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. Author manuscript; available in PMC: 2024 Apr 1.
Published in final edited form as: Laryngoscope. 2022 Jun 26;133(4):977–983. doi: 10.1002/lary.30263

Voice Therapy Improves Acoustic and Auditory-Perceptual Outcomes in Children

Robert Brinton Fujiki 1, Maia Braden 2, Susan L Thibeault 1,2
PMCID: PMC9790974  NIHMSID: NIHMS1817292  PMID: 35754165

Abstract

Purpose:

This study employed acoustic measures as well as auditory-perceptual assessments to examine the effects of voice therapy in children presenting with benign vocal fold lesions.

Methods:

A retrospective, observational cohort design was employed. Sustained vowels produced by 129 children diagnosed with benign vocal fold lesions were analyzed, as well as connected speech samples produced by 47 children. Treatment outcome measures included Consensus of Auditory-Perceptual Evaluation of Voice (CAPE-V), jitter, shimmer, Noise-to-Harmonic Ratio (NHR), cepstral peak prominence (CPP) and Low-to-High Ratio (LHR) on sustained vowels, and CPP and LHR on connected speech.

Results:

Following voice therapy, significant improvements in CAPE-V ratings (p<.001) were observed. Additionally, jitter (p=.041), NHR (p=.019), and CPP (p<.01) on sustained vowels, and CPP (p=.002), and LHR (p=.008) on connected speech significantly improved following voice therapy. CPP increased with age in males but did not change in females. CAPE-V ratings and perturbation measures indicated that dysphonia was more severe in younger children pre and post therapy.

Conclusions:

Auditory-perceptual and acoustic measures demonstrated improved voice quality following voice therapy in children with dysphonia. CPP effectively quantified voice therapy gains and allowed for analysis of connected speech, in addition to sustained vowels. These findings demonstrate the value of CPP as a tool in assessing therapy outcomes and support the efficacy of voice therapy for children presenting with vocal fold lesions.

Keywords: pediatric voice, acoustics, cepstral peak prominence

Introduction

Voice disorders impede communication and adversely affect quality of life for many children.1 The prevalence of pediatric dysphonia is unknown but has been estimated to be somewhere between 1.4% and 23.9%.2–7 These numbers likely underestimate the true prevalence of laryngeal pathology as many voice problems in children go unreported.7 Pediatric voice disorders often have negative emotional and social ramifications,8 as dysphonia can impact the way children are perceived by their peers9 and teachers.10,11 Although causes of pediatric dysphonia vary, benign vocal fold lesions constitute the most common etiology.12–15 Voice therapy is considered to be the primary treatment for these disorders.16–19

Preliminary evidence suggests that voice therapy is effective for children.20–22 Auditory-perceptual assessments following therapy have indicated functional improvements in overall voice quality, as well as reduced roughness, breathiness, and strain.23–25 In addition, voice therapy has been effective in decreasing vocal fold lesion mass and improving voice-related quality of life.20,26,27 Acoustic measures have documented positive changes following voice therapy,25,28 however, studies have typically considered small Ns and have often been limited to basic perturbation measures.29 Jitter and shimmer, measures of cycle-to-cycle variation in frequency and amplitude,30,31 have been the most reported acoustic outcomes in children.29,32 Noise-to-Harmonic Ratio (NHR), the proportion of noise in the voice signal, has also been employed.33,34 Significant improvements in jitter, shimmer, and NHR have only been inconsistently documented following voice therapy in children with benign vocal fold lesions.26,29,35 Although findings may reflect the relatively small samples of these studies, there are some basic concerns about the effectiveness of perturbation measures for assessing dysphonic voices.36 For example, these measures are restricted to the analysis of sustained vowels, and do not use the richer data available in naturalistic connected speech samples.37 Additionally, basic perturbation measures rely on frequency tracking and are therefore limited in their ability to analyze aperiodic voices, 38–40 or those with strong subharmonics.41 They are also susceptible to sex and loudness effects,31 and cannot always differentiate dysphonic and normophonic adults.42,43

