Cepstral and perceptual investigations in female teachers with functionally healthy voices

1 Cepstral and Perceptual Investigations in Female Teachers with Functionally Healthy Voice Ketaki Vasant Phadke1, Anne-Maria Laukkanen2, Irma Ilomäki2, Elina Kankare3, Ahmed Geneid 4, Jan G Švec1 1Voice Research Laboratory, Department of Biophysics, Faculty of Science, Palacký University Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic 2Speech and Voice Research Laboratory, Faculty of Education, University of Tampere, Tampere, Finland 3 Ear and Oral Diseases, Department of Phoniatrics, Tampere University Hospital, Tampere, Finland 4 Department of Ear, Nose and Throat and Phoniatrics - Head and Neck Surgery, University of Helsinki and Helsinki University Hospital, Helsinki, Finland Corresponding author information Anne-Maria Laukkanen, Ph.D. Speech and Voice Research Laboratory, Faculty of Education, University of Tampere, Åkerlundinkatu 5, 33100 Tampere, Finland Phone Number: +358 (0)50 3635152, Email Address: [email protected] Jan G. Švec, Ph.D. et Ph.D. Palacký University Olomouc, Faculty of Science, Dept. Biophysics, Voice Research Lab, 17. listopadu 12, 771 46 Olomouc, Czech Republic Phone Number: +420 58 563 4151, Email Address: J[email protected] This is the accepted manuscript of the article, which has been published in Journal of Voice, 2020, 34(3), 485.e33-485.e43. https://doi.org/10.1016/j.jvoice.2018.09.010 © <2020>. This manuscript version is made available under the CC-BY-NC-ND 4.0 license 2 Cepstral and Perceptual Investigations in Female Teachers with Functionally Healthy Voice ABSTRACT Purpose: The present study aimed at measuring the smoothed and non-smoothed cepstral peak prominence (CPPS and CPP) in teachers who considered themselves to have normal voice but some of them had laryngeal pathology. The changes of CPP, CPPS, sound pressure level (SPL) and perceptual ratings with different voice tasks were investigated and the influence of vocal pathology on these measures was studied. Method: Eighty four Finnish female primary school teachers volunteered as participants. Laryngoscopically, 52.4 % of these had laryngeal changes (39.3 % mild, 13.1 % disordered). Sound recordings were made for phonations of comfortable sustained vowel, comfortable speech and speech produced at increased loudness levels as used during teaching. CPP, CPPS and SPL values were extracted using Praat software for all three voice samples. Sound samples were also perceptually evaluated by five voice experts for overall voice quality (10 point scale from poor to excellent) and vocal firmness (10 point scale from breathy to pressed, with normal in the middle). Results: The CPP, CPPS and SPL values were significantly higher for vowels than for comfortable speech and for loud speech compared to comfortable speech (p<0.001). The loud speech was perceived to be firmer and have a better voice quality than comfortable speech. No significant relationships of the laryngeal status with cepstral values, perceptual ratings or voice SPLs were found (p>0.05). Conclusion: Neither cepstral measures nor perceptual evaluations could clearly distinguish teachers with laryngeal changes from laryngeally healthy teachers. Considering no vocal complaints of the subjects, the data could be considered representative of teachers with functionally healthy voice. Key words: Teachers’ voice, Voice SPL, CPP, CPPS, Perceptual evaluation, laryngeal pathologies 1. INTRODUCTION 3 Cepstral Peak Prominence (CPP) and the Smoothed Cepstral Peak Prominence (CPPS)1, 2 are considered to be rather robust acoustic measures of overall severity of dysphonia.3, 4 CPP is a measure of the relative cepstral peak amplitude (in decibels) of the voice signal.1, 2 It is obtained by finding out the difference between the maximum cepstral peak value occurring within the boundaries of the expected phonational quefrencies and the corresponding value on the regression line fitted on the cepstrum. CPP was originally developed to analyze sustained vowels and measures the degree of harmonic organization (periodicity) of the signal over the “noisiness” in the voice signal. The Smoothed Cepstral Peak Prominence (CPPS) is a modification of the CPP measure, where the individual cepstra are smoothed across time and quefrency domains, which was developed for greater prediction accuracy particularly in speech signals.2 The CPP and CPPS measures were shown to be more reliable than the traditional perturbation