseline (some as late as 7 days after fitting), 6, 12 weeks postfitting	Required at least 6–8 hr/day	Self-reported	FAAF for quiet, normal, and raised speech levels	N	Yes	The aided scores improved in the fitted ear across all presentation levels: approximately 1.0%, 2.6%, and 3.6% at 55, 62, and 69 dB SPL, respectively. Conversely, the control ear saw consistent negative changes, typically around −2.0%. The unaided scores initially were 55% with speech at 55 dB SPL but increased to approximately 71% with speech at 69 dB SPL. The results confirmed acclimatization, indicating improved auditory performance over time. However, this effect was evident only at higher presentation levels, not the lowest. Therefore, the conclusion is that acclimatization in FAAF depends on speech presentation level, with greater improvements seen at higher levels.
Amorim & Almeida (2007)	18 weeks	Baseline, 4, 16/18 weeks postfitting	Not reported	—	Speech discrimination	Q	No	The study noted that the word discrimination scores in aided and nonaided ears across the study's phases, showed no significant differences between groups (p < .05). Unilateral users saw a decline in speech recognition in their nonaided ear and an improvement in their aided ear, which was linked to the audibility of the HAs. These results lacked statistical significance. Conclusions: Short-term benefits were obtained with the use of hearing aids; however, the phenomenon of acclimatization was not observed through the SRPI
Choi et al. (2011)	6 months	Baseline & 6 months postfitting	Not reported	—	WIN	N	Yes	Comparison of recognition scores between the study and control groups showed that after 6 months, the study group improved, from 11.7 ± 1.9 to 13.1 ± 1.5 (p < .05) while the control group's score stayed the same.
Song et al. (2011)	Between 5 months & 12 years	Baseline vs. postfitting value ranged between 5 months & 12 years	Not reported	—	WRS	Q	No	When aiding the better WRS ear, 17 out of 21 experienced a decrease, and two improved. For those with worse WRS, 12 out of 23 saw a decrease in control ear scores. In patients with equal WRS in both ears, control ears mostly declined (15 out of 22), while aided ears remained stable or improved in only 2 out of 22 cases. Overall, regardless of which ear was aided, WRS generally stayed the same or decreased.
Dawes et al. (2014a)	12 weeks	Baseline & 12 weeks postfitting	7.5 hr/day	Objective (datalogging feature)	FAAF	N	No	A small, statistically significant improvement was observed in the study group as well as in the control group, likely due to a general practice effect. This underscores the importance of using a control group to avoid misinterpreting improvements as acclimatization effects. No interaction between time and group, stimulus intensity, or ear of aiding was observed. Unilateral and bilateral fitting yielded no differences in improvement. No change in error patterns or group-level acclimatization was found. No notable improvements in speech recognition at higher stimulus intensities, contradicting the “stimulus novelty” hypothesis from prior studies.
Dawes et al. (2014b)	12 weeks	Baseline & 12 weeks	Required at least 6 hr/day; M = 10 hr/day	Objective (datalogging feature)	FAAF	N	No	No interactions or correlations were found between time, level, group (new users vs. control group), speech recognition changes, and N1/P2 latency or amplitude, age, average hearing loss, or HA use (Pearson's |r| between .03 and .44; all p > .05).
Lavie et al. (2015)	14 weeks	Baseline, 4, 8, & 14 weeks postfitting	Not reported	—	Dichotic tests, SIN, Monosyllabic Word Identification in Quiet	Dichotic & SIN: N Mono syllabic: Q	Dichotic & SIN: yes Monosyllabic: no	Dichotic test: Improvements in less dominant ear, with no changes for the dominant ear, and no changes were observed in the control group. Bilateral amplification appeared more effective than monaural use, as it notably reduced ear asymmetry in dichotic listening tests. SIN: Enhancements were noted, while no such gains were seen in the control group. Monosyllabic-quiet: No significant changes and also for the control group. The study concluded that even a short period of hearing aid use can improve auditory processing in older adults, particularly in speech-in-noise and dichotic listening tasks, suggesting that the central auditory system retains a capacity for beneficial plasticity.
Dawes & Munro (2017)	1 month	Baseline, 1, 7, 14, and 30 days postfitting	Subgroup (> 6 hr/day)	Objective (datalogging feature)	SIN	N	Subset: yes	At day 30, new users did not significantly improve their SIN scores compared to experienced users. However, a subset of new users with moderate hearing loss, using their HAs for at least 6 hr daily, showed a significant SIN improvement of around 3-dB SNR when compared to experienced users. Notably, this SIN enhancement in new users was associated with consistent HA use and more severe HL. Experienced users did not exhibit significant SIN improvement over time. No discernible link was found between SIN improvements and cognitive ability or auditory distraction.
Habicht et al. (2018)	24 weeks	Baseline, 12, & 24 weeks postfitting	At 24 weeks = New: 8.2 hr/day Ex: 10.7 hr/day	NovHA: datalogging ExpHA: self-reported	Processing times & SRT	SRT: N	Processing times: yes SRT: no	Novice users improved processing times by 30%, showing HA benefits for speech comprehension. Experienced users saw no changes. SRT80s were similar across sentence types and user groups, with no significant differences.
Karawani et al. (2018b)	6 months	Baseline and 6 months postfitting	Required to wear 8 hr/day	Objective (datalogging feature)	QuickSIN	N	No	QuickSIN scores improved in both aided and unaided conditions for the experimental group and in the control group's unaided condition. However, there were no main effects of time or group observed (p > .08). The improvement in both groups likely reflects a practice effect rather than a true change due to HA use.
Megha & Maruthy (2020)	2 months	Initial, 1, & 2 months postfitting. Control: recorded twice, 2 months apart	8.1 hr/day	Objective (datalogging feature)	SIN	N	Yes	An improvement in SIN scores for HA users after 2 months. The control group showed no statistically significant changes in SIN. Enhanced ALLRs do not guarantee automatic improvements in speech perception in noise.
Wright & Gagné (2021)	10 months	Baseline, 2, 4, 6, 8, 14, 22, 38 weeks postfitting	12 hr/day	Objective (datalogging feature)	HINT	N	Yes	Over a 4-week period, new HA users experienced a notable improvement in speech recognition in noisy environments, with an average gain of approximately 2 dB in SNR. In contrast, experienced HA users showed no change in their performance.

Note. HA = hearing aid; WRS = Word Recognition Score; FAAF = Four Alternative Auditory Feature Test; Q = quiet; N = noisy; SRT = speech recognition threshold; SIN = Speech in Noise; NST = Nonsense Syllable Test; SPIN = Speech Perception in Noise; CST = Connected Speech Test; SPAC = Speech Pattern Contrast; PSRT-N = Performance SRT-N; SSRT-N = Subjective SRT-N; WIN = Words in Noise; ANOVA = Analysis of Variance; SRPI = speech recognition performance index; ALLRs = Auditory Late Latency Responses; HINT = Hearing in Noise Test; SNR = signal-to-noise ratio.

Table 4.

Summary of studies on self-reported outcomes (n = 8).