Best practice dictates that acoustic measures be used in the evaluation of voice.44 As such, research is needed to expand the battery of acoustics used in children. For this reason, measures such as cepstral peak prominence (CPP) have been developed.45–47 CPP, a measure of peak cepstral amplitude,45,48 correlates strongly with breathiness,47,49,50 one of the auditory-perceptual voice parameters most common in dysphonic children.36,51 Additionally, studies suggest that Low-to-High Ratio (LHR), an estimation of spectral tilt above and below 4kHz, effectively supplements CPP for characterizing dysphonic voices.48 Even though initial studies suggest that CPP can reliably distinguish between dysphonic and non-dysphonic voices in children,52 the effectiveness of measures like CPP and LHR for assessing therapy induced voice changes is not understood. Further study is warranted considering that these measures are increasingly available, are capable of characterizing both sustained vowels and connected speech,45 and are potentially effective markers of therapeutic voice progress.

This investigation employed both acoustic and auditory-perceptual voice assessment to examine changes following voice therapy in children with benign vocal fold lesions. Specifically, this study sought to determine if pediatric voice therapy produced improvements in jitter, shimmer, NHR, CPP, and LHR. It was hypothesized that voice therapy would improve auditory-perceptual and acoustic measures of voice in children with benign lesions.

Materials and Methods

Study Design

This study employed a retrospective, observational cohort design. All study procedures were approved by the University of Wisconsin-Madison Institutional Review Board (IRB#2020-0693). Information pulled from medical records included age, diagnosis, voice evaluation findings, and audio recordings taken at voice evaluations.

Study Population

Children diagnosed with benign vocal fold lesions, by a pediatric otolaryngologist, were identified using the audio-recording database from voice evaluations at a large children’s hospital ambulatory voice and swallow clinic between the years of 2009 and 2020. Eligibility for the study was confirmed using electronic medical records (EMR). Patients, under 18 years of age, underwent laryngeal imaging, auditory-perceptual, and acoustic evaluations of voice before and after a minimum of two voice therapy sessions. Exclusionary criteria included cancer, voice disorders aside from benign vocal fold lesions, and previous voice therapy. Voice therapy was individualized to the needs and developmental level of each child, as is common practice.53,54 All participants received a combination of direct and indirect therapy consisting of vocal hygiene education, semi-occluded vocal tract exercises, resonant voice, and assigned home practice. Repeated-measure sample size calculations were performed using CPP connected-speech data from 10 participants (mean difference=.89). It was determined that an N of 37 was needed to detect the effect of time (pre/post therapy) with medium effect size (Cohen’s d=.42) and .8 power.

Outcome Measures

Voice recordings were collected using the Computerized Speech Lab by Kaypentax (Model 4500). Acoustic analyses were performed using the Multi-Dimensional Voice Program and the Analysis of Dysphonia in Speech and Voice. The following measures were calculated (acoustics) or retrieved from the EMR (CAPE-V).

  • Consensus of Auditory-Perceptual Evaluation of Voice (CAPE-V)55,56 assessments were performed by speech-language pathologists (SLP) during formal voice evaluations before and after voice therapy. Complete CAPE-V ratings were extracted from the medical record. Only ratings of overall voice quality were included in the analysis as these ratings are most reliable55 and best correlated with objective voice measures.36 Evaluating SLPs were all specialized in the treatment of pediatric voice.

  • Jitter, Shimmer, Noise-to-Harmonic Ratio (NHR), and fundamental frequency (F0) were calculated on the vowel /a/ at comfortable loudness. Three vowels were produced, and the middle vowel analyzed. The middle 3 seconds of the vowel was selected for analysis and mic to mouth distance was 6 inches. These measures were selected because they are commonly employed in voice clinics, and normative pediatric data is available.57–59

  • Cepstral peak prominence (CPP), CPP standard deviation (CPPSD), and Low-to-High Ratio (LHR) were analyzed on both the same sustained /a/ vowel used for perturbation measures and on connected speech. For connected speech, the sentence “we were away a year ago” from the CAPE-V protocol55,56 was analyzed at comfortable loudness.

