Biomechanics of sound production in high-pitched classical singing.

Voice production of humans and most mammals is governed by the MyoElastic-AeroDynamic (MEAD) principle, where an air stream is modulated by self-sustained vocal fold oscillation to generate audible air pressure fluctuations. An alternative mechanism is found in ultrasonic vocalizations of rodents, which are established by an aeroacoustic (AA) phenomenon without vibration of laryngeal tissue. Previously, some authors argued that high-pitched human vocalization is also produced by the AA principle. Here, we investigate the so-called "whistle register" voice production in nine professional female operatic sopranos singing a scale from C6 (≈ 1047 Hz) to G6 (≈ 1568 Hz). Super-high-speed videolaryngoscopy revealed vocal fold collision in all participants, with closed quotients from 30 to 73%. Computational modeling showed that the biomechanical requirements to produce such high-pitched voice would be an increased contraction of the cricothyroid muscle, vocal fold strain of about 50%, and high subglottal pressure. Our data suggest that high-pitched operatic soprano singing uses the MEAD mechanism. Consequently, the commonly used term "whistle register" does not reflect the physical principle of a whistle with regard to voice generation in high pitched classical singing.

Simulation of Multiple Source Vocalization in the Larynx: How True Folds, False Folds, and Aryepiglottic Folds May Interact.

PURPOSE: This study was a modest beginning to determine dominance and entrainment between three soft tissues in the larynx that can be set into flow-induced oscillation and act as sound sources. The hypothesis was that they interact as coupled oscillators with observable bifurcations as energy is exchanged between them. METHODOLOGY: The true vocal folds, the ventricular (false) folds, and the aryepiglottic folds were part of a soft-walled airway that produced airflow for sound production. The methodology was computational, based on a simplified Navier-Stokes solution of convective and compressible airflow in a variable-geometry airway. RESULTS: Three serially connected sources could all produce flow-induced self-oscillation with soft wall tissue and small cross-sectional area. When the glottal cross-sectional areas were similar, bifurcations such as subharmonics, delayed voice onset, and aphonia occurred in the coupled oscillations. CONCLUSIONS: Closely spaced sound sources in the larynx are highly interactive. They appear to entrain to the source that has the combined advantage of small cross-sectional glottal area and proximity to a downstream vocal tract that supports oscillation with acoustic inertance.

Vocal Demands of Musical Theatre Rehearsals: A Dosimetry Study.

OBJECTIVE: To investigate singers' vocal load by documenting three types of vocal doses (time, cycle, and distance doses) and sound pressure levels during the four phases of rehearsal and how the vocal doses vary between singers across rehearsals in the musical Nine, written by Maury Yeston. METHODS/DESIGN: Five student-singers participating in the musical Nine gave informed consent to participate in the study. All five participants were assigned female at birth and female-identifying individuals. They attached a KayPENTAX APM 3300 dosimeter sensor to their lower neck and wore the accelerometer during four three-hour rehearsals throughout the rehearsal process (the music learning phase, the choreography learning phase, the blocking learning phase, and the dress rehearsal) of the musical. The dosimeter records neck vibrations at a rate of 20 samples per second. but it does not record linguistic content. RESULTS: A dosimetric analysis of five student singers identified variability in voice production throughout the rehearsal process. According to the dosimetry findings, singers employed extensive low-frequency voicing below the first passaggio, with belting and mixed vocal strategies as the predominant stylistic choices when performing in Nine. Additionally, the singers used an occasional head voice effect at specific moments. The roles of Carla, Saraghina, La Fleur, and Ensemble One and Two required specific vocal ranges due to the musical score. CONCLUSIONS: Researchers have yet to establish a safe baseline vocal dose for singers. The vocal dose is affected by many factors, such as duration of phonation, frequency range, SPL, and styles of vocalism required by the score. Louder and heavier vocalization produces larger distance doses, representing the cumulative load placed on vibrating tissue. The cycle dose, distance dose, and SPL reported in this study varied within and between singers. The phonation density graphs show this variability and the low tessitura required by the score. Time doses ranged from 4% to 7% of rehearsal time; this short dose suggests that the rehearsals provided healthy conditions for the successful rehearsal process with efficient attention to the vocalization of a score that requires heavy vocal styles, including belting. While the rehearsal pace was not alarming, the demands of the score alone may prove to be much greater than the vocal dose reported through the rehearsal. Further studies are needed to establish the overall dose of each Broadway role to serve as parameters for vocal pacing and voice care.