Study	Duration of study	Time points measured	HA use	How HA use was measured	Assessment measures	HA acclimatization	Outcomes
Bentler et al. (1993b)	1 year	Initial fitting, 6 & 12 months postfitting	Part time: 2.57 hr/day. Full time: 10.61 hr/day	Self- reported	HPI, Expectation Checklist, Qualitative Judgments & Satisfaction Questionnaire	No	HPI: Score remained stable over time. However, items related to communication ability in a quiet background showed reduced difficulties over the year. Expectation Checklist: No changes in the overall scores, but when looking at cohesive items separately, self-perceived communication performance exceeded expectations over time. Qualitative Judgments & Satisfaction Questionnaire: No changes over time. The statistical analyses confirm that while there were trends in improved outcomes for both groups, there were no statistically significant differences between new and experienced HA users in most measures, except for the significantly higher daily use time reported by experienced users.
Horwitz & Turner (1997)	18 weeks	Initial fitting, 3, 6, 10, 14, & 18 weeks postfitting	Not reported	—	PHAB	Yes	The simple main effects tests indicated a significant decrease in aided frequency of problems over five visits for new users, F(4, 96) = 2.610, p < .05, while no significant change was observed for long-standing users, F(4, 96) = 0.804, p > .05, indicating that there was a significant interaction between HA experience and time, F(4, 96) = 3.049, p < .025. The average unaided results for both groups showed stable frequencies of problems over time. A two-way repeated-measures ANOVA found no significant effects of time, F(4, 96) = 0.948, p > .05, or HA experience, F(1, 24) = 0.062, p > .05, nor was there a significant interaction between HA experience and time, F(4, 96) = 0.679, p > .05. In summary, while subjective benefit improvement (unaided minus aided conditions) was not significant, there was a significant improvement over time in the aided condition for new HA users.
Vestergaard (2006)	13 weeks	1, 4, & 13 weeks postfitting	First-time HA users (> 4 hr/day) Experienced users (< 4 hr/day)	Not reported	GHABP, IOI-HA, HAPQ, & SADL	GHABP & IOI-HA: Yes SADL & HAPQ: No	First-time HA users who wore their devices for over 4 hr daily reported improved outcomes over time compared to users with prior HA experience or those using their HAs for less than 4 hr daily. Notably, this improvement was observed in the GHABP, F(1, 23) = 5.95, p = .023, and IOI-HA, F(1, 23) = 5.13, p = .033, scales, which address general aspects of HA benefit and satisfaction. In contrast, there was no such change over time in the SADL, F(1, 23) = 3.86, p = .062, and HAPQ, F(1, 23) = 0.54, p = .472, scales, which focus on HA performance in specific situations.
Metselaar et al. (2009)	12 months	Baseline, 2, 6, & 12 weeks, 6 & 12 months postfitting	Not reported	—	HHDI, APHAB	Yes	HHDI: New HA users experienced significant improvements in various aspects immediately after fitting, with sustained results over a year. Notably, new users showed better results in “withdrawal,” while fitting procedures generally didn't differ significantly. In summary, the new HA users reported lower withdrawal and greater benefit from their HAs compared to experienced users. APHAB: After the initial fitting, users reported significant benefits in most subscales, except for aversiveness. These benefits remained consistent at the 26-week follow-up, and no significant differences were found between fitting procedures or levels of speech recognition. First-time users of HAs reported significantly greater benefit at 6 and 26 weeks compared to experienced users in subscales EC, BN, and RV. However, in subscale AV, the benefit was negative: First-time users reported encountering more problems compared to experienced users.
Dawes et al. (2014a)	12 weeks	Baseline and 12 weeks postfitting	7.5 hr/day	Objective (datalogging feature)	SSQ	Yes	The mean SSQ-D scores for new unilateral users (26.2, SD = 14.5) and new bilateral users (20.5, SD = 14.6) showed positive performance improvements over time, contrasting with experienced users (−0.2, SD = 11.7) who showed no significant change, F(2, 46) = 17.2, p < .01. The new unilateral and bilateral groups did not significantly differ from each other (p = .65), while the experienced user group scored significantly lower than both (p < .01). Additionally, no significant correlations were found between SSQ-D scores and aided performance changes on the FAAF for any listening condition among new users (rs ranged from .00 to .10, all ns).
Chang et al. (2016)	3 months	HHIE: Prefit and 1 & 3 months IOI-HA: 1 & 3 months postfit	Not reported	—	HHIE & IOH-HA	HHIE: Yes IOI-HA: No	HHIE scores decreased from prefitting (54.07 ± 27.35) to 1 month (43.25 ± 27.87) and 3 months (37.79 ± 27.07), with statistically significant differences between prefitting and both 1-month and 3-month scores. Comparison of IOI-HA scores at 1 month and 3 months showed no statistically significant difference. The control group also showed no change over time.
Dawes & Munro (2017)	1 month	Initial vs. Day 30	Required at least 6 hr/day	Objective (datalogging feature)	IOI-HA & questionnaire: “annoyance”	No	No changes were noted in self-reported annoyance or distraction ratings of background noise during the task. Experienced users found the noise more bothersome and distracting than new users, despite no age or hearing differences.
Karawani et al. (2018b)	6 months	Pre vs. 6 months post	Required to wear 8 hr/day	Objective (datalogging feature)	APHAB & SSQ	APHAB: Yes SSQ: No	APHAB: Statistically significant improvements in the EC, RV, and BN subscales with HA use. However, AV to sounds increased with HAs. Notably, despite heightened awareness of aversive sounds, the improvements in the EC, RV, and BN subscales indicate enhanced communication with HA use over the 6-month period. SSQ: The paired-samples t test showed a significant improvement in the experimental group's speech subscale scores from pre- to post-assessment, t(16) = 2.183, p = .044, although this significance was lost after multiple comparison correction. No significant differences were found in the control group's pre and post scores, t(12) = 0.699, p = .498.

Note.  HA = hearing aid; HPI = Hearing Performance Inventory; PHAB = Profile of Hearing Aid Benefit; ANOVA = analysis of variance; GHABP = Glasgow Hearing Aid Benefit Profile; IOI-HA = International Outcome Inventory for Hearing Aids; HAPQ = Hearing Aid Performance Questionnaire; SADL = Satisfaction with Amplification in Daily Life; HHDI = Handicap and Disability Inventory; APHAB = Abbreviated Profile of Hearing Aid Benefit; EC = Ease of Communication; BN = Background Noise; RV = Reverberation; AV = Aversiveness; SSQ = Speech, Spatial and Qualities of Hearing Scale; SSQ-D = Speech, Spatial and Qualities of Hearing Scale–Difference version; FAAF = Four Alternative Auditory Feature Test; ns = ; HHIE = Hearing Handicap Inventory for the Elderly.

Table 5.

Summary of studies on electrophysiological outcomes (n = 7).

