The Use of Frequency Lowering Technology in the Treatment of Severe-to-Profound Hearing Loss: A Review of the Literature and Candidacy Considerations for Clinical Application

Abstract

This article provides a review of the current literature on the topic of frequency lowering hearing aid technology specific to the treatment of severe and profound levels of hearing impairment in child and adult listeners. Factors to consider when assessing listener candidacy for frequency lowering technology are discussed. These include factors related to audiometric assessment, the listener, the type of hearing aid technology, and the verification and validation procedures that can assist in determining candidacy for frequency lowering technology. An individualized candidacy assessment including the use of real-ear verification measures and carefully chosen validation tools are recommended for listeners requiring greater audibility of high-frequency sounds, when compared with amplification via conventional hearing aid technology.

Learning Outcomes: As a result of this activity, the participant will be able to (1) list five factors to consider when assessing candidacy for frequency lowering hearing aids; (2) explain and apply a verification method for use in the assessment of frequency lowering hearing technology.

Frequency lowering (FL) is a hearing aid signal processing technique that aims to improve audibility of high-frequency sounds for hearing impaired (HI) listeners by shifting high-frequency sounds to a lower output frequency. Effectively, FL may offer greater audibility for an extended range of frequencies for some listeners. Included under the FL umbrella term are many different types of clinically available technologies, each of which provides a unique lowering effect. A growing body of literature exists on the topic of FL, addressing listener performance and candidacy factors across various types. Benefit is often assessed through comparison of listener performance with FL technology to that of conventional hearing aid technology (i.e., with FL disabled). This incorporates known fitting challenges that can accompany conventional hearing aid fittings, including limited high-frequency gain, limited receiver bandwidth, and/or acoustic feedback. These limit the hearing aid's ability to make sounds loud enough for listeners with high-frequency hearing loss, especially when considering severe and profound levels of hearing loss in the high frequencies.

Most of the literature reporting studies of FL technology has focused on the evaluation of speech perception. Considering the studies reporting benefit with FL in hearing aid trials with adult and child listeners, many have included listeners with sloping hearing loss of a severe or profound degree in the high frequencies. ^{1 2 3 4 5 6 7 8 9 10} This literature also suggests that some listeners may not benefit from FL and may even experience adverse speech perception effects. ^{11 12 13} In addition to speech perception, some of the more recent studies have focused on how FL may affect outcomes of sound quality and objective measures of listening effort for listeners with high-frequency hearing loss. For example, perceived sound quality of speech may improve with FL, compared with conventional fittings. ^{2 14} Recently, Kirby and colleagues evaluated sound quality using a questionnaire and found better self-reported word understanding. In addition, listening effort was not negatively affected for the one type of FL tested. ¹⁵ In other studies, HI listeners have reported a broad range of settings in which the quality of speech is similar to that of conventional technologies and that FL can adversely affect sound quality, with poorer ratings reported for stronger FL settings. ^{16 17} Findings for music perception suggest ratings are unaffected, on average, ^{14 17} or are negatively affected by stronger settings. ¹⁸ This relates to the concept of FL, which is aimed at improving audibility of high-frequency sound, but introduces distortion to the aided signal. It is important to note, however, that the listeners in the study of Mussoi and Bentler did not present with severe or profound hearing loss. ¹⁸ Therefore, the results may not generalize to the population of interest in this article.

Fig. 1 illustrates factors that influence FL benefit in the current literature. These include the listener's degree and configuration of hearing loss, their age, language development needs, cognitive ability, acclimatization time with FL, the type of FL and fitting parameters available in the device, the verification procedures used when fitting FL, and/or the validation procedures used to measure performance. This review article will focus on the topic of FL in the treatment of severe or profound hearing loss for children and adults, with discussion of candidacy for clinical application. Relevant literature will be reviewed within the context of a patient-centered approach to fitting FL technology.

Figure 1.

Candidacy factors to consider when fitting frequency lowering to adults and children with severe-to-profound hearing loss.

