Assessment of professional singers using laryngeal, respiratory, and airflow measurements

Purpose: In the pedagogy of classical vocal singing, it can be difficult to determine the human voice fach, especially for the voice of aspiring vocalists. Hence, an objective metric-based system for the determination of the human voice is needed. In the present study, we investigated the anthropological and aerodynamic parameters for 60 professional singers with a professionally confirmed singing range. Methods: Amongst the 60 included professional singers, there were ten participants each for sop-ranos, mezzo-sopranos, altos (female vocal fach), and tenors, baritones, basses (male vocal fach). Airflow measurements were recorded using spirometry whilst anthropological measurements were taken using CT scans. Appropriate statistical analyses were done using the Mann-Whitney U test and Kruskal Wallis H test with post-hoc tests and Bonferroni correction. p < 0.05 was considered statistically significant. Results: Soprano singers, who have the highest pitch, were found to be the shortest and least heavy, whilst basses, who have the lowest pitch, were found to be tallest and heaviest amongst the study participants. Furthermore, sopranos had the smallest lung volumes while the basses had the largest lung volumes (raw spirometry measures). However, when normalized ratios were considered, no differences were observed. Finally, laryngeal size showed sexual dimorphism due to developmental changes. Conclusions: A mix of anthropological and aerodynamic measurements may be useful to assist singers and vocal pedagogues to assess and determine voice types before the beginning of their vocal studies.


Introduction
Western classical vocal pedagogy has a long and rich history of identifying and categorizing the human voice into different voice types [1,2].Historically, vocal pedagogues have used vocal range (pitch or frequency), tessitura (comfortable vocal range) and timbre (color) as determinants of a singer's voice type [3].Male voices have been categorized as tenors (C 3 -C 5 ), baritones (F 2 -F 4 ), and basses (E 2 -E 4 ), and female voices as sopranos (C 4 -C 6 ), mezzo-sopranos (A 3 -A 5 ), altos (F 3 -F 5 ), and contraltos (E 3 -F 5 ) [4].However, this approach's effectiveness is constrained because the pitch limits of each voice type overlap, making precise classification challenging.For instance, the ranges of bass and baritone singers overlap by greater than two-thirds [5].
An alternate classification system, widely used in Europe, is the German Fach system.The system employs a combination of the traditionally used voice parameters like range and timbre with physical characteristics of the singer like age and physical build [6].It also considers the experience, desire, and frequency of the performances to classify singers into different voice types [6].Nonetheless, the system is known for being too complicated (e.g.multiple subcategories) and one that does not factor in the changes in the physical characteristics as one ages and matures [7,8].Others attempting to determine singing voice type through voice analysis parameters have suggested using vocal pitch as the primary factor, supplemented with formant or resonant frequency [5].Pitch is determined by vocal fold vibrations (source), which in turn, depends on the vocal fold length (VFL).Studies in the past have relied on a variety of approaches (cadaveric, endoscopic, radiologic) to measure the VFL and have found a correlation between the VFL and the pitch of the voice [9][10][11][12].Furthermore, VFL was found to be inversely related to the width of the voice range, with sopranos having the widest ranges and basses having the narrowest ranges [9].
Voice formants, on the other hand, remain less investigated with respect to fach.Formants are determined by the size and shape of the vocal tract (filter) [13,14] and can be tuned by adjusting the shape of the tract and using articulatory structures such as the tongue, lips, and soft palate.For example, formant resonance can be amplified by jaw opening, changing of lip and tongue position, and by slight smiling [15].Previous research in non-human primates has shown correlation between body size, vocal tract length, and formant frequencies [16].In humans, M€ urbe et al. measured the length of the resonating structures above the glottis (pharyngeal, velar, and oral segments) and found it to be shortest in sopranos and longest in basses [17].Furthermore, these differences were reported to be most prominent for the pharyngeal length.In another study, authors reported pharyngeal and laryngeal length and volume to be significantly larger for men than for women [18].
Additionally, several other physiologic factors like the geometry and material properties of the lung, larynx, and vocal tracts could influence the complex fluid-structuralacoustic interplay that is responsible for voice production.Recent studies have employed magnetic resonance imaging (MRI) to measure the length of the vocal tract [19,20].However, no study has investigated the role of the dimensions of sub-laryngeal structures including trachea and bronchi in affecting the voice range.The exhaled air from lungs is responsible for vibrating the vocal folds and subsequent production of voice and speech.It has been shown that in male singers, a higher fundamental frequency (pitch) requires higher phonation threshold pressure (due to stiffer vocal folds), and hence higher subglottic pressure (Ps) [21].While muscles and the elasticity of the thoracic-pulmonary unit are primarily responsible for modulating the Ps, we hypothesize that differences in the trachea-bronchial tree measurements (Poiseuille's equation) could possibly play a minor yet significant role in modulating physiological aerodynamics, contributing to different voice ranges [22,23].
It has been shown that during expiration, larynx and trachea-bronchial tree move upwards with the degree of this displacement being dependent on the lung volume [24].Amongst untrained singers, higher lung volumes have been shown to be associated with a lower-positioned larynx, higher Ps (increased vocal loudness) and a lower pitch [24,25].Apart from the positioning of the larynx, the external size of the larynx has also been shown to correlate strongly with the VFL and subsequently the vocal range of the individual [26].It is well known that smaller drums, wind instruments, and violins with a smaller volume can produce a higher pitch sound than instruments with a larger volume.Based on this observation, we hypothesized that same could be also true for the volume of the trachea-bronchial tree.Hence, in the present study, we examined the airflow measurements using spirometry (lung volumes) and respiratory tract measurements of larynx, trachea, and bronchi to understand the dynamics of voice production amongst professional singers.We believe the results from the present study could aid in accurate determination of the vocal fach based on measurements of aerodynamic and anthropometric data before the beginning of vocal studies.