Study	Duration of study	Time points measured	HA use	How HA use was measured	Assessment measures	Assessing	HA acclimatization	Outcomes
Dawes et al. (2013)	3 months	Baseline vs. 3 months	Required at least 6 hr/day	Objective (datalogging feature)	ABR (click stimulation)	Latency and amplitude: Wave V	No	No latency changes were observed over time in the aided ears of new user groups, with high reliability indicated by the high correlation between T1 and T2 latency. There were no differences between T1 and T2 in mean amplitude for any group, suggesting lower reliability as indicated by the low correlation between T1 and T2 amplitude. In summary, the study found no changes in either the latency or amplitude of ABR Wave V in the aided ears of both groups after 3 months of HA usage.
Dawes et al. (2014b)	12 weeks	Baseline & 12 weeks	Required at least 6 hr/day. M = 10 hr/day	Objective (datalogging feature)	CAEPs	Latency and amplitude: N1 and P2	No	In the new unilateral (NU) group, level significantly affected N1 and P2 amplitude, but no time-related effects were observed. For the new bilateral (NB) group, both N1 and P2 latencies were significantly influenced by level, and ear also significantly affected amplitudes, with no time-related changes. The experienced user group showed significant effects of level on N1 latency and N1/P2 amplitudes but no time-related effects. No significant correlations were found between changes in N1/P2 measures and factors such as age, hearing loss, or HA use in new users. Correlations between baseline and Week 12 amplitudes and latencies were significant across all groups. Overall, no significant changes over time were detected in N1 or P2, suggesting that acclimatization effects related to HA use were not observed.
Habicht et al. (2018)	24 weeks	Baseline, 12 & 24 weeks postfitting	New: 8.2 hr/day. Ex: 10.7 hr/day	NovHA: datalogging ExpHA: self-reported	Late ERPs	Latency and amplitude: N1, P2, N2, P3	No	The study detected no changes in ERP measurements over time for both new and experienced HA users. Latencies of the N2 and P3 components remained consistent across groups, with no differences or changes observed in P3 amplitudes between the two groups.
Karawani et al. (2018a)	6 months	Baseline & 6 months postfit	Required to wear 8 hr/day	Objective (datalogging feature)	CAEPs	Amplitude: P1, N1, P2	Yes	The experimental group showed increased N1 and P2 amplitudes. In quiet conditions, an interaction between time, amplification, and group was observed, with amplified N1 peak amplitudes in the experimental group. No changes were found in noise conditions. Additionally, P2 amplitudes increased in aided conditions for both quiet and noise, but not without HAs. However, no interaction was found for time and group. No changes were observed in the control group. Overall, these findings suggest HAs may enhance auditory signals and cognitive function.
Karawani et al. (2018b)	6 months	Baseline & 6 months postfit	Required to wear 8 hr/day	Objective (datalogging feature)	FFR	Peak latency and amplitude: Transition and steady-state regions of the response	Yes	Significant differences in peak latencies were found between groups for quiet listening conditions (65 dB: t(13) = 3.357, p = .005; 80 dB: t(13) = 3.684, p = .003), with delayed latencies in the control group and no change in the experimental group, t(17) < 0.868, p > .397. Amplification led to earlier latencies (65 dB: p = .002; 80 dB: p < .001; noise: p = .002). F0 amplitude decreased in the experimental group, particularly at 65 dB, interaction: F(1, 30) = 5.890, p = .021. A main effect of amplification, F(1, 12) = 5.758, p = .023, and region, F(1, 12) = 14.585, p = .001, was observed, with no significant interactions. In summary, while the experimental group showed no latency changes, the control group exhibited delayed latencies. Physiological changes, such as reduced F0 magnitude, were seen in the experimental group, suggesting HAs may mitigate neural timing delays and affect subcortical processing.
Megha & Maruthy (2020)	2 months	Baseline, 1 & 2 months Control: Recorded twice (2-month interval)	8.1 hr/day	Objective (datalogging feature)	CAEPs	Latency and amplitude: P1, N1, P2	Yes	The clinical group displayed reduced latency for P1 and N1, while P2 remained stable with the Friedman's test, indicating a significant main effect of session on the peak latency of P1, χ2(2, 30) = 38.64, p < .001, W = .64, and N1, χ2(2, 30) = 24.88, p < .001, W = .41, but not in P2, χ2(2, 30) = 0.47, p = .79, W = .008. The amplitude also increased in P1 and N1 in ALLRs over three sessions, while P2 latency remained stable. In contrast, the control group exhibited minimal alterations in both latency and amplitude of ALLRs across the two sessions, with no significant differences between the first and third session. Overall, the clinical group showed acclimatization to the HAs with decreased latency and increased amplitude of P1 and N1, while the control group displayed minimal changes.
Karawani et al. (2022)	6 months	Baseline, 2, 6, 12, 18, & 24 weeks	9,31 hr/day	Objective (datalogging feature)	CAEPs	Latency and amplitude: P1, N1, P2	Yes	Neural changes in CAEPs were noticeable as early as 2 weeks after initial fitting. Specifically, there were significant increases in N1 amplitude (p = .031), indicating early neural adaptation. However, it took 6 weeks of HA use to see similar amplitude increases in the P2 component (p = .033), reflecting stimulus detection and identification. Furthermore, a significant increase in P2 amplitudes in noise was observed in the final session—24 weeks following the initial HA visit (p = .012), but N1 in noise amplitudes did not show any significant changes (p > .3). Taken together, these results demonstrate evidence of neuroplasticity in N1 amplitudes earlier (2 weeks following HA fitting) than P2 amplitude changes (6 weeks after HA fitting). In contrast, the control group exhibited no significant changes in the amplitudes and latencies of P1, N1, and P2 peaks between Sessions 1 and 6 in both quiet and noise conditions.

Note. HA = hearing aid; ABR = auditory brainstem response; CAEPs = cortical auditory evoked potentials; NovHA = novice HA users; ExpHA = experienced HA users; ERPs = event-related potentials; FFR = frequency following response; ALLRs = auditory late latency responses.

Tables 3 –5 detail each study's speech recognition, self-reported, and electrophysiological outcomes, respectively. Studies measured various factors hypothesized to impact acclimatization, including listening conditions (aided and unaided), stimulus levels in different environments, and ear comparisons. Additionally, research explored the effects of hearing aid features, fitting protocols, usage duration, degree of hearing loss, age, fitting type (bilateral or unilateral), and cognitive factors. The following sections present study outcomes across the three main domains.

Summary of Studies on Speech Recognition Outcomes

This systematic review analyzed 18 studies on speech recognition, with 11 focusing on recognition in noisy environments, two in quiet, and five in both conditions (see Supplemental Material S1). In total, 10 out of the 18 studies observed an acclimatization effect, while seven studies did not. Additionally, one study (5%), Habicht et al. (2018), reported improvement in processing times but not in speech recognition threshold (SRT; see Table 3).

Aided Speech Recognition in Quiet

The evidence for changes in aided speech recognition in quiet varied across studies. Silman et al. (1993) reported significant improvement in the fitted ear of new, monaurally aided users, while the nonfitted ear slightly declined, suggesting an acclimatization effect for monaural hearing aid users. No changes were observed in the control group of non–hearing aid users, indicating that the improvement in the aided ear was specific to hearing aid use. In contrast, Bentler et al. (1993a) found no significant time effect on the Nonsense Syllable Test (NST) in quiet conditions for both part-time and full-time hearing aid users over a year, with minimal score changes under 5%, suggesting stable performance without substantial acclimatization. Similarly, Saunders and Cienkowski (1997) found no differences in speech recognition scores in quiet (SRT-Q) over 3 months between new and experienced users, and Amorim and Almeida (2007) also reported no significant differences in word discrimination scores in quiet for new unilateral and bilateral hearing aid users over the study period.

Aided Speech Recognition in Noise

The studies showed more consistent improvement in aided speech recognition in noise, often attributed to acclimatization. Gatehouse (1992) found a clear pattern of improvement in speech recognition for new unilateral hearing aid users in noise conditions, with aided scores in the fitted ear improving significantly over 12 weeks, while the nonfitted ear remained stable. Silman et al. (1993) reported similar findings, where monaural users demonstrated improved Speech Perception in Noise test scores in the aided ear, with a decline observed in the nonfitted ear. Additionally, Cox et al. (1996) found a gradual increase in noise benefit from approximately 4% to 8% over 12 weeks for new hearing aid users, with no change in the control group. The Speech Pattern Contrast test also indicated enhanced performance between 6 and 9 weeks, suggesting an acclimatization effect.