Statistical Analysis

All statistical analyses were performed in SPSS (Version 28, 2021). Mixed level modeling was used to examine the effects of voice therapy on auditory-perceptual and acoustic outcomes. Fixed factors in the model included time (pre and post therapy), age (children ages ≤6 years, children 7 through 10 years, and children ≥11 years), sex (male or female), and all appropriate interactions. Age and sex were included in the model as these factors have been shown to impact cepstral/spectral measurements of voice.52,58,60 Patient was included as a random factor in order to account for individual variation. An alpha of .05 was used to determine significance and Bonferroni corrections were made on all post-hoc comparisons.

Results

A total of 129 children between the ages of 3.11 and 17.5 years met inclusion criteria (Mean age=8.4 years, SD=3,72, 81 males, 48 females). All patients underwent auditory-perceptual voice assessment and produced sustained vowels. Forty-seven of the 129 children also produced connected speech stimuli, as allowed by age and equipment availability. All children were diagnosed with benign vocal fold lesions. Diagnoses included vocal fold nodules, and vocal fold polyps or cysts with reactive lesions. For many children, benign bilateral lesions were appreciated, but lesion type remained unclear. Patients received an average of 6 (SD=2.5) therapy sessions between voice evaluations (range 2 to 13; over the course of 2–18 weeks). Population demographics by age group are presented in Table 1. Age groups were divided based on previous study.61 Significant findings are presented below.

Table 1.

Population Demographics

Demographics
Age group ≤6 years 7–10 years ≥11 years
N 63 36 30
Mean Age (SD) 5.45 (.95) 8.73 (.97) 14.21 (2.1)
Males/Females 39/24 26/10 16/14
Mean Number of Therapy Sessions (SD) 6.10 (2.63) 6.04 (2.11) 6.05 (2.78)
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Auditory-Perceptual Assessment

Means and standard errors for CAPE-V data are presented in Figure 1. For CAPE-V ratings of overall voice quality, a significant effect of time was observed (F1,123=100.01, p<.001). This occurred as CAPE-V scores became significantly smaller following voice therapy (p<.001). Additionally, a significant effect of age was observed (F2,127.6=8.48, p<.001). Older children (aged ≥11) were perceived to be significantly less dysphonic both pre and post therapy than children ≤6 (p<.001) or between 7 and 10 (p=.002).

Figure 1.

Figure 1.

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Means and Standard Errors for CAPE-V Ratings (millimeter, mm) of Overall Vocal Quality Pre and Post Voice Therapy Across Age Groups

Sustained Vowels

F0.

A significant interaction between sex and age was observed for F0 (F2,120.1=6.5, p=.032). F0 was lower for males in the ≥11 age group than for males in the younger groups (p<.01), or females in any age group (p<.01). Means and standard errors for all acoustic measures are presented in Table 2.

Table 2.

Means and Standard Errors for Acoustic Data Pre and Post Therapy Across Age Groups