Materials and methods
The study protocol for the present study was approved by the Research Ethics Committee of the Riga Stradin� s University (ethical approval no.703).All patients provided written consent for participation in the study.

Participant profiles
The present study included a cohort of 30 male and 30 female professional singers in the age range of 19 to 77 years (median age 38 and inter-quartile range 13.02 years).All singers had a professionally confirmed voice fach.The participants had been working as soloists at the Latvian National Opera, Latvian Radio, open choirs and/or were students at the vocal department of Jazeps Vitols Latvian Academy of Music, Latvia.To minimize random variations in the dataset, the study protocol required the participants to have professional singing experience of five years or more in their respective voice type and be able to sing with ease in that range.Prior to beginning the experimental procedure, all participants were required to fill-in a questionnaire regarding their vocal fach.
The vocalists were asked to mark the lowest and highest sounds of their voice range in which they had been singing for the past three years.Their singing voice range was recorded along with their phonation range that included notes beyond which the singer would typically use in performance.Accordingly, amongst the 30 male participants, there were 10 tenors (T), 10 baritones (Ba), and 10 basses (B).Amongst the 30 female participants, there were 10 sopranos (S), 10 mezzo-sopranos (Mz) and 10 altos (A).Participants suffering from acute cold, respiratory illness, or having history of a disease/surgical intervention involving the vocal cords or other laryngeal structures (including oncological lesions) were excluded from the present study.Non-professional amateur singers were also excluded from the present study.

Airflow measurements
The baseline measurements including body weight and height were measured using appropriate instrumentation.The spirographs were used to measure the peak expiratory flow (PEF; measurement of airflow) and the vital capacity of the lungs for all participants following standard clinical procedures.Before making measurements, all participants were provided with a small introduction of the examination procedure and were made to rest both physically and vocally for about 10-15 min.