Horwitz and Turner (1997) noted an increase in benefit from 7 to 14 rationalized arcsine units (rau) among new unilateral users over five follow-up visits, while long-term users saw a slight decrease from 10 to 9 rau over 18 weeks. Munro and Lutman (2003) reported further confirmation of acclimatization in noise, with aided scores in the fitted ear improving by 1.0%, 2.6%, and 3.6% at 55, 62, and 69 dB SPL, respectively, over 12 weeks, while the nonfitted ear showed consistent negative changes around −2.0%. Choi et al. (2011) found that new bilateral users showed improvement in the Words in Noise test, with scores increasing from 11.7 to 13.1 after 6 months, while the control group remained unchanged. Dawes and Munro (2017) reported a subset of new users with moderate hearing loss using hearing aids for at least 6 hr daily, who showed a significant improvement of approximately 3 dB in SNR over 30 days, particularly for speech-in-noise (SIN) tasks. However, experienced users did not show a similar improvement, highlighting the role of consistent use in achieving acclimatization effects in noise environments. Megha and Maruthy (2020) also observed improvements in SIN performance over 2 months in bilateral hearing aid users. Wright and Gagné (2021) reported a 2-dB SNR improvement over 4 weeks in new bilateral users, with no change in experienced users.

On the other hand, some studies did not find evidence of acclimatization. Dawes et al. (2014a) observed a statistically significant improvement in speech recognition over 12 weeks in new bilateral and unilateral hearing aid users, as well as in a control group of experienced users, likely due to a general practice effect rather than acclimatization. Habicht et al. (2018) found no differences in SRT80s between new and experienced hearing aid users, with only a marginal preference for low linguistic complexity sentences after 24 weeks. Karawani et al. (2018b) reported improved QuickSIN scores in both aided and unaided conditions for the experimental group and in the control group's unaided condition, but no main effects of time or group were observed, suggesting the improvements were due to practice effects rather than hearing aid use.

Unaided Speech Recognition in Quiet

Similar to Saunders and Cienkowski (1997), who reported no significant differences in unaided listening across visits, other studies also observed stability in unaided speech recognition over time. For instance, Song et al. (2011) found that unaided word recognition scores generally remained the same or decreased, while Lavie et al. (2015) reported no changes in unaided monosyllabic word identification in quiet across a 14-week period. Similarly, Amorim and Almeida (2007) found no significant differences in unaided speech recognition scores between aided and non-aided ears, though a slight decline was observed in the non-aided ear for unilateral users.

Unaided Speech Recognition in Noise

In unaided speech recognition in noise, most studies reported either no changes or slight declines over time. Gatehouse (1992) observed a decline in the fitted ear's performance over 12 weeks, while the nonfitted ear showed no significant changes. Cox et al. (1996) found no difference in unaided Connected Speech Test scores between baseline and 12 weeks for both the study and control groups. Similarly, Saunders and Cienkowski (1997) observed no changes in unaided listening across visits. Munro and Lutman (2003) also found a slight, nonsignificant decrease in mean unaided speech recognition scores in noise over time. In contrast, Lavie et al. (2015) reported improvement in unaided speech recognition for new hearing aid users over 14 weeks, with no gains in the control group. However, Karawani et al. (2018b) noted improved QuickSIN scores in both experimental and control groups, but these improvements were likely attributable to a practice effect rather than true acclimatization.

In summary, evidence for acclimatization was inconsistent for aided speech recognition in quiet, with some studies (e.g., Silman et al., 1993) showing improvements in the fitted ear, while others (e.g., Bentler et al., 1993a; Saunders & Cienkowski, 1997) reported stable performance. In aided speech recognition in noise, most studies (e.g., Gatehouse, 1992; Munro & Lutman, 2003) showed consistent improvements, though some (e.g., Dawes et al., 2014a; Karawani et al., 2018b) attributed gains to practice effects. Unaided speech recognition in quiet was generally stable (e.g., Saunders & Cienkowski, 1997), with occasional declines in nonfitted ears, while unaided speech recognition in noise showed stability or slight declines (e.g., Munro & Lutman, 2003), with rare improvements probably due to practice effects (e.g., Karawani et al., 2018b).

Summary of Studies on Self-Reported Outcomes

The self-reported measures encompassed a diverse array of questionnaires and assessments, each evaluating different aspects of hearing and hearing aid use. Measures of self-reported hearing disability included the HHIE (Ventry & Weinstein, 1982) and the Speech, Spatial, and Qualities of Hearing Scale (SSQ; Gatehouse & Noble, 2004), both of which assess the perceived impact of hearing loss on daily life (i.e., self-reported hearing disability). Hearing aid benefit was evaluated using tools such as the Profile of Hearing Aid Benefit (PHAB; Cox & Gilmore, 1990), the APHAB (Cox & Alexander, 1995), and the GHABP (Gatehouse, 1999), which measure improvements in hearing ability and reductions in disability with hearing aid use. Hearing aid satisfaction was assessed through the International Outcome Inventory for Hearing Aids (IOI-HA; Cox & Alexander, 2002) and the Satisfaction With Amplification in Daily Life (SADL; Cox & Alexander, 1999), both of which gauge user contentment with their devices. Additionally, hearing performance and user expectations were explored through the Hearing Performance Inventory (HPI; Giolas et al., 1979). The Hearing Aid Performance Questionnaire (HAPQ) is a visual analogue scale consisting of 18 items that assess hearing aid performance in various situations. It has been used in multiple studies conducted at Eriksholm. Of the eight studies reporting on self-reported outcomes, three showed clear evidence of acclimatization, three demonstrated evidence on certain measures but not others, and two indicated no evidence of acclimatization.

Horwitz and Turner (1997) found that new hearing aid users experienced significant improvement in the aided condition on the PHAB scale across multiple visits, while experienced users showed no change; notably, no changes were observed in the unaided condition for either group. Vestergaard (2006) observed that first-time hearing aid users who wore their devices over 4 hr daily reported improvements on the GHABP and IOI-HA scales, but no change was noted on the SADL and HAPQ scales. Metselaar et al. (2009) noted improvements in new users on the Handicap and Disability Inventory and APHAB scales, especially in areas like “withdrawal,” ease of communication, background noise, and reverberation. Dawes et al. (2014a) found significant improvements on the SSQ for new unilateral and bilateral users, with no change observed for experienced users. Additionally, no correlation was found between SSQ-D scores and performance changes on the Four Alternative Auditory Feature Test for any listening condition among new users. Chang et al. (2016) reported significant improvements on the HHIE scale from prefitting to 1 and 3 months postfitting, while IOI-HA scores showed no significant differences. Karawani et al. (2018b) observed statistically significant improvements in the APHAB subscales of Ease of Communication, Reverberation, and Background Noise but noted an increase in awareness of aversive sounds; no improvements were observed on the SSQ.

Bentler et al. (1993b) tracked both part-time and full-time hearing aid users over 1 year. While HPI scores remained stable overall, items related to communication in quiet environments showed reduced difficulty over time. The Expectation Checklist revealed no changes in overall scores, but self-perceived communication performance exceeded expectations over time. Qualitative judgments and satisfaction measures showed no significant changes. Importantly, while trends in improved outcomes were noted across groups, the only statistically significant difference was the higher daily use time reported by experienced users. Dawes and Munro (2017) found no evidence of acclimatization, as no significant changes were reported in annoyance or distraction ratings for either new or experienced users during a background noise task.

In summary, acclimatization was most frequently detected in subjective measures assessing general hearing aid benefit, such as PHAB, GHABP, and APHAB. In contrast, measures like the HPI, Expectation Checklist, Qualitative Judgments & Satisfaction Questionnaire, SADL, HAPQ, and IOI-HA showed no evidence of acclimatization. These findings suggest that while overall improvements in hearing aid benefit can be observed, specific measures—such as aversiveness, distraction, expectations, satisfaction, and situational responses—may not consistently reflect acclimatization (see Table 4).