Means and Standard Errors
≤6 YEARS AGE GROUP
Speech Type Measure Pre-Therapy Post-Therapy p Value
Sustained /a/ Jitter (%) 3.10 (27) 2.04 (.26) *.001
Shimmer (%) 6.05 (.47) 5.85 (.61) .179
NHR .217 (.01) .148 (.01) *<.01
CPP (dB) 8.38 (.36) 8.58 (.48) *.015
CPP SD (dB) 1.09 (.08) .717 (.08) *<.01
LHR (dB) 25.6 (.72) 26.1 (1.0) .061
F0 (Hz) 253.8 (25.6) 274.6 (30.8) .321
Connected speech CPP (dB) 5.62 (.53) 6.61 (.50) *<.01
CPP SD (dB) 2.75 (.21) 2.81 (.27) *.025
LHR (dB) 23.3 (1.5) 24.9 (1.4) *.008
7–10 YEARS AGE GROUP
Speech Type Measure Pre-Therapy Post-Therapy p Value
Sustained /a/ Jitter (%) 2.26 (.22) 1.96 (.22) *.030
Shimmer (%) 5.48 (.36) 3.96 (.54) .109
NHR .140 (.01) .132 (.01) *<.01
CPP (dB) 8.21 (.28) 9.12 (.42) *.012
CPP SD (dB) .88 (.07) .70 (.07) *<.01
LHR (dB) 26.5 (.58) 28.1 (.90) .062
F0 (Hz) 222.9 (37.6) 256.25 (45.3) .234
Connected speech CPP (dB) 5.44 (.38) 6.25 (.35) *<.01
CPP SD (dB) 2.60 (.16) 2.72 (.18) *.019
LHR (dB) 26.6 (1.1) 27.1 (1.0) *.008
≥ 11 YEARS AGE GROUP
Speech Type Measure Pre-Therapy Post-Therapy p Value
Sustained /a/ Jitter (%) 1.39 (.30) 1.30 (.310 .361
Shimmer (%) 3.82 (.63) 4.56 (.64) .178
NHR .141 (.01) .120 (.01) *<.01
CPP (dB) 10.71 (.49) 11.91 (.50) *.041
CPP SD (dB) .86 (.10) .63 (.10) *<.01
LHR (dB) 28.6 (1.0) 28.1 (1.0) .055
F0 (Hz) 163.39 (32.1) 183.9 (36.8) .286
Connected speech CPP (dB) 7.07 (.57) 7.98 (.49) *<.01
CPP SD (dB) 3.62 (.24) 2.93 (.25) *.032
LHR (dB) 25.4 (1.7) 29.5 (1.4) *<.01
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*

Indicates statistical significance, p-values represent pre-post voice therapy comparisons, CPP=cepstral peak prominence, NHR=Noise-to-Harmonic Ratio, LHR= Low-to-High Ratio, F0=fundamental frequency

Perturbation Measures.

A significant interaction between time and age was observed for jitter (F2,122.1=3.27, p=.041). Jitter values were significantly smaller in children ≥11 (p=.002) and children 7–10 (p=.021), as compared with those ≤6 years of age. In addition, jitter significantly improved/decreased following voice therapy for children ≤6 years of age (p=.001) and those between 7–10 (p=.03), however, no change in jitter was observed for children ≥11 (p=.361). Similarly, a significant interaction between time and age was observed for NHR (F2,122.7=4.09, p=.019). Significant improvements in NHR were observed for all age groups (p<.001), however, the magnitude of this change was significantly greater for the children ≤6 than for those between 7–10 (p=.003) or those >11 (p=.012). For shimmer, only a significant effect of age was observed (F2,125.4=4.96, p=.008), as shimmer values were significantly larger for children aged ≤6 (p=.05), and significantly smaller for the ≥11 group (p=.011), when compared with the 7–10 group.

CPP.

A significant interaction between sex and age was observed for CPP (F2,125.6=4.6, p=.012). CPP was greater for males in the ≥11 age group than for males in the younger groups, or females in any age group (p<.01). There was also a significant effect of time (F2,123.1=10.4, p=.002), as CPP values increased post-therapy (p=.002). Means and standard errors for CPP across age and sex can be found in Figure 2. Additionally, CPPSD became significantly smaller following voice therapy (F2,123.2=16.5, p<.01).

Figure 2.

Figure 2.

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Means and Standard Errors for CPP Data (decibel, dB) on Sustained Vowels Pre and Post Voice Therapy Across Age Groups and Sex

LHR.