Factors to Consider when Fitting Frequency Lowering

Audiometric Factors

Degree of Hearing Loss

Analyses of the degree and configuration of hearing loss in a typical caseload suggest that approximately 15% of clinical cases have severe and profound levels of impairment. ¹⁹ Sloping hearing loss is a commonly reported hearing configuration in adult and child listeners, with poorer thresholds in the high frequencies. ^{19 20} FL is considered when fitting hearing aids to listeners with severe and profound hearing loss because it aims to improve audibility of high-frequency sounds that may not be available via conventional fitting. Should the hearing aid fitter therefore consider all listeners with severe or profound hearing levels in the high-frequency candidates for FL? Although there are many studies suggesting FL benefit for listeners with greater degrees of HI, a mixed overall message is presented on this topic. Large variability in aided listening performance is reported for listeners with severe and profound, high-frequency hearing loss even with conventional fittings. Individual performance with hearing aids can be influenced by both the audibility of the signal and the listener's proficiency in extracting useful information from that signal, as well as the type of measure used to assess benefit (see below). However, studies also have demonstrated that some listeners with severe and profound high-frequency hearing loss experience negative effects on speech understanding performance when provided with an extended high-frequency bandwidth without FL. ^{21 22 23} When presenting with similar high-frequency thresholds, adult listeners with flat or moderately sloping hearing loss benefit more from information in amplified high-frequency speech components than individuals with steeply sloping hearing losses. ²⁴ On the other hand, the literature suggests that some listeners with sloping hearing loss experience significantly improved speech understanding, especially for noisy listening environments, when provided with extended high-frequency information. ^{25 26}

Degree of hearing loss predicts or relates to FL benefit, ^{27 28} with greater degrees of hearing loss (e.g., high-frequency thresholds in the severe range) being associated with better performance with FL. ^{3 27} Some studies including listeners with milder levels of impairment (e.g., mild–moderate degree of high-frequency loss) report poorer performance and/or less benefit with FL. ^{28 29 30 31} This likely relates to listeners with mild–moderate high-frequency hearing loss experiencing fewer limitations with conventional technology and/or requiring weaker FL settings or smaller amounts of lowering. Results describing FL benefit by degree of hearing loss are therefore related to improvements in aided audibility with FL, or lack thereof, when compared with conventional technology.

Configuration of Hearing Loss

The literature is mixed when considering hearing loss configuration as a candidacy factor for FL. Listeners in the study of Glista et al, presenting with high-frequency steeply sloping hearing loss, were more likely to benefit from FL technology than those with flatter configurations. Less favorable results were reported in a previous study, ¹¹ which used different fitting methods and prototype hearing instruments. Degree and configuration of hearing loss may remain important factors in determining candidacy for FL. In general, it is more difficult to provide audibility of high frequencies for steeply sloping hearing losses. Improvement in aided audibility with FL is often considered for such cases. However, not all listeners benefit equally, or at all, from FL. This article will focus on the idea that the fitter needs to assess candidacy for FL on an individual basis. Candidacy consideration includes many factors beyond the audiometric factors, many of which will be discussed in this article.

Cochlear Dead Regions

Several studies have considered whether advanced hearing assessment, specifically those related to the detection and characterization of cochlear dead regions (DRs), may be useful in determining candidacy for FL. A review article offers comprehensive information on the diagnosis, perceptual consequences, and implications of fitting hearing aids in the presence of DRs. ³² Clinically, DRs can be estimated using the threshold-equalizing noise test, now calibrated for use in dB HL. ³³ Research evaluating FL for participants with diagnosed DRs is emerging. This is not surprising given that DRs are more commonly associated with steeply sloping losses and with hearing loss more than 90 dB in the high frequencies. Furthermore, it has been recommended that consideration be given to the use of FL for listeners diagnosed with DRs at high frequencies. ³² Research in this area is divided into studies that have incorporated information of DRs in the FL fitting process ^{10 34 35} and those that have not. ^{3 4 6} The fitting methods, type of FL technology used, and participant groups all differ across the reported studies, making conclusions related to fitting FL with DRs difficult. Recently, one study found FL to produce a moderate degradation in sound quality for a group of listeners with extensive DRs, ³⁵ suggesting that the presence of DRs, the type of FL, and the settings used may influence benefit. However, a study by Cox et al reported speech recognition benefit from improvements in high-frequency audibility, on average, for both listeners with and without DRs. ³⁶ More research is needed to assess the impact of DRs on FL benefit using a study design that compares back to a control group and across various types of FL.