Summary of Studies on Electrophysiological Outcomes

A total of seven studies examined electrophysiological outcomes, specifically ABR, cortical auditory evoked potentials (CAEPs), and late ERPs, to assess the effects of hearing aid use on neural adaptation and auditory processing over varying time frames. Among the studies, four provided evidence supporting acclimatization, whereas three did not find such effects. All studies assessed both latency and amplitude values to determine whether acclimatization occurred over time, except for the study by Karawani et al. (2018a). In this study, only the amplitudes of the CAEP P1, N1, and P2 peaks were measured.

The findings on acclimatization to hearing aids through electrophysiological outcomes were mixed, with some studies showing improvements in neural measures, while others found no significant changes over time. In Karawani et al. (2018a), the experimental group demonstrated increased N1 and P2 amplitudes, particularly in quiet conditions, suggesting that hearing aids had a positive impact on auditory processing. This improvement was driven by a significant interaction between time, amplification, and group, with the experimental group showing enhanced N1 and P2 amplitudes in aided conditions compared to the control group, which showed no significant changes. Similarly, in Karawani et al. (2018b), the experimental group exhibited stable peak latencies, while the control group showed delayed latencies, suggesting that hearing aids helped to mitigate neural timing delays, particularly for the F0 magnitude. Megha and Maruthy (2020) also found evidence of acclimatization, with the experimental group showing reduced latency and increased amplitude for P1 and N1 components, indicating neural adaptation, while P2 latency remained stable. In contrast, the control group showed minimal changes in latency and amplitude. In Karawani et al. (2022), neural changes were observed as early as 2 weeks postfitting, with significant increases in N1 amplitude, followed by similar increases in P2 amplitudes by 6 weeks. This early change in N1 amplitude is indicative of neuroplasticity.

However, three studies found no significant evidence of acclimatization. Dawes et al. (2013), for example, found no changes in the latency or amplitude of ABR Wave V after 3 months of hearing aid use, despite participants using the devices for at least 6 hr a day. Similarly, Dawes et al. (2014b) observed no significant changes in N1 and P2 latencies or amplitudes after 12 weeks, suggesting that acclimatization did not occur. Habicht et al. (2018) also found no changes in ERP measures (N1, P2, N2, P3) over 24 weeks, indicating that the prolonged use of hearing aids did not result in significant neural adaptations in either new or experienced users (see Table 5).

Factors Influencing Acclimatization

The factors potentially influencing acclimatization to hearing aids, as explored across the various controlled studies, include hearing aid usage patterns, degree of hearing loss, cognition, age, and presentation levels (see Table 6). Some studies found a positive link between consistent hearing aid use (> 4 hr/day) and improved self-reported outcomes (r = .37, p < .02; Bentler et al., 1993b; Vestergaard, 2006) but showed limited impact on electrophysiological measures, as no significant correlations have been found between hearing aid usage and changes in N1/P2 amplitude (Dawes et al., 2014b). Regarding degree of hearing loss, some evidence suggests that new hearing aid users with severe hearing loss experience greater SIN improvements than those with milder losses. For example, Dawes and Munro (2017) found that individuals with severe hearing loss improved by 4.1 dB in SIN performance over 30 days (p < .01), whereas those with milder losses or inconsistent use showed negligible changes.

Table 6.

Factors influencing acclimatization.

Factors		Influence on acclimatization
Hearing aid use	Speech recognition	Bentler et al. (1993a) found no difference in speech recognition improvements between part-time (< 4 hr/day) and full-time (> 4 hr/day) hearing aid users. Dawes et al. (2014a, 2014b) found no correlation between hearing aid use and changes in speech recognition performance. Dawes & Munro (2017) showed that new users with severe hearing loss and consistent use improved speech recognition by 4.1 dB in 30 days, while experienced users improved by 0.8 dB. No improvement was seen for new users with milder loss or inconsistent use. Wright & Gagné (2021) found no link between hearing aid use and the acclimatization period, likely because all participants used their aids for at least 9 hr/day.
	Self-reported outcomes	Vestergaard (2006) found that first-time users wearing hearing aids for over 4 hr daily had better benefit and satisfaction over time, but the study couldn't confirm a direct cause-and-effect relationship between usage and long-term outcomes. Bentler et al. (1993b) found a correlation between daily hearing aid use and satisfaction ratings (r = .37, p < .02).
	Electrophysiological measures	Dawes et al. (2014b) found no correlations between changes in N1/P2 amplitude or amount of hearing aid use.
Degree of hearing loss	Speech recognition	Bentler et al. (1993a) found no change in speech recognition performance over time, despite participants having mild-to-moderate hearing loss. Dawes et al. (2014a, 2014b) found no correlation between hearing loss severity and changes in speech recognition performance. Dawes & Munro (2017) found that new hearing aid users with severe hearing loss and consistent use showed improvement in speech-in-noise performance over time, F(3, 22) = 7.21, p < .01. Wright & Gagné (2021) found that more severe hearing loss was associated with greater improvement in speech perception in noise.
	Self-reported measures	Bentler et al. (1993b) found no change in the HPI-38 total score over time, regardless of hearing loss severity, suggesting that hearing loss severity did not impact self-reported outcomes. Metselaar et al. (2009) found that while the degree of hearing loss affected HHDI scores, it did not correlate with the perceived benefit from hearing aids (APHAB scores). This indicates that different levels of hearing loss might lead to similar benefits from proper hearing aid fitting.
	Electrophysiological measures	Dawes et al. (2014b) found no correlation between changes in N1/P2 amplitude or latency and average hearing loss levels for new hearing aid users.
Cognition		Dawes et al. (2014a) and Dawes & Munro (2017) found no correlation between cognitive factors (reaction time, working memory) and changes in speech recognition or acclimatization outcomes. They suggested that acclimatization shifts attention, making background sounds noticeable. As acclimatization advances, these sounds are ignored, reducing masking, improving speech recognition, and enhancing background noise tolerance. Wright & Gagné (2021) also found no correlations between working memory, processing speed, and acclimatization.
Age		Dawes et al. (2014b) found no correlations between age and changes in speech recognition, N1/P2 latency, or amplitude. Wright & Gagné (2021) found no correlation between age (63–75 years, M = 70.2) and the extent of acclimatization over 22 weeks.
Presentation levels		Munro & Lutman (2003) found that acclimatization to hearing aids occurs at higher speech levels, leading to greater improvements, while lower levels showed no changes. Dawes et al. (2014a) reported no improvements at higher stimulus intensities.

Note.  HHDI = Handicap and Disability Inventory; APHAB = Abbreviated Profile of Hearing Aid Benefit.

Cognitive factors and age do not appear to be significant predictors of acclimatization. Multiple studies have reported no significant correlations between acclimatization effects and cognitive abilities such as working memory or reaction time (Dawes et al., 2014a; Dawes & Munro, 2017; Wright & Gagné, 2021). Similarly, age has not been found to influence acclimatization outcomes, as no significant associations have been observed between age and speech recognition improvements, self-reported benefit, or electrophysiological responses (Dawes et al., 2014b; Wright & Gagné, 2021). Findings on presentation levels have been inconsistent. Munro and Lutman (2003) reported that acclimatization effects were greater at higher presentation levels, whereas Dawes et al. (2014a) found no significant changes at increased stimulus intensities. Overall, while some studies suggest that consistent hearing aid use and greater hearing loss severity may facilitate acclimatization—particularly for SIN recognition—other factors such as cognition, age, and presentation levels remain inconclusive or show mixed results.