measures such as jitter, shimmer and noise to harmonic ratio (NHR).5-7 A higher CPP amplitude value can be found in highly periodic signals and lower CPP amplitude value in less periodic or aperiodic signals.1, 5 From previous clinical studies, CPP and CPPS measures have been found to correlate strongly with perceptual evaluations of voice.2, 6, 8 Applications of CPP measures have been extended to the analysis of different phonation types. It has been reported that CPP values are higher for pressed and normal (modal) phonation compared to, breathy type of phonations.9 These findings have been attributed to larger open quotient values of glottal waveform during breathy phonations which lead to increased spectral noise.9 Wolfe and Martin10 classified dysphonic patients into breathy, hoarse and strained voice types based on four parameter model including cepstral peak prominence. The CPP values were lower for hoarse and breathy voice compared to strained voice type.10 Lower CPP values have been reported to differentiate rough from normal voice based on the increased amplitude of noise components in relation to fundamental frequency in rough voice.11 The CPP measure has also been useful to differentiate hypofunctional from normal voice.12 Perceptual evaluation of strain severity has as well shown moderate to high correlation with the cepstral measures.13 CPP and other cepstral based measures have also been reported to be useful in assessing voice quality in various voice disorders,14 vocal nodules15 and unilateral vocal fold paralysis.16 CPPS has been recommended for voice screening purposes as it has a high predictive value for voice disorder status.17 CPP as well as CPPS have been used to analyze both sustained vowels and continuous speech samples in assessing dysphonic voices. Hillenbrand and Houde2 reported that both CPPS and CPP were good predictors of breathiness rating, while CPPS showed slightly better results over CPP for both sustained vowel and continuous speech samples. In a study by Hasanvand et al.18 CPPS and CPP were shown to be significantly reduced in female dysphonic subjects compared to nondysphonic subjects for both vowel and speech (reading) samples. Comparing dysphonic to nondysphonic males, the authors showed that CPPS from vowel and speech and CPP from only speech sample were significantly reduced. Authors advocate use of both CPP and CPPS for differentiating dysphonic and non-dysphonic individuals. In another study, Brinca et al.19 reported both CPP and CPPS measures to differentiate between dysphonic and normal individuals for sustained vowel sample, but only CPP from continuous speech sample to help differentiate between the two groups. 4 These authors as well report use of both CPP and CPPS as a promising acoustic measure of dysphonia. Moers et al.20 reported the reading-based CPP and CPPS to correlate well with perceptual rating of dysphonic voice. Based on all these results we explore in this paper the use of both CPP and CPPS measures for both vowel and speech samples. Occupational voice users normally demand more attention than non-occupational voice users. CPP measures have been applied to assess voice quality in vocally healthy occupational voice users, such as radio broadcasters21 and in Indian classical singers.22 The Indian Carnatic classical singers had a higher CPP compared to nonsingers, attributing it to stronger harmonic organization in the singers. However, in the study on radio broadcasters, there was no difference between the radio performers and non-radio performers on the cepstral measures indicating no differences in the strength of harmonic content in the voice signal between the two groups. One of the largest groups of professional voice users are teachers. Teacher’s voice is vulnerable to disorders as a result of prolonged voice use and heavy vocally loading conditions.23 Poor environmental23-26 and working conditions,27, 28 unawareness of appropriate vocal hygiene29 and lack of voice training,30 all may contribute to the development of voice disorders in teachers. Several studies have shown a high prevalence of frequently occurring symptoms of vocal overloading and fatigue in teachers.27, 30-32 Studies have shown that in presence of unfavorable environmental conditions such as background noise, teachers tend to raise their voice, and speak with increased vocal loudness leading to increased vocal effort and strain in these teachers.33-35 There have been some indications, that