Listener Factors

Adult/Child Differences and Language Development

The age of the listener may play a role in determining candidacy for FL, especially when considering the listener's language development needs. High-frequency speech energy provides listeners with important linguistic information. For example, speech sounds such as /s/ and /z/ are important grammatical markers that denote plurality and possession in the English language. ³⁷ The spectral peak frequency for the phoneme /s/ is at approximately 5 kHz for male speech, between 6 and 9 kHz for female speech, and at 9 kHz for child speech. ^{38 39} It is therefore important to provide audibility across a broad bandwidth of frequencies when attempting to make all types of speech audible for children. Consideration should be given to the hearing aid pass-band when attempting to provide audibility of high-frequency speech cues. ⁴⁰

Significant adult/child differences in the bandwidth required for accurate fricative recognition for listeners with moderate to moderately severe hearing loss are reported in the literature. ^{37 39 41} In these studies, children required greater high-frequency bandwidth, compared with adults (i.e., above 5 kHz) to achieve similar speech recognition scores for the phoneme /s/. This suggests that children require audibility of a broad bandwidth of speech for optimal access to fricatives and when developing speech production and overall language abilities. Children provided with extended bandwidth perform better on short-term word learning tasks than children who are provided with a limited bandwidth, regardless of hearing status. ⁴² In infants, hearing loss is linked to a delay in fricative and affricate production. ^{37 43} Furthermore, children often communicate with female caregivers and with their peers, indicating the need for consistent exposure to high-frequency sound.

Only two studies have directly compared adult–child benefit with FL. Glista et al reported children receiving greater FL benefit, when compared with adults ³ and McCreery et al reported similar FL benefit across child and adult listeners. ³⁰ Several factors are discussed within these articles as important to consider when making comparisons of adult–child FL benefit. First, children may have more hearing loss than adults in typical caseloads and slight but possibly important differences were noted by Glista and colleagues. Second, results may be confounded by differences in the underlying hearing aid prescription and fitting, because children typically use higher levels of gain and output in their hearing aids, which affects both audibility with conventional fittings and the strength of FL setting that may be required. These factors may affect speech recognition ability with conventional amplification. The study by McCreery and colleagues included listeners with HI sloping to a moderately severe degree of impairment, on average. Speech recognition was reported to be similar across age groups. ³⁰ Finally, the clinical goal of pediatric fitting may be driven in part by the need to provide access to speech cues to promote auditory development, while adult fittings may be more driven by listener preference. More remains to be learned about age differences, and children and adults may differ in their ability to benefit from FL. An individualized approach to FL fitting will be discussed later.

Cognitive Ability

On the other end of the age spectrum, recent research has focused on whether cognitive ability predicts speech perception benefit when listening to frequency-lowered signals. This expands on the idea that FL technologies inherently introduce distortion to the aided output signal by modifying the signal in the frequency domain. Depending on the type of FL, modifications can include reduced spacing between harmonics, altered spectral peak levels, and modified spectral shape. ⁴⁴ Work by Souza and colleagues suggests that the fitting of FL technology can be viewed as a trade-off between improved audibility versus increased distortion, and that this pattern varies across individuals. ¹⁶ A review article by Akeroyd suggests that cognition can be a useful predictor of hearing-aid benefit, in some circumstances. Working memory is known to be highly related to language comprehension, with measures of reading span suggested as being more effective than measures of IQ or general ability. ⁴⁵ Ellis and Munro studied the effect of cognition in predicting frequency-lowered speech recognition in normal-hearing adults using the reading span test (RST) and the trail-making test. Results were poorer in the FL condition; however, this was not found to significantly relate to cognitive ability. ⁴⁶ In contrast, research by Arehart et al suggests that older HI listeners with poor working memory (evaluated with the RST) are susceptible to hearing aid distortion. Specifically, a laboratory-generated FL algorithm was found to negatively affect intelligibility of noisy speech, to a larger extent in older listeners presenting with hearing loss and poor working memory. ⁴⁷ Given that stronger FL settings will degrade an aided signal more, it makes sense that the strength of the FL setting influenced the results reported in both of the earlier-mentioned studies. The study by Arehart et al introduces a further possible source of signal degradation, that being hearing loss, which may explain the difference in the results noted across the studies of Ellis and Munro as well as Arehart et al. The findings reported on this topic suggest a need for further research to evaluate whether cognitive ability predicts speech recognition with listeners wearing clinically available FL technologies and after allowing for a period of acclimatization time (see later). Furthermore, this research highlights the need to avoid unnecessarily strong settings in individualized fittings.