An Introduction to the Science of Information Transfer, Uncertainty, and a Personal View of Organizational Intelligence.

An introduction to the concepts of entropy, information transfer, and the uncertainty in transfer of information is given. The specific target is information transfer in voice and speech. The entropies of a square wave, a sinusoid, and a sawtooth are calculated because these waveshapes approximate information carriers in vocalization. This is followed by a less-known concept that the increase of organizational intelligence gleaned from physical systems is proportional to the gradient of entropy. That leads to a multi-dimensional interpretation of diversity. Finally, a personal meta-physical extension is made to a universal reservoir of intelligence from which species can draw to advance civilizations.

Simulation of Vocal Loudness Regulation with Lung Pressure, Vocal Fold Adduction, and Source-Airway Interaction.

In speaking, shouting, and singing, vocal loudness is known to be regulated with lung pressure, but the degree to which vocal fold adduction and airway shape play a role in loudness control is less well known. When loudness is quantified in sones instead of sound pressure level (SPL), the regulatory mechanisms are even less obvious. Here it is shown computationally that loudness is insensitive to changes in SPL produced with variable adduction. A trade-off exists between a reduction in glottal flow amplitude and a flatter spectral slope. When the airway configuration is changed from a uniform tube to a "belt" or "call" shape, loudness can increase with a slight decrease in SPL. When the airway configuration is changed from a uniform tube to an operatic "ring" shape, loudness is increased with only a small increase in SPL.

Integrative Insights into the Myoelastic-Aerodynamic Theory and Acoustics of Phonation. Scientific Tribute to Donald G. Miller.

In this tribute article to D.G. Miller, we review some historical and recent contributions to understanding the myoelastic-aerodynamic (MEAD) theory of phonation and the related acoustic phenomena in subglottal and vocal tract. At the time of the formulation of MEAD by van den Berg in late 1950s, it was assumed that vocal fold oscillations are self-sustained thanks to increased subglottal pressure pushing the glottis to open and decreased subglottal pressure allowing the glottis to close. In vivo measurements of subglottal pressures during phonation invalidated these assumptions, however, and showed that at low fundamental frequencies subglottal pressure rather tends to reach a maximum value at the beginning of glottal closure and then exhibits damped oscillations. These events can be interpreted as transient acoustic resonance phenomena in the subglottal tract that are triggered by glottal closure. They are analogous to the transient acoustic phenomena seen in the vocal tract. Rather than subglottal pressure oscillations, a more efficient mechanism of transfer of aerodynamic energy to the vocal fold vibrations has been identified in the vertical phase differences (mucosal waves) making the glottal shape more convergent during glottis opening than during glottis closing. Along with other discoveries, these findings form the basis of our current understanding of MEAD.

How can vocal folds oscillate with a limited mucosal wave?

Self-sustained vocal fold vibration is possible with either or both of two mechanisms: (1) a mucosal wave propagating along the medial surface of the vocal folds and (2) a vocal tract that offers inertive reactance. A quantitative comparison shows the mucosal wave mechanism has a lower threshold pressure and a higher glottal efficiency, but the supraglottal inertance mechanism can assist in the oscillation and is effective in optimizing the two mechanisms. It is concluded that optimal parameters are a mucosal wave velocity on the order of 1 m/s and a diameter of the larynx canal (epilarynx tube) on the order of 0.8 cm.

Optimizing Diameter, Length, and Water Immersion in Flow Resistant Tube Vocalization.