Quality (Risk of Bias) Assessment and Level of Evidence for the Systematic Review

Quality ratings, as detailed in Supplemental Material S2, showed an average score of approximately 62% on the NIH Quality Assessment Scale. Of the 25 studies reviewed, four (16%) were rated as good quality, 20 (80%) as fair quality, and one (4%) as poor quality.

Among the studies rated as “good” quality, three out of four reported evidence of auditory acclimatization, particularly in speech recognition in noise and electrophysiological measures. Munro and Lutman (2003) observed improvements in speech perception in noise, while Karawani et al. (2018a, 2018b) reported neural changes associated with acclimatization, including increased N1 and P2 amplitudes. Wright and Gagné (2021) also found a 2-dB improvement in SIN performance over 4 weeks. In contrast, studies rated as “fair” showed more variability, with some reporting acclimatization effects (e.g., Cox et al., 1996; Gatehouse, 1992; Silman et al., 1993), while others did not (e.g., Choi et al., 2011; Dawes et al., 2013, 2014a, 2014b; Saunders & Cienkowski, 1997). This pattern suggests that methodological rigor may influence the ability to detect acclimatization, but even among the highest quality studies, results remain inconsistent.

Common factors contributing to lower quality included the lack of randomization, absence of blinding in group assessments, and insufficient reporting on sample size considerations to ensure adequate statistical power for detecting significant differences in primary outcomes. Notably, only eight of the 25 studies conducted sample size estimations or power analyses. Regarding the OCEBM levels of evidence, where Level 1 represents the highest quality of evidence and Level 5 represents the lowest, out of the 25 studies analyzed, one (4%) was classified as Level 4, 20 (80%) as Level 3, and four (16%) as Level 2, representing true experimental studies (see Supplemental Material S2).

Discussion

Is There High-Quality Evidence of Systematic Improvements Outcome Measures Consistent With Acclimatization Following Hearing Aid Use?

Despite the extensive scope of this review, which spans over 30 years of research, the findings align with previous reviews indicating that evidence for auditory acclimatization is generally inconsistent and, when observed, tends to be small to moderate in magnitude. This conclusion is consistent with the observations of Turner and Bentler (1998), who, in their letter on hearing aid acclimatization, indicated that across many studies, observed improvements in speech recognition were minimal and often difficult to differentiate from task-specific learning or natural variability.

The current review adds to previous reviews on this topic by providing an up-to-date synthesis of the literature and by including studies that made use of a controlled design. Some frequently cited studies, such as Gatehouse (1993, 1995), were excluded from this review as they involved participants with previous hearing aid experience (12–15 months and 1–4 years, respectively). Since this review focuses on acclimatization in new hearing aid users, studies involving experienced users for the study group were not included.

Among the studies focusing on speech recognition outcomes, there was a tendency toward acclimatization, with 10 studies reporting evidence of acclimatization and seven reporting no such effect. Speech recognition in noise showed the strongest support for acclimatization, as controlled studies frequently reported improvements in the aided ear over time (e.g., Gatehouse, 1992; Munro & Lutman, 2003). However, aided speech recognition in quiet and unaided conditions generally demonstrated limited or inconsistent improvements, with some gains attributed to practice effects rather than true acclimatization (e.g., Dawes et al., 2014a). Self-reported measures provide partial evidence of acclimatization, with findings varying across studies and assessment tools. Specifically, three studies observed clear acclimatization effects, while three others reported mixed results, showing acclimatization in certain measures but not others. In contrast, two studies found no evidence of acclimatization. In terms of electrophysiological outcomes, out of the seven studies, four provided evidence for acclimatization, while three did not.

It is important to note that the number of studies available, particularly in the self-reported and electrophysiological category, may not be sufficient to allow for robust comparisons. Therefore, while there is a trend indicating that more studies report acclimatization rather than no acclimatization, the evidence remains inconsistent and lacks robustness across different study designs and outcome measures.

In conclusion, systematic improvements consistent with acclimatization are most evident in speech recognition in noise and selected self-reported measures, but they are inconsistent in quiet conditions, unaided settings, and electrophysiological outcomes. This variability underscores the complex interplay between individual, environmental, and measurement factors in auditory acclimatization research.

If Changes Were Reported, What Is Their Magnitude and Are They Likely to Be Clinically Relevant?

Speech Recognition

Studies that reported the magnitude of change in speech recognition performance following hearing aid use include Munro and Lutman (2003), who observed aided improvements in the fitted ear across all presentation levels, with gains of 1.0%, 2.6%, and 3.6% at 55, 62, and 69 dB SPL, respectively. The control ear consistently showed declines of around −2.0%. Cox et al. (1996) documented a mean benefit in noise, rising from 4% at fitting to 8% after 12 weeks, with significant improvements occurring between 6 and 9 weeks. Horwitz and Turner (1997) found that new hearing aid users experienced an increase in NST benefit from 7 rau at fitting to 14 rau after 18 weeks, while long-standing users showed no significant change. Dawes and Munro (2017) reported a 3-dB improvement in SIN scores after 30 days for new hearing aid users with moderate hearing loss and consistent use, compared to experienced users. Megha and Maruthy (2020) observed significant improvements in SIN scores after 2 months, while Wright and Gagné (2021) found a 2-dB gain in SNR for new users. Habicht et al. (2018) reported a 30% improvement in processing times for novice users. Overall, while statistically significant changes are observed, the clinical impact depends on the context, measures, and user conditions. Most improvements are modest.

Self-Reported Outcomes

Studies on self-reported measures further illustrate the magnitude of changes associated with hearing aid use. Horwitz and Turner (1997) found a significant decrease in aided problems for new users, F(4, 96) = 2.610, p < .05, while experienced users showed no change, F(4, 96) = 0.804, p > .05. Vestergaard (2006) reported significant improvements on the GHABP, F(1, 23) = 5.95, p = .023, and IOI-HA, F(1, 23) = 5.13, p = .033, scales for first-time hearing aid users who wore their devices for more than 4 hr daily. Metselaar et al. (2009) found significant improvements on the APHAB subscales Ease of Communication, Background Noise, and Reverberation at 6 and 26 weeks for new users, although Aversiveness to Sounds showed a negative effect. Dawes et al. (2014a) found significant improvements in SSQ-D scores for new unilateral (M = 26.2, SD = 14.5) and bilateral (M = 20.5, SD = 14.6) users over 12 weeks, F(2, 46) = 17.2, p < .01, while experienced users showed no significant change (M = −0.2, SD = 11.7). Chang et al. (2016) observed significant reductions in HHIE scores from prefitting (54.07 ± 27.35) to 1 month (43.25 ± 27.87) and 3 months (37.79 ± 27.07), indicating reduced hearing-related difficulties (p < .01). Karawani et al. (2018b) noted significant improvements on the APHAB subscales Ease of Communication, Background Noise, and Reverberation (p < .05), although aversiveness scores increased for some users. These findings highlight that new hearing aid users often experience statistically significant improvements, with communication-related benefits being the most clinically relevant. Most observed changes are small to moderate in magnitude, with some showing larger effects depending on the measure and user group.