Hearing Aid Factors

Hearing aid bandwidth has traditionally been restricted in the high frequencies, thereby restricting the range of aided high-frequency hearing. More recently, hearing aid manufacturers have advertised high-frequency responses extending to or exceeding 10 kHz; this usually refers to the input bandwidth and/or signal processing abilities of acoustically amplified devices. For listeners with severe and profound levels of hearing loss, the specifications around input bandwidth do not match up with the reality of the output bandwidth that can be achieved, especially for high-powered devices. Modern devices still impose restrictions when it comes to achieving high gain levels across a broad frequency bandwidth due to the physical limitations imposed by the receiver. If feedback is present in the fitting, further gain limitations may be imposed as well. Although today's hearing aids offer a better gain response across a wider range of frequencies for severe-to-profound hearing loss, compared with what was available 10 years ago, audible bandwidth is still limited to below 10 kHz for most listeners, particularly those with overlapping candidacy regions between high-powered hearing aids and cochlear implants. The maximum audible bandwidth that can be achieved for a given listener can be seen as a moving target, as it depends on a host of factors related to the current state of the technology, in combination with listener's hearing levels.

Types of Frequency Lowering Technology

Over the decades, FL has evolved to include many different types, each providing unique lowering effects. A review article by Simpson offers an overview of the historical evolution of FL. ⁴⁸ A more recent discussion comparing the details of different modern FL technologies is also available, and includes adaptive processors. ⁴⁹ The complexity of FL has evolved considerably over the last decade, to the point where it has become difficult to identify and keep up with the different options. Some manufacturers have FL as a default setting that is automatically turned on. In addition, many studies include FL presented via hearing aid simulators, using research laboratory-only methodologies, of which the details will not be covered for the purpose of this article. For the purpose of this review article, discussion will be given to the components that can differ across FL technologies. These include differences relating to how the signal is analyzed (static vs. adaptive processors), the type of lowering used (linear vs. nonlinear), the channel structure imposed (single vs. multiple, sometimes overlapping), and the components included in the final output bandwidth (reduced vs. fixed to include the original signal). These components will be discussed for the different types that are available in today's hearing aid market. The purpose of this section is not to introduce a comprehensive list of FL technologies, but to make the reader aware of some of the components that may be important to consider when fitting and verifying FL technologies.

There are four categories of FL technologies that emerge in the literature: compression, transposition, composition, and translation. In frequency compression , upper and lower channels are defined (i.e., the output is divided into multiple channels), using a cut-off frequency. In the upper channel, compression is applied by a specified ratio, which is nonlinear. This overall bandwidth of the output signal appears reduced when looking at electroacoustic measurements (this will be discussed in greater detail in section “Verification Procedures”). This describes the static form of frequency compression technology included as a hearing aid feature that has been studied in a vast collection of research studies. ^{2 3 6 7 8 13 28 50 51} Frequency compression has evolved to include an adaptive form, which employs more than one cut-off frequency, depending on the input signal (e.g., low-frequency sounds would get a weaker FL setting and high-frequency sound a stronger setting). Adaptive frequency compression has started to appear in the literature related to FL benefit with children and adults. ^{5 52}

Frequency transposition differs from compression given its use of linear lowering techniques. A study by McDermott used electroacoustic analyses to characterize the similarities and differences across transposition and compression, confirming that each FL scheme was effective at lowering certain high-frequency acoustic signals, but that they each achieved a unique lowering effect that could be altered depending on the settings chosen in the programming software. ⁴⁴ Transposition is like compression in that it separates an incoming signal into multiple channels; however, the sound from the upper channel is mixed with that present in the lower channel. The processor does this by locating a prominent peak of sound located in the upper channel, band-pass filtering this, and shifting it down by a specific amount (e.g., one octave). ⁹ Both static and adaptive forms of transposition have been available over the years, with several studies published in the literature. ^{1 9 34 53}

Frequency composition , sometimes referred to as multilayered lowering, applies adaptive signal analysis techniques to the input signal. In a recent study, composition was described as a variation of compression and transposition, in which signal processing is used to locate and copy multiple adjacent segments of high-frequency bands from the upper channel to compress and overlap these multiple bands with the lower channel. ¹⁵ This type of lowering has been evaluated in research including listeners with cochlear DRs. ¹⁰ Like frequency transposition, some recent signal processors allow the conventional hearing aid bandwidth to be preserved (as a fitting option) and mixed with the frequency lowered signal. This effectively adds back the entire passband of sound available for a given device to be used in the fitting process. Further research is needed to explore the effects of restoring the conventional passband of the hearing aid versus presenting a reduced one (due to FL) on the HI listener and on the fitting process.