Anthropological measurements
The length, diameter, area, and volume were measured for the larynx, trachea, and bronchi for each participant.For bronchial measurement, an average of the measurements from right and left side were used in the study.The measurements were taken using radiotherapy simulation computed tomography (CT) equipment named XIMA-TRON CX (Varian Medical Systems, California, USA).The simulator is equipped with a digital image converter that converts DICOM (Digital Imaging and Communications in Medicine) files to standard image files like JPEG and simultaneously transmits them to the monitor for visualizations.The medical operator or researcher then simply needs to mark the borders of the anatomical structures that they are interested in measuring (Figure 1).
The software performs calculations and provides measurement values within approximately five minutes.The borders for larynx, trachea, and bronchi were decided and approved by a group of experts with three radiologists, two otorhinolaryngologists and one medical physicist.The initial anatomical borders were marked by the two otorhinolaryngologists together and then were sent for adjustments/agreements to the radiologists and the physicist.After the input from radiologists and the physicist, all experts held a concilium to reach the final borders.The input of the equipment technician was also factored in during the benchmarking of the anatomical borders.In the present study, supra-glottal intralaryngeal space was defined as the space from the vocal folds to the upper border of thyroid cartilage while sub-glottal intralaryngeal space was defined as the space from the vocal folds to the lower border of thyroid cartilage.

Data processing and analysis
All participant data was anonymized, summarized, and stored using MS Excel (Microsoft Office 365, Windows 11).Normality was checked for all quantitative variables using Shapiro Wilk test and visually using histograms and Q-Q plots.All variables were found to have non-Gaussian distribution.Kruskal-Wallis H ANOVA was therefore used for analyzing inter-vocal range comparisons with appropriate post-hoc tests and Bonferroni's correction.For comparison of data between male and female participants, Mann-Whitney U test was used.A stepwise elimination multivariable linear regression model was performed along with Spearman's correlation with laryngeal volume as the dependent variable.Model parameters such as Akaike Information Criterion (AIC), Schwarz Bayesian Criterion (SBC), and Mallow's Prediction Criterion (Mallow's Cp) were used to select the most precise model.p < 0.05 was considered statistically significant.The statistical analysis was done using SPSS (IBM Corp. Released 2020; IBM SPSS Statistics for Windows, Version 27.0; Armonk, NY, USA: IBM Corp).

Baseline characteristics
The baseline characteristics for the male and female study groups are summarized in Table 1.There were significant differences in the distributions of height, weight, and BMI of  the participants based on gender (p < 0.001).Males were generally taller and heavier than female participants (Table 1).
No differences in the distribution of age were found.Amongst the male participants, there were significant differences in the distribution of height between different voice ranges.Post-hoc tests revealed significant differences in distribution of height between tenor and bass (adjusted p ¼ 0.019) and tenor and baritones (adjusted p ¼ 0.037).No significant differences were found in distributions of age, weight, and BMI of the male participants (Table 2).
Similar to the male participants, female participants showed significant differences in the distribution of height.Post-hoc tests revealed significant differences in height distribution between alto and sopranos (adjusted p ¼ 0.002) and alto and mezzo-sopranos (adjusted p ¼ 0.015).Female participants also reported significant differences in the distribution of weight.Post-hoc tests showed significant results for differences between sopranos and mezzo-sopranos (adjusted p ¼ 0.035) and sopranos and altos (adjusted p ¼ 0.030).No differences in distributions of age and BMI were noticed among the female participants (Table 3).

Airflow measurements
During spirometry, we measured the following parameters-forced vital capacity (FVC; the total volume of air exhaled during a maximal forced expiration), forced expiratory volume in one second (FEV 1 ; the volume of air exhaled in the first second under force after a maximal inhalation), maximal expiratory flow at 75%, 50%, and 25% of FVC (MEF 75 , MEF 50 , and MEF 25 ), and peak expiratory flow (PEF; the maximal flow that can be exhaled during maximal forced expiration).The individual participant values so obtained need to be calibrated against normal values for a person of the same gender, height, and age.This yields us the variable "% predicted".Based on the % predicted values, no significant differences in the distributions were noted for all assessed parameters between males and females (Table 4).Similar results were noted for the % predicted values for all parameters in both the male and female sub-groups (Tables 5 and 6).