cepstral peak prominence values are influenced by vocal loudness, or more explicitly by sound pressure level (SPL) of voice.36 This relationship has not yet been well explored and deserves more attention, however. The present study applies cepstral (CPP and CPPS) and perceptual evaluations to assess voice quality of female primary school teachers who are serving in a vocally loading profession and have not been seeking for help for any voice problems. These teachers considered themselves to have normal voice, but in some of them pathological findings in the larynx were discovered through laryngoscopy, which did not make it impossible for them to work as a teacher. The questions addressed in this study are: (1) What is the perception of the voice quality and firmness for the sustained vowel, comfortable and loud speech in teachers who consider themselves to have normal voice? (2) What are the representative CPP, CPPS and SPL values for sustained vowel, comfortable and loud speech in these teachers? (3) How are these CPP and CPPS values related to the measured voice SPLs? (4) In case of laryngeal pathologies, are these perceivable by voice expert listeners and detectable by the CPP, CPPS and voice SPL measures? 2. MATERIALS AND METHOD 2.1 Participants and their laryngeal status The material for this study has been derived from an earlier study,37 which investigated the relationship between self-reported voice symptoms, working conditions, background factors (such as noise and air quality) and phoniatric evaluation but did not attempt using cepstral measures in 5 these teachers. A total of 84 Finnish female primary school teachers volunteered as subjects for the present study. The mean age was 42.6± 8.9 years. The mean years in profession were 16.5±9.4 years. The mean number of teaching hours per week was 31.3±7.3 hours. All the participants considered themselves to be vocally healthy and capable of carrying their profession. Some laryngeal changes were found in 44 (52.4%) teachers; 33 of them (39.3 %) had mild and 11 (13.1 %) had substantial changes that were evaluated by an experienced phoniatrician on a three point scale (1-healthy; 2-mild changes; 3-disorderd). This laryngeal status rating was based on case history and indirect mirror laryngoscopy. Mirror laryngoscopy was used out of practical reasons, since the laryngeal inspections were mostly made in field conditions and no portable rigid endoscopy system was available for that purpose. The mild laryngeal changes consisted of mild vocal fold erythema, arytenoid erythema, mild edema and mild glottal closure insufficiency. The more substantial findings (disordered group) included individuals having nodules, polyps, chronic laryngitis, laryngeal reflux disease, moderate to severe glottal closure insufficiency.37 Table 1 lists the laryngeal findings in the study participants diagnosed via indirect laryngoscopy. Table 1: Diagnostic distribution of study participants. (Some participants presented with more than one finding) Laryngeal status Category (No. subjects) Laryngeal findings Number of subjects Healthy (40) Normal laryngeal findings 40 Mild changes (33 ) Mild redness of vocal folds 4 Mild swelling of VF 5 Beginning vocal nodule 1 Mild redness in arytenoids 7 Slight amount of thick mucus 2 Slight hoarseness 7 Incomplete glottal closure in phonation 10 Mild false VF medialization 2 Slight hyperkinesia 3 Disordered (11) Nodules 4 Polyps 2 Chronic laryngitis 1 Vocal fold atrophy 2 Reflux disease 1 Moderate to severe closure insufficiency 1 2.2. Recordings and tasks Teachers were asked to sustain three times a prolonged vowel [a:] for 5 seconds, followed by reading of a text containing 213 words (no sibilants were included in the text to prevent any speech 6 noise components in the signal considering cepstral sensitivity to noise) at comfortable loudness as in conversational speaking. Additionally, the teachers were asked to read the same text at increased loudness levels as they used during teaching in a large noisy classroom. The voice recordings were carried out in primary schools, in teacher’s own classrooms with minimal ambient noise (approximately about 35 dB(A)). Recordings were made using a portable digital recorder (Sony TCD-D8, Sony Corporation, Tokyo, Japan) and an omnidirectional head-mounted microphone (AKGC477, AKG, Vienna, Austria), selected according to the recommendations by Švec and Granqvist (2010)38 which was maintained at a constant distance of 6 cm, at an angle of 45ο from the side of the subject’s mouth. The voice recordings were then calibrated using a sound level meter (type 2206 Bruel & Kjaer, Copenhangen) to obtain the true sound pressure level (SPL) of vowel and speech samples. 