OBJECTIVE: The objective was to quantify the range of airflow resistance and oral pressure attainable with variation of length, diameter, and water immersion depth of tubes and straws. STUDY DESIGN: Pressure-flow equations for tubes, determined previously for variable tube geometries, were used to calculate oral pressure ranges. Human subjects were then recruited to use the variable tube geometries to produce oral pressures, which were quantified with commercial manometers. RESULTS: Nomograms for airflow resistances and oral pressures are plotted as a function of tube length, tube diameter, and water insertion depth. CONCLUSIONS: It is shown that tube diameters in the range of 2.5-3.0 mm with tube lengths of 10-40 cm produce oral pressures in the range of 10-40 cm H2O. Insertion of the distal end into water adds a pressure in the amount of the depth of insertion. Maximum power transfer with different tube geometries is discussed.

Vocal Dose and Vocal Demands in Contemporary Musical Theatre.

OBJECTIVE: To document and quantify vocal dose and student-singers' self-assessment during rehearsals for a contemporary musical theater production. METHODS/DESIGN: Six student singers fastened the sensor from the KayPentax APM 3200 dosimeter to the lower neck to capture neck vibration data during their preparation for the musical Wonderland by Frank Wildhorn. Data were collected during 8-hour periods, at four different stages throughout the rehearsal process: beginning (music and choreography learning phase), middle (staging phase), and end (running the entire show/dress rehearsal phase), plus a post-production day once the production had concluded to establish a baseline vocal load. Students concurrently completed the EASE questionnaire1 after each data collection day. RESULTS: The EASE score (Appendix 1) and demographics/perceptual questionnaire (Appendix 2) revealed that all subjects (three males and three females) found the singing role vocally and physically demanding but only two found the roles to be emotionally challenging. The musical score demanded a higher usage of chest register (judged perceptually) than mixed register from lead singers. All subjects' maximum fundamental frequency range exceeded the pitch range required by the score. The mean vibration dose (distance dose) across all singers for rehearsal days was higher for females, 1,100,000 meters, than males, 900,000 meters, and the subjects' self-ratings on the EASE were not correlated with the distance dose. CONCLUSIONS: A review of Wonderland's score and perceptual judgment of the singers' performances revealed extensive use of chest register, with belting and mix vocal strategies being the predominant stylistic choices. Students described the singing roles' vocal and physical requirements as more challenging than the character's emotional components. This pilot study provides information on the vocal dose for lead and ensemble singers in rehearsal for a Contemporary Musical Theatre production. Singers and voice professionals may find dosimetry a valuable tool for monitoring the vocal dose during rehearsals and performances.

Regulation of laryngeal resistance and maximum power transfer with semi-occluded airway vocalization.

Steady airflow resistances in semi-occluded airways as well as acoustic impedances in vocalization are quantified from the lungs to the lips. For clinical and voice training applications, the primary focus is on two airway conditions, an oral semi-occlusion and a semi-occlusion above the vocal folds. Laryngeal airflow resistance is divided into glottal airflow resistance and epilaryngeal airway resistance. Maximum aerodynamic power is transferred to the vocal tract if the glottal airflow resistance is reduced while the epilaryngeal airway resistance is increased. A semi-occlusion at the lips helps to set up this condition. For the acoustic power transfer, the epilaryngeal airway also serves to match the impedance of the source to the impedance of the vocal tract.

Vocalization with semi-occluded airways is favorable for optimizing sound production.

Vocalization in mammals, birds, reptiles, and amphibians occurs with airways that have wide openings to free-space for efficient sound radiation, but sound is also produced with occluded or semi-occluded airways that have small openings to free-space. It is hypothesized that pressures produced inside the airway with semi-occluded vocalizations have an overall widening effect on the airway. This overall widening then provides more opportunity to produce wide-narrow contrasts along the airway for variation in sound quality and loudness. For human vocalization described here, special emphasis is placed on the epilaryngeal airway, which can be adjusted for optimal aerodynamic power transfer and for optimal acoustic source-airway interaction. The methodology is three-fold, (1) geometric measurement of airway dimensions from CT scans, (2) aerodynamic and acoustic impedance calculation of the airways, and (3) simulation of acoustic signals with a self-oscillating computational model of the sound source and wave propagation.

