Articulation and vocal tract acoustics at soprano subject's high fundamental frequencies

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.

Articulatory and acoustic differences between lyric and dramatic singing in Western classical music

Within the realm of voice classification, singers could be sub-categorized by the weight of their repertoire, the so-called "singer's Fach." However, the opposite pole terms "lyric" and "dramatic" singing are not yet well defined by their acoustic and articulatory characteristics. Nine professional singers of different singers' Fach were asked to sing a diatonic scale on the vowel /a/, first in what the singers considered as lyric and second in what they considered as dramatic. Image recording was performed using real time magnetic resonance imaging (MRI) with 25 frames/s, and the audio signal was recorded via an optical microphone system. Analysis was performed with regard to sound pressure level (SPL), vibrato amplitude, and frequency and resonance frequencies as well as articulatory settings of the vocal tract. The analysis revealed three primary differences between dramatic and lyric singing: Dramatic singing was associated with greater SPL and greater vibrato amplitude and frequency as well as lower resonance frequencies. The higher SPL is an indication of voice source changes, and the lower resonance frequencies are probably caused by the lower larynx position. However, all these strategies showed a considerable individual variability. The singers' Fach might contribute to perceptual differences even for the same singer with regard to the respective repertoire.

Voice efficiency for different voice qualities combining experimentally derived sound signals and numerical modeling of the vocal tract

Purpose: Concerning voice efficiency considerations of different singing styles, from western classical singing to contemporary commercial music, only limited data is available to date. This single-subject study attempts to quantify the acoustic sound intensity within the human glottis depending on different vocal tract configurations and vocal fold vibration. Methods: Combining Finite-Element-Models derived from 3D-MRI data, audio recordings, and electroglottography (EGG) we analyzed vocal tract transfer functions, particle velocity and acoustic pressure at the glottis, and EGG-related quantities to evaluate voice efficiency at the glottal level and resonance characteristics of different voice qualities according to Estill Voice Training^®. Results: Voice qualities Opera and Belting represent highly efficient strategies but apply different vowel strategies and should thus be capable of predominate orchestral sounds. Twang and Belting use similar vowels, but the twang vocal tract configuration enabled the occurrence of anti-resonances and was associated with reduced vocal fold contact but still partially comparable energy transfer from the glottis to the vocal tract. Speech was associated with highly efficient glottal to vocal tract energy transfer, but with the absence of psychoactive strategies makes it more susceptible to noise interference. Falsetto and Sobbing apply less efficiently. Falsetto mainly due to its voice source characteristics, Sobbing due to energy loss in the vocal tract. Thus technical amplification might be appropriate here. Conclusion: Differences exist between voice qualities regarding the sound intensity, caused by different vocal tract morphologies and oscillation characteristics of the vocal folds. The combination of numerical analysis of geometries inside the human body and experimentally determined data outside sheds light on acoustical quantities at the glottal level.

Investigation of resonance strategies of high pitch singing sopranos using dynamic three-dimensional magnetic resonance imaging

Resonance-strategies with respect to vocal registers, i.e., frequency-ranges of uniform, demarcated voice quality, for the highest part of the female voice are still not completely understood. The first and second vocal tract resonances usually determine vowels. If the fundamental frequency exceeds the vowel-shaping resonance frequencies of speech, vocal tract resonances are tuned to voice source partials. It has not yet been clarified if such tuning is applicable for the entire voice-range, particularly for the top pitches. We investigated professional sopranos who regularly sing pitches above C6 (1047 Hz). Dynamic three-dimensional (3D) magnetic resonance imaging was used to calculate resonances for pitches from C5 (523 Hz) to C7 (2093 Hz) with different vowel configurations ([a:], [i:], [u:]), and different contexts (scales or octave jumps). A spectral analysis and an acoustic analysis of 3D-printed vocal tract models were conducted. The results suggest that there is no exclusive register-defining resonance-strategy. The intersection of fundamental frequency and first vocal tract resonance was not found to necessarily indicate a register shift. The articulators and the vocal tract resonances were either kept without significant adjustments, or the f_R1:f_o-tuning, wherein the first vocal tract resonance enhances the fundamental frequency, was applied until F6 (1396 Hz). An f_R2:f_o-tuning was not observed.

Are source-filter interactions detectable in classical singing during vowel glides?

In recent studies, it has been assumed that vocal tract formants (F_n) and the voice source could interact. However, there are only few studies analyzing this assumption in vivo. Here, the vowel transition /i/-/a/-/u/-/i/ of 12 professional classical singers (6 females, 6 males) when phonating on the pitch D4 [fundamental frequency (ƒ_o) ca. 294 Hz] were analyzed using transnasal high speed videoendoscopy (20.000 fps), electroglottography (EGG), and audio recordings. F_n data were calculated using a cepstral method. Source-filter interaction candidates (SFICs) were determined by (a) algorithmic detection of major intersections of F_n/nƒ_o and (b) perceptual assessment of the EGG signal. Although the open quotient showed some increase for the /i-a/ and /u-i/ transitions, there were no clear effects at the expected F_n/nƒ_o intersections. In contrast, ƒ_o adjustments and changes in the phonovibrogram occurred at perceptually derived SFICs, suggesting level-two interactions. In some cases, these were constituted by intersections between higher nƒ_o and F_n. The presented data partially corroborates that vowel transitions may result in level-two interactions also in professional singers. However, the lack of systematically detectable effects suggests either the absence of a strong interaction or existence of confounding factors, which may potentially counterbalance the level-two-interactions.