Laryngeal measurements
There were statistically significant differences (p < 0.001) observed in the distributions of laryngeal parameters including length, diameter, area, and volume between male and female participants (Table 7).Differences in subglottal and supra-glottal parameters are also summarized in Table 7.It is interesting to note that the spread of laryngeal length, area, and volume was wider in males in comparison with the females (as seen by the higher IQR).This could be explained by the larger variability in body build parameters among the male participants (Table 1) in comparison with the female participants.The post-hoc tests (Table 8) revealed that within the male participants, there were significant differences between the distribution of laryngeal length between tenors-baritones (adjusted p < 0.001) and tenors-basses (adjusted p ¼ 0.041).For laryngeal diameter however, post-hoc tests revealed significant differences in distribution for baritones-basses (adjusted p ¼ 0.038) and baritones-tenors (adjusted p ¼ 0.005).Laryngeal area showed differences similar to those for laryngeal diameter (baritones-basses and baritonestenors; adjusted p ¼ 0.021 and < 0.001, respectively).The distribution of laryngeal volumes also showed significant differences between baritones-basses and baritones-tenors (adjusted p ¼ 0.011 and < 0.001, respectively).
Within the female participants, post-hoc tests (Table 9) revealed there were significant differences between the distribution of laryngeal length between altos and sopranos (adjusted p ¼ 0.001).Similar differences between these two groups were noted for laryngeal diameter (altos-sopranos; adjusted p < 0.001).For laryngeal area and volume, there were significant differences in distribution between altosmezzo-sopranos and altos-sopranos (for area -adjusted p ¼ 0.049 and < 0.001, respectively; for volume-adjusted p ¼ 0.031 and < 0.001, respectively).
For supra-glottal area, post-hoc tests revealed significant differences in distribution between altos-sopranos and altos-mezzo-sopranos (adjusted p < 0.001 and 0.026, respectively) in the female group and between baritonesbasses and baritones-tenors (adjusted p ¼ 0.006 and < 0.001, respectively) in the male group.For supra-glottal volume, in the female group, post-hoc tests were significant for altos-sopranos and altos-mezzo-sopranos (adjusted p < 0.001 and 0.041, respectively) and, in the male group, between baritones-basses and baritones-tenors (adjusted p ¼ 0.003 and < 0.001, respectively).
For sub-glottal length, post-hoc tests were significant for altos-sopranos and altos-mezzo-sopranos (adjusted p ¼ 0.004 and 0.033, respectively) amongst the females and for baritones-tenors (adjusted p ¼ 0.001) amongst the males.The differences in the distributions of sub-glottal area and volume were significantly differently distributed between altossopranos and altos-mezzo-sopranos (area -adjusted p < 0.001 and 0.029, respectively; volume -adjusted p < 0.001 and 0.038, respectively), and between baritonesbasses and baritones-tenors (area -adjusted p ¼ 0.033 and < 0.001, respectively; volume -adjusted p ¼ 0.036 and < 0.001, respectively).

Tracheal measurements
Between males and females, there were significant differences in tracheal length (p < 0.001; male median 13.79 cm, IQR 2.28 compared with female median 11.84 cm, IQR 2.12), tracheal diameter (p ¼ 0.033; male median 1.86 cm, IQR 0.42 compared with female median 1.68 cm, IQR 0.16), Amongst the males, no significant differences were noted for tracheal length, diameter, area, and volume (Table 10).In the females group however, significant differences in distributions of tracheal area and volume were noted (Table 11) between the altos-sopranos (area -adjusted p ¼ 0.021; volume -adjusted p ¼ 0.010) and altos-mezzo-sopranos (area -adjusted p ¼ 0.038; volume -adjusted p ¼ 0.016).