2.3. SPL calibration procedure and measurement Calibration was made by using a standard complex sound source (BOSS-TU 120), and a sound level meter (SLM, type 2206, Brüel & Kjaer, Denmark), placed at the same distance and angle from the sound source as the microphone was from the subject's lips. For SLM the slow time averaging and C frequency weighting was used. After the recording, the sound calibration signal was then loaded in Praat software. For calibrating the sound levels in the Praat, the procedure mentioned by Boersma and Weenink (2013)39, 40 in the Praat manual was used, where the recorded signal was mathematically amplified to obtain the true sound pressure levels (that corresponded to the waveform values in pascals) using the multiplication factor 10�∆𝐿𝐿 20� where ∆𝐿𝐿 was the difference level (difference between the true sound pressure level read in the SLM and the uncalibrated level depicted in the Praat software). In Praat this was done by selecting the signal and choosing the option ‘Multiply’ from the ‘Modify’ menu and supplying the multiplication factor. After the calibration, the steps involved in obtaining the SPL value in Praat were as follows: Voice sample of interest (vowel or speech) was selected in the ‘View and edit’ window of Praat. From the ‘Intensity settings’ the intensity contour was obtained by selecting the option ‘Show intensity’. The following intensity settings were used: view range 40-120 dB, “mean energy” averaging method, and ‘subtract mean pressure’ chosen (as in standard settings). The final representative SPL value was obtained using the ‘Get intensity’ option. The final single SPL value obtained this way represents a close approximation of the time-averaged (equivalent) C-weighted sound level for the entire voice sample selected as measured by the sound level meter.40 Briefly, the time-averaged sound level of a voice signal is equivalent to SPL of a steady sound of the same duration and energy as the selected voice signal, and C-weighting is used by sound level meters when approximating the human hearing for loud sounds. 40 2.4. Cepstral analysis 7 Sustained vowel [a:] at comfortable loudness for 3 seconds and 2 first sentences of continuous speech samples (23 syllables) at comfortable and increased loudness were analyzed for all teachers for CPP and CPPS data using software Praat. The vowel samples were chosen from the middle and most stable part of the second vowel from the row of three trials recorded. These selections were identical to those used for SPL analysis. The CPP values were obtained using standard Praat (version 5.4.05) settings while the CPPS values were extracted with settings recommended by Maryn and Weenink (2015).3 Table 2 shows the parameter setting from Praat software for the extraction of CPP and CPPS. Table 2: The steps and parameter setting in the Praat software for extraction of CPP and CPPS values for the vowel and continuous speech samples 2.5. Perceptual analysis The same samples of comfortable vowel phonation and comfortable and loud speech reading that were analyzed for cepstral measures were also perceptually analyzed by five experienced voice experts. They used headphones (Sony Stereo headphones MDR-CD480) in the evaluation task. They rated overall voice quality along a ten point unipolar scale from 0 = poor to excellent = 10. Additionally, they evaluated the vocal firmness along a bipolar axis from 0 = breathy through 5 = adequate to 10 = pressed. The listeners could listen to each sample as many times as they liked in Step 1) Select the vowel or speech sample Step 2) Go to ‘Analyse periodicity’ and click on to ‘To Power cepstrogram’ in the Praat Objects window. Step 3) Use the following settings for generating the power cepstrogram: Parameter setting CPP (standard settings for Praat version 5.4.05) CPPS3 Pitch floor (Hz) 60 60 Time step (s) 0.002 0.002 Maximum frequency (Hz) 5000 5000 Pre-emphasis from (Hz) 50 50 Step 4) On selecting the newly generated ‘powercepstrogram’ click on to ‘Query’and select ‘Get CPPS’ from the menu, and use the following settings: Parameter setting CPP (standard settings for Praat version 5.4.05) CPPS3 Select subtract tilt before smoothing Yes No Time averaging window (s) 0.001 0.01 Quefrency averaging window (s) 0.00005 0.001 Peak search pitch range (Hz) 60-330 60-330 Peak search tolerance (0-1) 0.05 0.05 Interpolation Parabolic Parabolic Tilt line quefrency range (s) 0.001-0.0 (=end) 0.001-0.0 (=end) Line type Exponential decay Straight Fit method Robust Robust 8 order to be sure of the evaluation. The individual listeners’ ratings were averaged for each sample to be used in the other statistical analyses. 