The fourth category, termed frequency translation , uses adaptive signal processing to lower high-frequency signals, while maintaining the conventional hearing aid signal (or bandpass) in addition to the transposed one. ¹² This type of FL is also referred to as spectral envelope warping, and is similar to transposition in that a portion of the high-frequency spectrum is translated down without compression; however, it is different from the other types in that high-frequency peaks are synthesized at lower frequencies by increasing the energy present in the destination channel. ⁴⁹ Transposition and translation both allow adjustments to the gain applied only to the lowered or translated signal, independently of the gain applied in the destination channel itself. ⁵⁴

Performance with Varying Frequency Lowering Types

For listeners who have a severe-to-profound hearing loss, trials with hearing aids may be an important part of candidacy determination. For some, fitting for the best aided condition may include the use of FL. The literature suggests that the effects of FL on the listener can differ across lowering techniques. Methods of FL that overlap the transposed frequencies with lower frequencies have been reported to degrade speech recognition in one study. ¹² Another study compared speech recognition and subjective ratings of performance for a group of adults who wore both transposition and compression devices, one of who later received a cochlear implant. ⁵⁵ Many of these participants had cochlear DRs. On average, few changes in speech recognition were noted, but individual listeners showed significant differences among conditions, along with acclimatization trends. Subjective improvements, such as reduction in perceived disability in noise, were noted for frequency compression hearing aids, as well as an overall subjective benefit for both transposition and compression hearing aids.

Flexibility and the Need for Fine-Tuning

Regardless of the type of FL, the fitter might also consider the flexibility with which FL parameters can be adjusted in the fitting software. Parameter adjustment abilities can differ considerably across manufacturers. In the fitting methods discussed later, the fitter should aim to avoid providing too weak or too strong FL settings, using careful measurement and fine-tuning of FL parameters. Overall, the literature suggests that FL parameter selections generating greater effective input bandwidth can result in more favorable speech recognition abilities for the listener; ^{29 40} these results are likely linked to the level of output bandwidth that can be achieved in a given fitting. ⁵⁴ Detrimental speech recognition effects and reduced quality ratings are reported in the frequency compression literature when considering the use of strong compression ratios and strong cut-off values (i.e., below ∼2,000 Hz). ^{6 16 17 54} For this reason, clinical FL devices may not provide enough lowering to render all high-frequency sounds audible for all listeners with profound hearing losses. ⁵ When sufficient audibility is not achieved, for example, in the case of some default manufacturer settings, benefit from FL may not be maximized. ⁷ FL with adaptive and/or mixing signal processing may offer different or stronger lowering effects in these cases. Overall, the literature suggests a need to select FL that allows for sufficient fine-tuning to achieve an appropriate amount of lowering for each individual listener.

Verification Procedures

Fine-tuned, individualized settings may be confirmed through verification, allowing the fitter to observe and evaluate whether the current processor and settings are providing high-frequency audibility. There are several protocols available in the literature that can be used to verify FL fittings and help determine candidacy for FL, some of which will be discussed later.

Fitting Assistants

One procedure uses “Frequency Lowering Fitting Assistants” that are available as a set of online tools. They provide information related to how much of the amplified signal falls within the audible bandwidth for a given fitting (conventional) and display several frequency input–output functions with FL applied. ⁵⁶ When using these tools, the fitter would first measure the hearing aid to determine the highest frequency in the amplified speech signal that is audible for a given listener, for the conventional fitting. This requires the fitter to assess the lowest frequency where the average of the long-term speech spectrum intersects the audiogram and has been termed the “maximum audible output frequency” (MAOF). ⁴⁰ This MAOF value is entered into the online tool, and the tool's displayed input/output plots are used to select specific FL settings thought to maximize audible bandwidth. ⁴⁰ The fitter can explore and assign parameter settings that can be used to achieve a certain MAOF, based on an estimation of how the FL technology of interest would perform in a hearing aid.