Bronchial measurements
No significant differences were noted for the distribution of bronchial measurements between the male and female participants for average length, diameter, area, and volume (p ¼ 0.099, 0.853, 0.186, 0.712, respectively).Amongst the male participants, no significant differences were noted in average bronchial length, diameter, area, and volume (Table 12).However, amongst the females, there were significant differences in distributions for all bronchial measurements (Table 13).Post-hoc tests revealed significant differences between altos and sopranos (diameter, volume -adjusted p ¼ 0.033) and altos and mezzo-sopranos (length, area, volume -adjusted p ¼ 0.003, 0.002, and 0.004, respectively).� For AIC and SBC, the lower the value, the better the model.For Mallow's Cp, the closer the value to the sum of the number of independent variables þ1 and the constant, the better the model.�� Statistically significant result (p < 0.05).

Regression model
A stepwise elimination multivariable linear regression was performed with laryngeal volume being the dependent factor (linear relationship was confirmed using curve estimator).Six significant models were found (Table 14).Across all these models five independent variables were identified-laryngeal area and diameter, supra-and sub-glottal volumes, and tracheal length.All other variables were excluded by the analysis.A further evaluation showed that Models 2, 3, and 6 were not acceptable due to the violation of multi-collinearity diagnostics (Variance Inflation Factor; VIF � 5).Model 1 was also not acceptable due to extremely high Akaike Information Criterion (AIC), Schwarz Bayesian Criteria (SBC), and Mallow's prediction criterion (Mallow's Cp).
Next, to select between Models 4 and 5, we looked at AIC, SBC, and Mallow's Cp.All the criteria supported the use of Model 5. We then used Spearman's correlation to evaluate the relationship between the variables.There was statistically significantly strong positive correlation between laryngeal volume and supra-glottal volume (q ¼ 0.964, 95% CI ¼ 0.939 to 0.978, p < 0.001) and laryngeal volume and sub-glottal volume (q ¼ 0.951, 95% CI ¼ 0.918 to 0.971, p < 0.001).However, laryngeal volume and tracheal length were found to be not significantly correlated (q ¼ 0.241, 95% CI ¼ −0.021 to 0.472, p ¼ 0.063).To understand whether tracheal length could be a suppressor variable, we looked at the correlations between tracheal length and the supra-and sub-glottal volumes.In both these correlations, no significant correlations were found (p ¼ 0.058 and 0.186, respectively).
Furthermore, the addition of tracheal length did not improve the adjusted R2 or ANOVA.The effect of tracheal length in terms of the linear equation and standardized beta was found to be of the order 10 −5 .The Durbin-Whatson also revealed lesser autocorrelation in the model without tracheal length.Hence, Model 4 was selected as the final model.Supra-and sub-glottal volumes and tracheal length significantly predict the laryngeal volume of the participants (ANOVA p < 0.001).This is interesting because it could be suggested that the height of the thyroid cartilage could be a more direct predictor of the voice range.This is also evident from the significant post-hoc differences noted in supra-and sub-glottal volumes and areas (Tables 8 and 9).It would be worth estimating the size of the cartilage itself in future studies.
With an increase of 1 cm 3 in supra-or sub-glottal volume, the laryngeal volume would also increase by 1 cm 3 , and this relationship is statistically significant (t-test p < 0.001).In terms of strength of relationship (standardized beta), supra-glottal volume had a stronger relationship (0.577) in comparison with the sub-glottal volume (0.484).When controlled for age, height, weight, and BMI (both individually and all together), partial Spearman's correlation showed no differences in the correlation strength or significance, suggesting that these confounding factors have no effect in determining the relationship between intra-laryngeal volumes and overall laryngeal volume.