Electrophysiological Outcomes

Several studies have shown measurable changes in auditory markers as a result of hearing aid use, reflecting physiological adaptation. Megha and Maruthy (2020) found significant reductions in P1 and N1 latencies (χ² = 38.64, p < .001, for P1; χ² = 24.88, p < .001, for N1) and increased amplitudes in both components, while the control group showed minimal changes. Karawani et al. (2018a) observed increased N1 and P2 amplitudes in quiet conditions for the experimental group, though no changes were observed in noisy conditions. Karawani et al. (2018b) reported earlier latencies for the experimental group in quiet conditions (65 dB, p = .002; 80 dB, p < .001), while the control group exhibited delayed latencies. A reduction in F0 amplitude, F(1, 30) = 5.890, p = .021, suggested that hearing aids might help mitigate neural timing delays. Karawani et al. (2022) found evidence of early neural adaptation, with significant N1 amplitude increases as early as 2 weeks postfitting (p = .031) and significant P2 amplitude increases at 6 weeks (p = .033) and at 24 weeks (p = .012) in noisy conditions, indicating neuroplasticity. In summary, these studies suggest that hearing aid use leads to significant changes in auditory processing, including reduced latencies and increased amplitudes in the P1, N1, and P2 components, indicating potential neuroplasticity. While these changes are of moderate magnitude, they highlight the possible long-term benefits of hearing aid use on auditory function.

Overall, acclimatization effects tend to result in small-to-moderate improvements in auditory performance. While the evidence for auditory acclimatization in new adult hearing aid users is inconsistent, some studies have reported these small-to-moderate effects. It is important to recognize, however, that some research shows improvements in performance over time. These gains, particularly in speech recognition in noise, self-reported communication, and early cortical electrophysiological responses, suggest that even modest improvements may have clinical significance by enhancing both daily functioning and neural adaptation.

However, given that the improvements from acclimatization are generally modest and not consistently observed, audiologists should prioritize addressing other factors that have a more substantial impact on patient outcomes. Focusing on barriers to hearing aid uptake and consistent use—such as patient education, proper device fitting, and expectation management—may lead to more meaningful and lasting improvements than the relatively modest benefits of acclimatization alone.

If Changes Occur, What Is the Time Course?

Study durations varied from 4 weeks to 1 year. Hearing aid acclimatization improvements for speech recognition generally occurred within the first few months. Studies show that new users often experience gradual gains in speech recognition, particularly in challenging environments like noise, with the most significant improvements observed between 4 and 6 weeks (Cox et al., 1996; Gatehouse, 1992; Wright & Gagné, 2021). Benefits continued over several months, with gradual improvements up to a year (Horwitz & Turner, 1997), though most enhancements were noted within the initial 3 months, emphasizing this period for maximizing hearing aid benefits. However, long-standing users generally show stable performance (Megha & Maruthy, 2020).

For self-reported measures, the time course of hearing aid outcomes varies, with most improvements occurring within the first few months. For new users, benefits such as reduced communication difficulties and increased satisfaction are often observed within the first 1–3 months, as seen in studies by Chang et al. (2016) and Horwitz and Turner (1997). Longer term improvements, particularly in communication and hearing benefit, can continue up to a year for users who wear their devices consistently, as shown in Vestergaard (2006) and Metselaar et al. (2009). Experienced users tend to show stable or minimal changes over time, with significant benefits less frequently observed beyond initial adjustments.

Electrophysiological studies showed significant improvements as early as 2 weeks postfitting, with increased N1 amplitude (Karawani et al., 2022) and further increases in P2 amplitude after 6 weeks (Karawani et al., 2022). Mid-term benefits, including reduced latency and increased P1 and N1 amplitudes, were observed at 3–6 months (Megha & Maruthy, 2020). Long-term changes continued beyond 6 months, with differences in peak latencies and P2 amplitudes in noise (Karawani et al., 2018b), reflecting neuroplastic changes from auditory stimulation.

The clinical implications for audiology patients and clinicians regarding the time course emphasize the importance of the first 3–12 weeks after fitting hearing aids. This period is typically when the most significant changes in patient performance and comfort are observed. Regular assessments provide clinicians with valuable information to track progress, identify challenges early, and adjust treatment plans accordingly. However, it is possible that some assessments may not be sensitive enough to detect subtle changes in auditory processing related to acclimatization, potentially overlooking important shifts in performance. While some studies have observed improvements over time, it remains unclear whether these changes plateau or continue beyond the typical assessment periods. Most studies demonstrating acclimatization have not systematically tested over an extended period to determine when performance stabilizes, leaving the time course to asymptote uncertain.

Regarding the timing of baseline measurements, it is crucial to capture data early enough to establish a clear starting point and ensure that any acclimatization that has already taken place is not missed. It is crucial to set realistic expectations through comprehensive counseling, reassuring patients that while improvements may be noticeable early on, additional benefits could develop over the following months.

Effective acclimatization may depend on interventions that mirror real-life experiences, as repeated exposure to relevant sounds can enhance adaptation. Prioritizing approaches that reflect daily auditory situations during these initial weeks may lead to better acclimatization, improving patient satisfaction and outcomes. Additionally, encouraging consistent use of hearing aids during this period can enhance long-term benefits and contribute to overall success. While acclimatization can occur naturally, training and counseling may support this process by reinforcing auditory exposure and device adherence. Perceptual training has shown small but significant effects on speech perception, while informational counseling improves hearing aid use, both of which may contribute to a more stable adaptation. However, the guideline notes that effects on broader outcomes like quality of life are minimal, suggesting training may supplement but not replace natural acclimatization (Basura et al., 2022).

Are Factors Such as the Duration and Severity of Hearing Loss, or the Length of Hearing Aid Use, Associated With Systematic Changes in Outcomes?

The factors influencing auditory acclimatization among hearing aid users are complex and present a range of perspectives across studies. For instance, the duration of hearing aid use has been examined with mixed results. Studies by Bentler et al. (1993a) and Dawes et al. (2014a, 2014b) found no significant correlation between the duration of hearing aid use and improvements in speech recognition. However, Dawes and Munro (2017) observed that consistent use among new users with severe hearing loss led to notable gains in speech recognition, suggesting that regular use might play a crucial role in acclimatization, particularly for those with more severe impairments.

The degree of hearing loss similarly yielded contrasting findings. While Bentler et al. (1993a) and Dawes et al. (2014a, 2014b) reported no significant relationship between the severity of hearing loss and changes in speech recognition, Dawes and Munro (2017) identified improvements in users with severe hearing loss, indicating that the extent of hearing impairment might influence the acclimatization process in certain cases.

While Munro and Lutman (2003) observed acclimatization effects at higher speech levels, Dawes et al. (2014a) found no improvements at higher stimulus intensities, highlighting variability in findings regarding stimulus level impacts. Among studies that used sentence-based tests, four out of eight (50%) reported evidence of acclimatization, while among studies that used word-based measures, six out of 12 (50%) found acclimatization effects. This comparable proportion indicates that acclimatization effects were not more prevalent in one type of material over another. Despite these differences, acclimatization effects varied within both categories rather than between them. This suggests that factors beyond stimulus type play a more significant role in whether acclimatization is observed. Further research is needed to determine whether specific material characteristics, such as spectral emphasis or phonetic complexity, contribute to individual differences in acclimatization outcomes. Of the studies that assessed acclimatization, seven studies evaluated speech perception in quiet, while 12 studies assessed it in noise. Among those that tested quiet conditions, 57% (four out of seven studies) reported acclimatization, whereas in noise, 75% (nine out of 12 studies) found evidence of acclimatization. This suggests that acclimatization is more commonly observed in noise than in quiet, although the difference is not extreme. The higher percentage in noise could indicate that individuals experience greater perceptual changes in more challenging listening environments over time.

Cognitive factors, often considered in rehabilitation contexts, appeared to have no significant impact on acclimatization outcomes (Dawes et al., 2014a; Dawes & Munro, 2017). Additionally, age did not emerge as a significant factor influencing acclimatization (Dawes et al., 2014b; Wright & Gagné, 2021), suggesting that the acclimatization process might be relatively independent of these variables.