Phonemic Verification

Another verification protocol for fitting FL also uses the MAOF as a starting point. Specific phonemic stimuli and fitting steps are used to assess and verify whether certain fricatives are audible, to determine appropriate settings for FL in accordance with the American Academy of Audiology fitting guideline. ⁵⁷ In a study by Scollie et al, specific /s/ and /ʃ/ stimuli were developed and evaluated for use with clinically available hearing aid analyzers. This study also examined the relationship between varying strengths of FL and found that increases in the audibility of fricatives were associated with better performance, particularly when the increased audibility maintained or maximized separation between fricatives. ⁵⁸ Measurements of the aided frequency responses for /s/ are described for four types of FL that used both static and adaptive processing, in addition to linear and nonlinear forms of lowering. In general, the frequency locations of the fricative peaks were found to vary across the different FL types. Results suggest that for processors that provide FL by applying a copy of high-frequency energy while maintaining the original conventional signal, a two-peaked configuration was sometimes observed in the aided response, depending on the strength of the processor (e.g., frequency translation and composition). At weak FL settings, the double-peaked /s/ was usually seen to overlap; at medium settings, they tended to combine to produce an /s/ that appeared broadened; and at strong settings, the double-peaked /s/ was seen. ⁵⁸ Therefore, when fitting FL to listeners with severe and profound losses, the fitter should apply knowledge of the processor used to ensure correct interpretation of verification results.

A fitting protocol that verifies the aided levels of phonemes, and compares these to the MAOF, is outlined in a step-by-step format with case examples in an article by Glista et al. ³ The steps in this protocol include (1) verification of the aided output of speech for the conventional hearing aid fitting; (2) measurement of the MAOF range to mark the upper limit of audibility. The MAOF range begins at where the long-term average speech spectrum crosses threshold, and ends where the peaks of speech cross threshold; (3) determination of candidacy for FL by measuring the aided response of a prerecorded and calibrated /s/ stimulus, with the conventional fitting, to determine if high-frequency speech is made audible within the MAOF range. If /s/ is not audible with conventional hearing aids, the fitter would proceed to steps 4 and 5; (4) enable and fine-tune FL to the weakest possible setting that provides audibility for /s/; (5) provide postfitting support. ⁵⁹ Illustrations are provided to guide the fitter through the identification of the MAOF range and in determining candidacy for FL. A follow-up article by Glista et al discusses the value in using this approach to capture the aided response for a specific FL fitting, while incorporating the individual listener's ear canal acoustics. The recommended stimuli for use with this protocol can be found in commercial hearing aid analyzers. The Audioscan Verifit2 also provides a tool that highlights the target MAOF range for the fitter, allowing for easier FL candidacy assessment. ⁶⁰ In the case of steeply sloping hearing loss of greater degrees of impairment, the /ʃ/ sound also can be measured. This is used to check /s-ʃ/ separation, along with a listening check of sound quality. However, feedback from the hearing aid wearer provided informally or with validation measures is preferred when assessing performance across FL settings.

Ear-Specific Fine-Tuning

For many years after the introduction of clinical FL technologies, fitting protocols recommended the use of one FL setting across ears and this was usually determined based on the better listening ear. This recommendation has shifted to include an approach that maximizes audibility for each ear separately and can therefore result in different FL settings per ear depending on the audiogram of the listeners. This also may mean that some listeners may have FL enabled in only one ear. An ear-by-ear fitting approach may be especially important when considering asymmetrical and/or steeply sloping losses ⁶¹ and has been suggested in the literature as an important factor to consider when fitting FL. ¹³