Effects of Nasalization on Vocal Tract Response Curve

BACKGROUND

Earlier studies have shown that nasalization affects the radiated spectrum by modifying the vocal tract transfer function in a complex manner.

METHODS

Here we study this phenomenon by measuring sine-sweep response of 3-D models of the vowels /u, a, ᴂ, i/, derived from volumetric MR imaging, coupled by means of tubes of different lengths and diameters to a 3-D model of a nasal tract.

RESULTS

The coupling introduced a dip into the vocal tract transfer function. The dip frequency was close to the main resonance of the nasal tract, a result in agreement with the Fujimura & Lindqvist in vivo sweep tone measurements [Fujimura & Lindqvist, 1972]. With increasing size of the coupling tube the depth of the dip increased and the first formant peak either changed in frequency or was split by the dip. Only marginal effects were observed of the paranasal sinuses. For certain coupling tube sizes, the spectrum balance was changed, boosting the formant peaks in the 2 - 4 kHz range.

CONCLUSION

A velopharyngeal opening introduces a dip in the transfer function at the main resonance of the nasal tract. Its depth increases with the area of the opening and its frequency rises in some vowels.

Printable 3D vocal tract shapes from MRI data and their acoustic and aerodynamic properties

A detailed understanding of how the acoustic patterns of speech sounds are generated by the complex 3D shapes of the vocal tract is a major goal in speech research. The Dresden Vocal Tract Dataset (DVTD) presented here contains geometric and (aero)acoustic data of the vocal tract of 22 German speech sounds (16 vowels, 5 fricatives, 1 lateral), each from one male and one female speaker. The data include the 3D Magnetic Resonance Imaging data of the vocal tracts, the corresponding 3D-printable and finite-element models, and their simulated and measured acoustic and aerodynamic properties. The dataset was evaluated in terms of the plausibility and the similarity of the resonance frequencies determined by the acoustic simulations and measurements, and in terms of the human identification rate of the vowels and fricatives synthesized by the artificially excited 3D-printed vocal tract models. According to both the acoustic and perceptual metrics, most models are accurate representations of the intended speech sounds and can be readily used for research and education.

Assessment of Tongue Position and Laryngeal Height in Two Professional Voice Populations

Purpose To advance our current knowledge of singer physiology by using ultrasonography in combination with acoustic measures to compare physiological differences between musical theater (MT) and opera (OP) singers under controlled phonation conditions. Primary objectives addressed in this study were (a) to determine if differences in hyolaryngeal and vocal fold contact dynamics occur between two professional voice populations (MT and OP) during singing tasks and (b) to determine if differences occur between MT and OP singers in oral configuration and associated acoustic resonance during singing tasks. Method Twenty-one singers (10 MT and 11 OP) were included. All participants were currently enrolled in a music program. Experimental procedures consisted of sustained phonation on the vowels /i/ and /ɑ/ during both a low-pitch task and a high-pitch task. Measures of hyolaryngeal elevation, tongue height, and tongue advancement were assessed using ultrasonography. Vocal fold contact dynamics were measured using electroglottography. Simultaneous acoustic recordings were obtained during all ultrasonography procedures for analysis of the first two formant frequencies. Results Significant oral configuration differences, reflected by measures of tongue height and tongue advancement, were seen between groups. Measures of acoustic resonance also showed significant differences between groups during specific tasks. Both singer groups significantly raised their hyoid position when singing high-pitched vowels, but hyoid elevation was not statistically different between groups. Likewise, vocal fold contact dynamics did not significantly differentiate the two singer groups. Conclusions These findings suggest that, under controlled phonation conditions, MT singers alter their oral configuration and achieve differing resultant formants as compared with OP singers. Because singers are at a high risk of developing a voice disorder, understanding how these two groups of singers adjust their vocal tract configuration during their specific singing genre may help to identify risky vocal behavior and provide a basis for prevention of voice disorders.