We analyzed whether acclimatization effects varied based on the type of control group used. Studies comparing first-time hearing aid users to long-term users often found no significant acclimatization effects. In contrast, studies that included non–hearing aid users with hearing impairment as a control group were more likely to report evidence of acclimatization. This suggests that the presence of a certain control group might influence the observed acclimatization effects.

Overall, the small number of available studies focusing on influencing factors limits the strength of making conclusions. Given this constraint, findings should be interpreted with caution, and further research is needed to better understand how these variables influence acclimatization.

Challenges of Comparison of Existing Studies

Comparing studies on auditory acclimatization is complicated by methodological inconsistencies across study designs, outcome measures, and participant characteristics. Many studies lack randomization, use different control groups (e.g., experienced hearing aid users vs. unaided individuals), or vary in measurement timeframes (ranging from 4 weeks to a year), making direct comparisons difficult. Additionally, speech recognition tests differ in intensity levels (e.g., 50 dB vs. 70 dB SPL), conditions (quiet vs. noise), and scoring methods (percentage correct vs. SNR), while self-reported outcomes rely on diverse questionnaires (APHAB, HHIE, IOI-HA), each assessing different dimensions of benefit. Electrophysiological measures also vary, with some studies focusing on cortical responses (P1, N1, P2) and others on brainstem activity (ABR Wave V), limiting consistency in findings.

Participant variability complicates interpretation, as differences in factors such as hearing loss severity, cognitive status, adherence to device use, hearing aid technology, and fitting protocols create variability across individuals. Furthermore, psychological and personality traits may affect the comparison of participants. For example, motivation, engagement, and openness to new experiences may enhance adaptation, while low motivation, negative attitudes, and unrealistic expectations could hinder it. Emotional factors, such as anxiety, frustration, or hearing-related stigma, may also play a role. Additionally, the quality of support and counseling from audiologists can impact hearing aid acceptance and long-term use.

Future Directions

While research on auditory acclimatization is extensive, there are still opportunities to improve study designs and ensure more consistent reporting of outcomes. Enhancing methodological rigor—particularly in areas such as study design, real-world relevance of interventions, adequate sample sizes, and measurement practices—could strengthen future studies. However, hearing aid acclimatization research faces inherent challenges with randomization and blinding due to individualized fittings, patient preferences, and real-world use (see Table 7). Traditional quality scales often penalize these studies for factors that are difficult to control. A hearing aid–specific quality scale could offer a more suitable evaluation by emphasizing key factors such as adequate sample size, statistical power, control of confounding variables (e.g., hearing loss severity), adherence monitoring (e.g., datalogging), standardized time points, and multiple outcome measures (e.g., speech recognition, self-reported benefit, electrophysiology). To improve rigor while addressing these challenges, alternative methods can be adopted, as follows:

Table 7.

Recommendations for designing and reporting of future studies on hearing aid acclimatization.

Area of recommendation	Suggestions for future clinical trials
Study design	Utilize longitudinal designs with at least 6 months duration for extended monitoring. Include control groups to differentiate hearing aid effects from natural adaptation. Prioritize randomization allocation of participants to different groups.
Testing relevance	Effective acclimatization occurs with interventions closely resembling real-life experiences. Optimal adaptation may be achieved through repeated exposure to relevant stimuli. In hearing aid studies, mirroring daily auditory experiences may enhance adaptation. Ensure hearing aid usage of at least 8 hr a day.
Sample size	Ensure study sample is appropriate for statistical power. Only 8 out of the 25 reviewed studies performed sample size estimation or power analysis.
Outcomes	Include different domains of outcomes such as self-reported, behavioral, and electrophysiological. Use standardized outcome measures that have been commonly used in hearing aid studies. Collect data on various factors (e.g., hearing aid fitting quality, daily usage patterns, and device compliance) to examine factors influencing auditory acclimatization.
Time points & baseline measurements	Take frequent measurements: baseline, 1 month, 3 months, 6 months, and 12 months. Avoid delays in baseline measurements to prevent underestimating treatment effects. Ensure follow-up assessments are long enough to observe initial improvements and long-term stabilization.
Additional factors	Investigate impact of hearing aid fitting quality and daily use on acclimatization outcomes. Examine effects of poorly fitted devices and low compliance on auditory, cortical, and cognitive improvements. Explore the influence of rehabilitation, auditory training, and advanced hearing aid features (e.g., directional microphones, noise reduction algorithms) on acclimatization outcomes.
Acclimatization yes/no	Ensure clarity in terminology to distinguish benefit or training effect from acclimatization. Differentiate acclimatization by observing group improvements. Experimental group improvement without control group changes indicates acclimatization. Both groups improving may suggest a practice effect, not acclimatization. No change or decreased performance in both groups suggests acclimatization absence.

• Blinding testers: Speech perception and electrophysiological assessors should be blinded to group assignments to reduce bias.

• Blinding participants: A double-dummy approach could be used, where all participants receive hearing aids programmed with either an experimental or placebo setting.

Implementing these strategies would reduce bias, improve reliability, and enhance the detection of genuine acclimatization effects. Additionally, better-powered studies are needed, as only eight of 25 controlled studies included sample size estimations or power analyses, raising concerns about underpowered research and inconsistent findings. Ensuring larger, well-controlled studies would enhance reliability and consistency in acclimatization research.

Limitations

A key limitation of this study was the inability to complete the meta-analysis due to missing data required for the meta-analysis. Additionally, variability in the timing and type of outcome measures across studies complicated the comparison and integration of findings.

Conclusions

This review highlighted the complexity of auditory acclimatization to hearing aids, a process that can be influenced by various factors. Evidence suggested that acclimatization occurred in some users and for certain outcomes, with the most consistent improvements observed in speech recognition in noise and self-reported measures, including the APHAB, HHIE, and GHABP questionnaires. However, overall changes in outcomes remained inconsistent, and when acclimatization was present, its effects were generally modest. Notably, the first 3–12 weeks after hearing aid use appeared to be the most likely period for acclimatization to occur. The variability and modest effects observed across numerous studies indicated that while acclimatization may have contributed to the adjustment process, it was unlikely to be the primary factor influencing overall hearing aid success.

Future research on auditory acclimatization should emphasize the inclusion of essential statistical metrics, such as correlation coefficients in within-subject designs, to facilitate robust meta-analyses. Studies should prioritize rigorous control group methodologies to accurately quantify the magnitude of changes directly attributable to acclimatization. Additionally, audiologists must continue to address barriers to hearing aid adoption by promoting consistent device use and implementing innovative, patient-centered strategies tailored to individual needs. While most clinicians already address these barriers, emerging research suggests that stigma related to hearing loss may be a primary concern (Scarinci et al., 2024). Addressing this stigma directly in consultations could further enhance patient engagement, leading to improved auditory outcomes and greater satisfaction with hearing aids.

Author Contributions

Clarissa Wentzel: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft. De Wet Swanepoel: Supervision, Writing – review & editing. Faheema Mahomed-Asmail: Supervision, Investigation, Writing – review & editing. Eldré Beukes: Formal analysis, Validation, Writing – review & editing. Piers Dawes: Writing – review & editing, Validation. Kevin Munro: Writing – review & editing, Validation. Ibrahim Almufarrij: Methodology, Writing – review & editing. Vinaya Manchaiah: Conceptualization, Supervision, Writing – review & editing.

Data Availability Statement

The data of the extracted studies are freely available on the University of Pretoria repository Figshare at the following link: http://doi.org/10.25403/UPresearchdata.28266458. For any questions regarding the data, contact the corresponding author.

Funding Statement

K. M. and P. D. are supported by the National Institutes for Health and Care Research Manchester Biomedical Research Centre.