2.6. Statistical analyses Kolmogorov-Smirnov test was used to check normal distribution of voice SPL, cepstral measures (CPP and CPPS) and perceptual ratings (voice quality and firmness) for all the three voice samples. To check the inter-rater reliability for the perceptual ratings, Cronbach’s alpha test was used. Paired t-test was used to compare voice SPL, CPP, CPPS, voice quality rating and firmness ratings between comfortable vowel and comfortable speech and between comfortable speech and loud speech. Pearson’s product moment correlation test was used to find correlation between voice SPL and cepstral measure. Spearman’s rank order correlation test was used to find correlations between perceptual ratings, cepstral measures and voice SPL across the three laryngeal status categories (healthy, mild changes and disordered). One way ANOVA was used to compare the voice quality rating, firmness rating, cepstral measures and voice SPLs across the three laryngeal status categories. All the statistical analyses were carried out using SPSS 22 software (IBM SPSS Statistics v. 22 for Windows, Armonk, NY). Significance level was set at p < 0.05 in the statistical analyses. MATLAB R2016a was used for scatterplots. 2.7. Ethical approval Permission for data collection was obtained from school administration and social services departments in the districts in question. Participants volunteered in the study and signed a written consent, which informed them about the aim and procedure in the studies, and stated that the participants may withdraw from the study at any point without any consequences. Handling and preservation of the research material follows the Personal Data Act (523/1999) of Finland. 3. RESULTS All the measures, CPP, CPPS, SPL, voice quality rating, and rating of firmness for all three voice samples, were normally distributed based on Kolmogorov-Smirnov Test. 3.1. Reliability of perceptual evaluation The inter-rater reliability of the perceptual evaluation (Table 3) was regarded as adequate based on results of Cronbach’s alpha except for rating of voice quality for loud speech which was lower (0.6), and was found questionable, as normally the cutoff value of 0.70 is considered acceptable for reliability.41 Table 3: Inter-rater reliability for voice quality and firmness rating for three voice samples Voice samples Inter-rater reliabilityCronbach’s alpha (α) value Vocal quality Vocal firmness Sustained comfortable vowel 0.83 0.80 Comfortable speech 0.82 0.83 Loud speech 0.6 (low and questionable) 0.82 9 3.2. Acoustic and perceptual results for the three voice tasks: The results of the acoustic and perceptual evaluations for the three voice tasks are shown in Table 4. Furthermore, the results of the paired t-tests evaluating the significance of the differences between the different tasks are shown in Table 5. Together, these tables reveal that: a) The CPP, CPPS and SPL values were significantly larger for sustained vowels than for speech at comfortable loudness; b) The CPP, CPPS and SPL values were significantly larger for loud speech than for comfortable speech; c) Perceptually, the voice quality was found (marginally) significantly better for comfortable vowel than comfortable speech whereas vocal firmness did not show any significant differences here; d) The voices were found to have significantly better quality and more firmness/less breathiness for loud speech than for comfortable speech. Table 4: The evaluation results expressed through the mean and standard deviation values for the three voice samples Voice samples CPP (dB) CPPS(dB) Voice SPL(dB) Voice quality Vocal Firmness Sustained vowel 23.4±2.9 13.6±2.1 82.4±5.5 4.7±0.9 5.1±1.0 Comfortable speech 19.0±1.4 10.4±1.5 76.4±3.3 4.4±1.0 4.8±1.2 Loud speech 19.6±1.2 11.4±1.4 84.9±3.8 4.9±1.0 5.7±1.2 Table 5: P values for paired t-test comparing the evaluation results for vowel versus speech at comfortable loudness and for comfortable versus loud speech. Significant values (p<0.05) are indicated by *. Voice samples CPP CPPS SPL Voice