Discussion
The results from our present study support the hypothesis that soprano singers, who have the highest pitch, are the shortest and least heavy, while basses, who have the lowest pitch, are the tallest and heaviest amongst the study participants.These findings are in line with multiple previous studies that have highlighted the inverse relationship between pitch and physical attributes such as height and weight.A low-pitched voice has been associated with a larger body, especially in terms of weight and upper body musculature [13].For formant frequencies, a similar negative relationship has been reported with weight and height of the individual [13].Furthermore, it has been shown that height, weight, and BMI also correlate with the VFL [9].Developmental differences during puberty lead to preferential thickening and enlargement of the vocal folds in males under the influence of testosterone.This leads to larger VFL and a deep voice in males [18].
Although we observed no significant differences in spirometry results among male and female singers, airflow measurements of the respiratory tract may have some usefulness to determine the vocal fach.Firstly, aerodynamic, and physiological metrics correlate to some extent with the voice fach and are relatively easier to obtain than other metrics studied in prior works.Secondly, airflow measurements are non-invasive and can be obtained during standard clinical check-ups.Finally, the determination of the vocal fach according to the VFL may not prove to be accurate since VFL is dependent on vocal fold mass per unit of length which can vary when different fundamental frequencies are produced during phonation, making it difficult to compare data between study participants [10,27,28].
As for our results, we did not report any significant differences in spirometry as we compared the normalized values rather than the raw spirometry values.This was done since the raw values are not adjusted for individual's age, height, weight etc.Instead, when the non-normalized raw values were compared, we noticed statistically significant differences in the spirometry parameters (data not shown).Such a phenomenon has also been reported previously by other authors [29].
FVC is one of the most important parameters in spirometry, since the larger the FVC, the longer the phonation between inhalations and lesser the vocal effort needed for voice production [29].It has been shown that females have smaller lung volumes (hence lower FVC) and lower static recoil during exhalation in comparison with males (due to developmental differentiations) [30,31].This means that females require a higher fraction of lung volume to create the same lung pressure as males [29].Our results agree with these findings.Distribution of raw FVC values in females was statistically significantly different in females when compared with males (median females 3.99 IQR 1.02 compared with median males 5.90 IQR 1.15) using Mann Whitney test (p < 0.001).Sopranos had the smallest lung volumes while the basses had the largest lung volumes.Such differences were noted for all other raw spirometry values as well (data not shown).
In the context of the differences in the laryngeal sizes, most previous studies have investigated the VFL due to methodological limitations.However, with advances in radiological techniques (CT, MRI), non-invasive, painless, and accurate measurements of larynx are now possible, thereby arousing great interest from surgeons and clinicians [32,33].Previous investigations have revealed that laryngeal dimensions are greater in men than in women.However, when comparisons are made between the age groups of the same sex, there are no significant differences [32,33].These findings highlight two developmental phenomena-firstly, laryngeal size shows sexual dimorphism and secondly, there are few morphometric differences amongst the individuals of the same sex.Again, our results concur with these previous findings.In fact, while other studies investigated only the effects of age on intra-sex differences in laryngeal dimensions, our results indicate that height, weight, and BMI also are not significant parameters when considering the laryngeal dimensions.
Finally, for tracheal and bronchial measurements, we found significant differences among males and females.Such differences could be explained by sexual dimorphism.On the contrary, Welch et al. compared the vocal tract length and voice source behaviors of professional basses/baritones, tenors, and countertenors using xeroradiographicelectro-laryngography (XEL).XEL analysis combines two known techniques-soft tissue radiographic imaging (xeroradiography) and an analysis of voice source vibratory patterning (electro-laryngography). Their results indicated that in the bass/baritone sample the lower oropharynx and laryngeal airway dimensions expand systematically with increasing pitch; the principal differences between the bass/ baritone and tenor samples being the vertical dimensions of the vocal tract at rest and during phonation and in the voice source patterning; the relationships between the vocal tract dimensions of basses/baritones and tenors tend to be maintained across registers and there are significant differences between bass/baritone/tenors and countertenors for identical pitches, particularly at the source of the voice [34].
In our study, we also confirmed that there is no significant influence of the airflow in the middle and small bronchi on the pitch of the voice (data not shown).According to the Poiseuille's law, the speed of gas flow in a tube is directly proportional to difference of pressure on both ends of the tube and its radius to the 4 th power, in other words, the amount of air flowing in a definite time unit through tubes of larger diameter (bass, baritone) is greater than that flowing through smaller diameter (tenor).Vocalists with lower voices and larger volumes of the larynx, the trachea, and the big bronchial tubes will have a greater peak expiratory flow.
However, we would like to point out some limitations of the study findings.Since all measurements were taken by a single author, there could be some bias in the results.We did not assess intra-and inter-rater reliability measurements, and hence the results should be validated in a bigger sample size.Furthermore, our measurements come from largely bilingual Latvian-and Russian-speaking populations which could affect the pitch and voice parameters.It has been shown that when speaking different languages people tend to change pitch and voice quality.For example, when Finnish-speaking study group was asked to speak in English, a raise in the voice pitch was noticed.However, such an effect was not noted when English speakers were asked to speak in Finnish [35], highlighting the possible role of language in modulating voice parameters.Clearly, vocal fach cannot be precisely determined by just one aerodynamic test.All acquired data on the aerodynamic measurements should be considered, as well as the body weight and height, to recommend the vocalist to develop his or her voice in a certain voice category.