The Effect of Single Harmonic Tuning on Vocal Loudness.

The study addresses the benefit of tuning single harmonics with vocal tract resonances to increase vocal loudness. The loudness of theoretically constructed vocal sounds with variable levels of sound energy in the first, second, and third harmonics is computed on the basis of ISO standard 226:2003. In comparison to increased loudness with changes in overall spectral slope, it is shown that single harmonic tuning requires a greater range of SPL to produce a similar range of loudness. For example, a 10-40 dB increase in the level of a single harmonic produces less than two doublings of loudness, whereas a spectral slope change from -12 dB/octave to -3 dB/octave can produce a similar doubling of loudness with only a 5 dB SPL increase.

Vocal Tradeoffs in Anterior Glottoplasty for Voice Feminization.

OBJECTIVES/HYPOTHESIS: Anterior (Wendler) glottoplasty has become a popular surgery for voice feminization. However, there has been some discrepancy between its theoretical pitch-raising potential and what is actually achievable, and downsides to shortening the glottis have not been fully explored. In addition, descriptions of the surgery are inconsistent in their treatment of the vocal ligament. This study aimed to determine 1) how fundamental frequency (fo ) is expected to vary with length of anterior glottic fixation, 2) the impact of glottic shortening on sound pressure level (SPL), and 3) the effect of including the ligament in fixation. STUDY DESIGN: Computational simulation. METHODS: Voice production was simulated in a fiber-gel finite element computational model using canonical male vocal fold geometry incorporating a three-layer vocal fold composition (superficial lamina propria, vocal ligament, and thyroarytenoid muscle). Progressive anterior glottic fixation (0, 1/8, 2/8, 3/8, etc. up to 7/8 of membranous vocal fold length) was simulated. Outcome measures were fo , SPL, and glottal flow waveforms. RESULTS: fo increased from 110 Hz to 164 Hz when the anterior one-half vocal fold was fixed and continued to progressively rise with further fixation. SPL progressively decreased beyond 1/8 to 1/4 fixation. Inclusion of the vocal ligament in fixation did not further increase fo . Any fixation increased aperiodicity in the acoustic signal. CONCLUSIONS: The optimal length of fixation is a compromise between pitch elevation and reduction in output acoustic power. The simulation also provided a potential explanation for vocal roughness that is sometimes noted after anterior glottoplasty. LEVEL OF EVIDENCE: NA Laryngoscope, 131:1081-1087, 2021.

Analysis of Glottal Inverse Filtering in the Presence of Source-Filter Interaction.

The validity of glottal inverse filtering (GIF) to obtain a glottal flow waveform from radiated pressure signal in the presence and absence of source-filter interaction was studied systematically. A driven vocal fold surface model of vocal fold vibration was used to generate source signals. A one-dimensional wave reflection algorithm was used to solve for acoustic pressures in the vocal tract. Several test signals were generated with and without source-filter interaction at various fundamental frequencies and vowels. Linear Predictive Coding (LPC), Quasi Closed Phase (QCP), and Quadratic Programming (QPR) based algorithms, along with supraglottal impulse response, were used to inverse filter the radiated pressure signals to obtain the glottal flow pulses. The accuracy of each algorithm was tested for its recovery of maximum flow declination rate (MFDR), peak glottal flow, open phase ripple factor, closed phase ripple factor, and mean squared error. The algorithms were also tested for their absolute relative errors of the Normalized Amplitude Quotient, the Quasi-Open Quotient, and the Harmonic Richness Factor. The results indicated that the mean squared error decreased with increase in source-filter interaction level suggesting that the inverse filtering algorithms perform better in the presence of source-filter interaction. All glottal inverse filtering algorithms predicted the open phase ripple factor better than the closed phase ripple factor of a glottal flow waveform, irrespective of the source-filter interaction level. Major prediction errors occurred in the estimation of the closed phase ripple factor, MFDR, peak glottal flow, normalized amplitude quotient, and Quasi-Open Quotient. Feedback-related nonlinearity (source-filter interaction) affected the recovered signal primarily when f o was well below the first formant frequency of a vowel. The prediction error increased when f o was close to the first formant frequency due to the difficulty of estimating the precise value of resonance frequencies, which was exacerbated by nonlinear kinetic losses in the vocal tract.