quality Vocal Firmness Vowel versus comfortable speech P<0.001* P<0.001* P<0.001* P=0.040* P=0.085 (not significant) Comfortable speech versus loud speech P<0.001* P<0.001* P<0.001* P<0.001* P<0.001* 3.3. SPL versus CPP and CPPS for vowel and speech The next aim was to find the relationship between cepstral and voice SPL measures. Table 6 shows the Pearson’s product moment correlation between the cepstral and voice SPL measures. The results show a positive moderate correlation between voice SPL and both CPP and CPPS for vowel. Also a positive moderate correlation was obtained between voice SPL and CPPS for loud speech and a mild correlation with CPP for loud speech. No significant correlations were obtained between voice SPL and cepstral measures for comfortable speech. However, when the comfortable and loud speech data were pooled together the voice SPL again correlated moderately with both CPP and CPPS measures. Table 6: Pearson’s product moment correlation values and P values for correlations between cepstral measures and voice SPL measures. 16 Nevertheless, since in some of the teachers laryngeal pathologies were detected laryngoscopically, another goal of the present study was to find out whether the expert perceptual evaluations, cepstral measures and voice SPL measures could reveal some vocal changes due to the underlying pathology (section 3.4, 3.5 and 3.6 of results). The results of one way ANOVA test did not show any systematic differences for perceptual ratings across the laryngeal status categories (healthy, mild changes, disordered), and no significant correlations were found between the perceptual and laryngeal status evaluation. From Table 7 it can be seen that the voice quality and firmness ratings do not show any specific trends. This indicates that the laryngeal pathologies were not well perceivable by the voice expert listeners. Similar to perceptual evaluations, also the acoustic measures did not show statistically significant differences among or correlations with the laryngeal status categories for any of the vocal tasks. Nevertheless, a closer look at the results in Table 8 revealed that, in contrast to the perceptual evaluations, the cepstral measures (both CPP and CPPS) and SPL values (for all three voice samples), show a consistent decline in the mean values with increased severity of the laryngeal pathology. This suggests that the cepstral and SPL measures could be more sensitive to the underlying vocal pathology than the perceptual measures. However, the differences among the disordered vs. nondisordered groups were only around 0.2 dB for CPP and CPPS and 2 dB for SPL (Table 8) which are much smaller compared to the standard deviations which were above 1 dB (for CPP and CPPS) and above 2.9 dB (for SPL) within each category. This indicates that the CPP, CPPS and SPL variability among healthy larynges was larger than the influence of the underlying laryngeal pathology in our teachers. Considering this and the fact that all the teachers considered themselves to have a normal voice, they may be as such referred to having a functionally healthy voice. The large variability with respect to the small effect of the laryngeal pathology limits the possibility of using solely the CPP, CPPS and SPL measures for detecting the laryngeal pathology in individual teachers. Nevertheless, the trend of CPP, CPPS and SPL lowering with underlying laryngeal pathology can be explored in future for detecting differences among the groups of pathologic and control subjects. In this study, the pathologic group size was limited to only 11 teachers causing the standard error of the mean to be rather large for finding significant differences. Future studies may explore the differences with a larger number of subjects in which the standard error of the mean is expected to be smaller thus revealing better on potential significant differences among different subject groups. 5. CONCLUSION The present study brings basic information on CPP and CPPS values in teachers without vocal complaints and their relationships to voice SPL, voice quality, firmness of voice, and underlying laryngeal pathologies. The results show that with increased loudness and SPL, the cepstral values increased and the voice became firmer without becoming excessively pressed. Although the 17 teachers considered themselves vocally healthy, 52.4% of them had some laryngeal changes detected laryngoscopically. 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