Validation Factors

Assessing Performance with Frequency Lowering

Clinically, the fitter can assess a listener's performance with FL to assist in decisions related to the use of FL and the choice of settings. For example, FL benefit has been documented using a wide range of different stimuli including high-frequency phonemes, ^{3 4 5} words, ³⁰ and sentences. ⁶² Depending on the listener's degree of HI, consideration should be given to the difficulty of the measure. For example, if performance is reported at or close to ceiling (100%) with conventional technology, it will not be possible to evaluate relative performance with FL. Some researchers have included the use of background noise to increase the difficulty of speech perception testing. ³⁰ Ideally, performance in the conventional condition would be assessed at the midpoint of the performance range (50%) or slightly above, to leave room to measure any improvement or decrements with FL. Therefore, stimuli and outcome measures that include a larger proportion of high-frequency phonemes or that measure speech perception in noise, for example, are generally more sensitive to the assessment of high-frequency audibility. These types of measures, although sometimes less generalizable to real-world listening performance, lend themselves well to the evaluation of FL technology. For example, the California Consonant Test ⁶³ may provide a more sensitive measure of high-frequency audibility, when compared with a stimulus set with a phonemic balance. Other examples of clinically available outcome measures that have been used report FL benefit in the literature and include the Bamford-Kowal-Bench Test of Speech Recognition in Noise, ⁶⁴ the Ling-6(HL) Test of speech sound detection, ⁶⁵ the Phoneme Perception Test, ⁶⁶ and the University of Western Ontario Test of detection of word-final plurality markers. ⁶⁷ Outcome measures like the ones mentioned above can be used to compare performance at the time of the initial hearing aid fitting to that at a follow-up appointment, thereby assessing acclimatization effects. They also can be used to compare performance of conventional technology or across various FL settings resulting in different perceived sound quality effects. Such measures are especially informative when paired with measures of the real-ear–aided response specific to high-frequency sounds, offering validation in the case of a measured improvement in audibility. Alternatively, the fitter may learn of the need for further fine-tuning when assessing settings and levels of audibility in relation to actual listening performance.

It is interesting to look at measures of real-world performance that assess listening abilities outside of the sound booth. Several studies have examined real-world benefit with FL by having listener wear study aids outside of the laboratory. For example, Bohnert and colleagues asked adults to complete questionnaires about listening satisfaction and found significant improvements with FL compared with conventional processing. ² However, the participants in that study were not blinded to the processing manipulation in the study. More recently, Kirby and colleagues published a sound quality questionnaire tailored to the evaluation of FL benefit that has potential in a clinical context. ¹⁵ A real-world performance measure was used by Glista and colleagues to compare multimemory performance for FL and conventional hearing aid fittings. Individual preference for FL (compression) was found to relate to age group and to benefit; children were more likely to have preference for FL than were adults, and listeners were more likely to prefer FL if they benefited from it. ³ For children, speech and language outcomes are probably the most important real-world outcome measure. For example, parents can offer valuable information related to whether their child can hear /s/ and /ʃ/, and if this is different with FL compared with conventional hearing aids.

Acclimatization Time

Some types of FL are thought to alter the nature of fricatives substantially, especially for more aggressive settings, and may result in the listener initially perceiving the sound as a different phoneme or perhaps as unrecognizable. ⁴ In the case of steeply sloping, severe and profound high-frequency hearing loss, audibility of /s/ or /ʃ/ can be limited to only the lower shoulder of the frication band resulting in partial audibility of the sounds being tested. There are multiple studies that suggest that FL benefit increases after allowing for an acclimatization period with FL technology; recommendations from findings range from weeks to years. ^{1 4 62} However, others have found no evidence of acclimatization after a significant period of time. ⁷ Variability in the findings reported earlier may relate to the differences in FL technologies, prior experience with audibility and FL, fitting approach, length of acclimatization time, developmental status of the listener, and degree and/or configurations of the losses included in the study. It remains unclear what degree of acclimatization occurs, on average, for listeners wearing FL technology. The length of time needed to achieve maximum benefit from a FL fitting has been found to relate to the individual listener and the type of speech perception measurement used to evaluate benefit. ⁴ Overall, these findings suggest that results with FL technology, either reported or measured directly after a clinical hearing aid fitting, may not be indicative of the eventual benefit that a patient might receive. Assessing performance with FL at the time of the initial fitting will help both the fitter and the listener establish baseline performance. Listener performance can then be reassessed, after allowing for a period of time to get used to lowered and sometimes completely novel sounds presented via FL.

Summary

This article provides a review of the current literature on the topic of fitting FL to severe and profound levels of hearing loss. Considering modern hearing technology, the provision of a broad bandwidth of audibility, encompassing important high-frequency and environmental sounds, can be hard to achieve with conventional fittings. FL technology may be a suitable option for both adults and children presenting with severe and/or profound level of hearing loss in the high frequencies. The literature generally supports the use of FL with greater degrees of HI, including high-frequency thresholds in the severe-to-profound range, and reported benefit with FL for many of these listeners. There are many factors to consider when assessing candidacy for FL. Fig. 1 presents some of the important factors to consider when approaching FL fittings using a patient-centered approach. An individualized candidacy assessment including the use of real-ear verification measures and carefully chosen validation tools are recommended for listeners requiring greater audibility of high-frequency sounds.