Inertagrams for a Variety of Semi-Occluded Vocal Tracts.

Objective This investigation addressed the desirable source-airway interaction obtained with vocalization through a semi-occlusion at the mouth. The semi-occlusion was a flow-resistant tube of varying length and diameter. Method The methodology was strictly computational. Airway shapes resembling those obtained from magnetic resonance or computed tomography imaging were stylized. Supraglottal and subglottal acoustic impedances were calculated, from which inertagrams were plotted over a frequency range of 0-4000 Hz. The inertagrams predict which harmonics are likely to be strengthened. Results The combination of epilaryngeal airway narrowing and lengthening, a pharyngeal expansion, and an oral narrowing behind the tube produce the best overall inertagram for pitch glide exercises. This configuration supports harmonics of the source evenly over a range of 300-4000 Hz. Conclusions For clinical applications, theory predicts that the length of a tube or straw does not matter if the inner diameter is on the order of 3 mm or less. For wider open-ended tubes, greater length can compensate for the wider diameter for the beneficial inertance effect, but the desired steady pressure in the airways for vocal fold posturing cannot be maintained.

Vocal Loudness Variation With Spectral Slope.

Objective This investigation addresses the loudness variations in sones achievable with spectral slope variations (higher harmonic energy) in human vocalization and compares it to the sound pressure level (SPL) variations typically reported in the voice range profile (VRP). Method The primary methodology was computational. The ISO standard 226 was used to convert SPL values to sones for a 125- to 1000-Hz range of fundamental frequency and a -3 dB/octave to -12 dB/octave range of spectral slope. In addition, a retrospective analysis of human subjects' VRPs was conducted, and the experimental results were compared to the theoretical results. Results A very small range of SPL variation (less than 5 dB) in the VRP can produce a large range of loudness. The sensitivity can be on the order of 4 sones per dB SPL change. Conclusion For vocalization in the modal register, loudness variation is not well described by SPL change in dB, especially at high fundamental frequencies where the SPL range in the VRP becomes very small but sizeable loudness variations are still possible.

Effect of Vocal Fold Implant Placement on Depth of Vibration and Vocal Output.

OBJECTIVE: Most type 1 thyroplasty implants and some common injectable materials are mechanically stiff. Placing them close to the supple vocal fold mucosa can potentially dampen vibration and adversely impact phonation, yet this effect has not been systematically investigated. This study aims to examine the effect of implant depth on vocal fold vibration and vocal output. STUDY DESIGN: Computational simulation. METHODS: Voice production was simulated with a fiber-gel finite element computational model that incorporates a three-layer vocal fold composition (superficial lamina propria, vocal ligament, thyroarytenoid muscle). Implants of various depths were simulated, with a "deeper" or more medial implant positioned closer to the vocal fold mucosa and replacing more muscle elements. Trajectories of surface and within-tissue nodal points during vibration were produced. Outcome measures were the trajectory radii, fundamental frequency (F0 ), sound pressure level (SPL), and smoothed cepstral peak prominence (CPPS) as a function of implant depth. RESULTS: Amplitude of vibration at the vocal fold medial surface was reduced by an implant depth of as little as 14% of the total transverse vocal fold depth. Increase in F0 and decrease in CPPS were noted beyond 30% to 40% implant depth, and SPL decreased beyond 40% to 60% implant depth. CONCLUSIONS: Commonly used implants can dampen vibration "from a distance," ie, even without being immediately adjacent to vocal fold mucosa. Since implants are typically placed at depths examined in this study, stiff implants likely have a negative vocal impact in a subset of patients. Softer materials may be preferable, especially in bilateral medialization procedures. LEVEL OF EVIDENCE: N/A Laryngoscope, 130:2192-2198, 2020.