No significant effects or interactions were observed. LHR on the sustained vowel increased following therapy, however, this did not reach statistical significance (F1,45.8=7.79, p=.06).

Connected Speech

CPP.

CPP on connected speech significantly increased following voice therapy (F1,45.6=15.4, p<.01) indicating improved voice quality. In addition, a significant effect of age was observed (F2,64.5=4.2, p=.019), as CPP values were greater for the ≥11 age group (p=.017) than the younger groups. CPPSD on connected speech significantly decreased following voice therapy (F1,46.5=8.6, p=.005). There was also a significant effect of age for CPPSD (F1,65.5=4.2, p=.019), as values were greater for the ≥11 age group when compared with younger children (p=.019).

LHR.

LHR on connected speech significantly increased following therapy (F1,45.8=7.79, p=.008).

Discussion

This study employed acoustic voice measures and auditory-perceptual assessment to examine the outcomes of voice therapy in children with benign vocal fold lesions. Specifically, this study sought to determine if voice therapy improved jitter, shimmer, CPP and LHR measures on sustained vowels, as well as CPP and LHR calculated on connected speech. It was hypothesized that auditory-perceptual assessments, as well as perturbation and cepstral voice measures would reflect an improvement in voice quality following voice therapy. This hypothesis was confirmed as improvements in CAPE-V ratings, jitter, NHR, and CPP on sustained vowels were observed following voice therapy. Improvements in CPP and LHR on connected speech were also observed.

That CAPE-V ratings significantly improved following voice therapy is encouraging as it indicates that voice quality improved from a functional perspective. This finding supports past studies showing improved perceived voice quality following voice therapy in children.24,62–64 However, auditory-perceptual measures of voice quality, such as the CAPE-V, are susceptible to bias as clinicians frequently know that a child has been in therapy. It is therefore encouraging that improvements in auditory-perceptual assessments were corroborated by improvements in acoustic measures of voice.

The finding that CAPE-V ratings were significantly more severe for younger children at baseline has several possible explanations. First, it may be that young children were more dysphonic before their parents sought medical attention. This is likely as young children are less bothered by the negative attention associated with dysphonia, whereas the emotional impact is greater in older children.8 Referral for voice evaluations may occur at a lower threshold of dysphonia in school-aged children, as they have more interactions with unfamiliar adults (i.e., teachers, school SLPs etc.) who may be less accustomed to their voices and therefore report less severe dysphonia. Alternatively, articulatory context and phonological errors can impact perceptual judgements of voice65,66 and may artificially inflate CAPE-V scores in younger children. A third possibility is that younger children have undeveloped vocal mechanisms67 and may naturally be more dysphonic at baseline than older children. There are currently no studies characterizing auditory-perceptual features of normal pediatric voices, but studies have found higher soft phonation index in children under five,68 and lower harmonic-to-noise ratios in children under eight.69 Additionally, measures such as jitter, shimmer, and CPP have been observed to improve with increased age.58,61

Significant decreases in jitter and NHR support previous findings indicating that jitter and NHR decrease in children as improvements in voice quality are observed.26,64,70 Not all studies, however, have reported similar findings. For example, jitter and shimmer data for 19 children included in this study were previously published with no significant changes documented.25 This difference in findings is likely related to the larger N and therefore greater power, examined in this study. In children ≤6, jitter and NHR values improved but remained mildly elevated compared with previously reported norms.57 For children between 7 and 10, values returned to the upper end of normal after therapy,57,58 and for the ≥11 group, perturbation measures fell within normal range both before and after therapy.57,58 Improvements in shimmer did not reach statistical significance, which differs from previous work.26,64 Jitter is thought to be influenced by control of vocal fold oscillation, whereas shimmer is influenced by glottal resistance and lesion mass.71 As such, this finding may indicate that improvements in voice quality were driven more by functional gains, rather than reductions in laryngeal lesion size, however, as post-therapy stroboscopy was not performed on all patients – this is speculative. As previously discussed, jitter and shimmer are limited in their ability to analyze severely dysphonic voices,43 and improvements in NHR suggest that vocal fold closure was likely improved post-therapy.