Conclusions
A mix of anthropological and aerodynamic measures may be useful to assist singers and vocal pedagogues to assess and determine voice types.We found that soprano singers, who have the highest pitch, were the shortest and least heavy, whilst basses, who have the lowest pitch, were the tallest and heaviest amongst the study participants.Furthermore, sopranos had the smallest lung volumes while the basses had the largest lung volumes (raw spirometry measures).However, when normalized results were considered, no significant differences were observed.Our model also shows that only intralaryngeal spaces (possibly determined by the size of thyroid cartilage) are the relevant anatomical markings which may affect the voice type.All other anthropological measurements seem to have no significant effect on voice type.Finally, laryngeal size shows sexual dimorphism due to developmental changes.Such differences are not affected by age, height, weight, and BMI within the same gender.Similar results were derived for tracheal and bronchial measurements.

Informed consent & consent for publication
All participants in the study provided written and oral consent for participation in the study.Furthermore, all participants agreed on publications of the results obtained from the study.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
The author(s) reported there is no funding associated with the work featured in this article.

Figure 1 .
Figure 1.Screenshot of the digital images of the patient's larynx and trachea with the anatomical borders marked with green lines (taken from personal archive).(A) larynx of a tenor singer and (B) larynx of a bass singer.

Table 1 .
Baseline characteristic features of the study sample.

Table 2 .
Baseline characteristic features of the male study group.

Table 3 .
Baseline characteristic features of the female study group.

Table 4 .
Spirometry measurements for the study sample.

Table 6 .
Spirometry measurements for the female study group.

Table 5 .
Spirometry measurements for the male study group.

Table 7 .
Laryngeal measurements for the study sample.

Table 8 .
Laryngeal measurements for the male study group.

Table 10 .
Tracheal measurements for the male study group.

Table 11 .
Tracheal measurements for the female study group.

Table 14 .
Model summary for multivariable linear regression.

Table 13 .
Bronchial measurements for the female study group.Wallis H test is significant (p < 0.05).Abbreviations.IQR: inter-quartile range.Values presented are average of the measurements from right and left bronchi.

Table 12 .
Bronchial measurements for the male study group.Wallis H test is significant (p < 0.05).Abbreviations.IQR: inter-quartile range.Values presented are average of the measurements from right and left bronchi.