Jitter did not improve for the ≥11 age group, likely because values were within normal range at baseline.57,58 This finding reflected CAPE-V ratings indicating that this group was less dysphonic than the younger group when they presented to the clinic. This likely also explains the smaller magnitude change in NHR following voice therapy for this group.

Voice therapy yielded improvements in CPP on sustained vowels and connected speech. Increases in CPP suggest an improvement in voice quality,72 likely driven by decreased breathiness. Although improvements in CPP have been observed following voice therapy in adults,73,74 it has been unclear whether voice therapy produces similar results in children. This is a relevant finding as CPP allows for analysis of connected speech, which better approximates how children functionally communicate. These data suggest that CPP is useful for measuring the effects of voice therapy in children and may be considered a viable alternative to perturbation measures. This is advantageous as CPP has become increasingly available and can be analyzed using free software such as Praat. Normative CPP data in English-speaking children are sparse, however, CPP on connected speech returned to normative adult range following therapy for all age groups.75 CPP on sustained vowels only fell within normal range for children ≥11 following therapy, however, given that F0 and intensity impact CPP,76–78 adult norms are likely not valid for children.

The fact that CPP and CPPSD increased with age in males, but not in females,61 supports past study examining CPP in vocally healthy children.58,61 As the ≥11 group included teenagers, it is logical that these values would begin to approximate adult values as young men experience the decrease in F0 associated with puberty.79,80 The decrease in CPPSD following voice therapy was unexpected, as it has been previously reported that greater CPPSD reflects increased laryngeal flexibility.47,61,81 The fact CPPSD decreased in the current study may indicate that children employed less prosodic variation following voice therapy as they were concentrating on producing stimuli with forward resonance. Connected speech samples were short, however, and CPPSD should likely be interpreted with caution.

LHR improved on both sustained vowels and connected speech, however, improvements only reached statistical significance for connected speech. This is interesting, given the smaller sample size examined for connected speech. This would support previous study suggesting that measures taken on connected speech may more effectively assess dysphonia than sustained vowels.52 Future work is needed to confirm this finding.

This study has some limitations that warrant mention. First, further study is needed to verify the effects of voice therapy on acoustics and auditory-perceptual outcomes in children with a prospective clinical trial that includes a control group. Additionally, acoustic analysis of more extensive connected-speech samples is needed to confirm present findings and future work should include CAPE-V ratings performed by blinded listeners. Lastly, emerging evidence suggests that voice therapy reduces pediatric vocal fold lesion mass,26 however, large-scale studies confirming this finding with objective measures is needed. Lastly it should be acknowledged that the decreased vocal stability often observed in pubertal adolescents79,80,82 may impact auditory-perceptual and acoustic assessments. Thus, future work should examine the interaction between voice therapy, benign lesions, and puberty.

This study supports the efficacy of voice therapy for children with benign vocal fold lesions, as both auditory-perceptual assessment and acoustic measures improved following intervention. As such, voice therapy should be considered a viable treatment option for these children. In addition, these data suggest that acoustic analysis of connected speech can be used to quantify the effects of voice therapy in children. This is important as connected speech better resembles naturalistic communication, and auditory-perceptual voice assessments are most effective when coupled with objective voice measures.44 Further work should determine how cepstral/spectral voice measures can best serve this population.

Conclusion

Auditory-perceptual voice assessment and acoustic measures improved following voice therapy in children. CPP effectively quantified voice therapy gains and allowed for analysis of connected speech in addition to sustained vowels. These findings demonstrate the value of CPP as a tool in assessing therapy outcomes and support the efficacy of voice therapy for children presenting with vocal fold lesions.

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