Imaging the Vocal Folds: A Feasibility Study on Strain Imaging and Elastography of Porcine Vocal Folds

Vocal folds are an essential part of human voice production. The biomechanical properties are a good indicator for pathological changes. In particular, as an oscillation system, changes in the biomechanical properties have an impact on the vibration behavior. Subsequently, those changes could lead to voice-related disturbances. However, no existing examination combines biomechanical properties and spatial imaging. Therefore, we propose an image registration-based approach, using ultrasound in order to gain this information synchronously. We used a quasi-static load to compress the tissue and measured the displacement by image registration. The strain distribution was directly calculated from the displacement field, whereas the elastic properties were estimated by a finite element model. In order to show the feasibility and reliability of the algorithm, we tested it on gelatin phantoms. Further, by examining ex vivo porcine vocal folds, we were able to show the practicability of the approach. We displayed the strain distribution in the tissue and the elastic properties of the vocal folds. The results were superimposed on the corresponding ultrasound images. The findings are promising and show the feasibility of the suggested approach. Possible applications are in improved diagnosis of voice disorders, by measuring the biomechanical properties of the vocal folds with ultrasound. The transducer will be placed on the vocal folds of the anesthetized patient, and the elastic properties will be measured. Further, the understanding of the vocal folds’ biomechanics and the voice forming process could benefit from it.

Magnetic resonance imaging-based measurement of internal deformation of vibrating vocal fold models

A method is presented for tracking the internal deformation of self-oscillating vocal fold models using magnetic resonance imaging (MRI). Silicone models scaled to four times life-size to lower the flow-induced vibration frequency were embedded with fiducial markers in a coronal plane. Candidate marker materials were tested using static specimens, and two materials, cupric sulfate and glass, were chosen for testing in the vibrating vocal fold models. The vibrating models were imaged using a gated MRI protocol wherein MRI acquisition was triggered using the subglottal pressure signal. Two-dimensional image slices at different phases during self-oscillation were captured, and in each phase the fiducial markers were clearly visible. The process was also demonstrated using a three-dimensional scan at two phases. The benefit of averaging to increase signal-to-noise ratio was explored. The results demonstrate the ability to use MRI to acquire quantitative deformation data that could be used, for example, to validate computational models of flow-induced vocal fold vibration and quantify deformation fields encountered by cells in bioreactor studies.

How to precisely measure the volume velocity transfer function of physical vocal tract models by external excitation

Recently, 3D printing has been increasingly used to create physical models of the vocal tract with geometries obtained from magnetic resonance imaging. These printed models allow measuring the vocal tract transfer function, which is not reliably possible in vivo for the vocal tract of living humans. The transfer functions enable the detailed examination of the acoustic effects of specific articulatory strategies in speaking and singing, and the validation of acoustic plane-wave models for realistic vocal tract geometries in articulatory speech synthesis. To measure the acoustic transfer function of 3D-printed models, two techniques have been described: (1) excitation of the models with a broadband sound source at the glottis and measurement of the sound pressure radiated from the lips, and (2) excitation of the models with an external source in front of the lips and measurement of the sound pressure inside the models at the glottal end. The former method is more frequently used and more intuitive due to its similarity to speech production. However, the latter method avoids the intricate problem of constructing a suitable broadband glottal source and is therefore more effective. It has been shown to yield a transfer function similar, but not exactly equal to the volume velocity transfer function between the glottis and the lips, which is usually used to characterize vocal tract acoustics. Here, we revisit this method and show both, theoretically and experimentally, how it can be extended to yield the precise volume velocity transfer function of the vocal tract.

Relevance of the Implementation of Teeth in Three-Dimensional Vocal Tract Models

PURPOSE

Recently, efforts have been made to investigate the vocal tract using magnetic resonance imaging (MRI). Due to technical limitations, teeth were omitted in many previous studies on vocal tract acoustics. However, the knowledge of how teeth influence vocal tract acoustics might be important in order to estimate the necessity of implementing teeth in vocal tract models. The aim of this study was therefore to estimate the effect of teeth on vocal tract acoustics.

METHOD

The acoustic properties of 18 solid (3-dimensional printed) vocal tract models without teeth were compared to the same 18 models including teeth in terms of resonance frequencies (fRn). The fRn were obtained from the transfer functions of these models excited by white noise at the glottis level. The models were derived from MRI data of 2 trained singers performing 3 different vowel conditions (/i/, /a/, and /u/) in speech and low-pitched and high-pitched singing.

RESULTS

Depending on the oral configuration, models exhibiting side cavities or side branches were characterized by major changes in the transfer function when teeth were implemented via the introduction of pole-zero pairs.

CONCLUSIONS

To avoid errors in modeling, teeth should be included in 3-dimensional vocal tract models for acoustic evaluation.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.5386771.

Vocal Tract Morphology in Inhaling Singing: Characteristics During Vowel Production-A Case Study in a Professional Singer

BACKGROUND

A professional singer produced various vowels on a comfortable loudness and pitch in an inspiratory and expiratory phonation manner. The present study investigates the morphological differences and tries to find a link with the acoustical characteristics.

OBJECTIVES/HYPOTHESIS

We hypothesize that features, constantly present over all vowels, characterize inhaling phonation and that the formant frequencies reflect the morphological findings.

STUDY DESIGN

A prospective case study was carried out.

METHODS

A female singer uttered the vowels /a/, /e/, /i/, /o/, and /u/ in a supine position under magnetic resonance imaging, on a comfortable loudness and pitch, in both inhaling and exhaling manner. The exact same parameters as in previous reports were measured (1-3). Acoustical analysis was performed with Praat.

RESULTS

Wilcoxon directional testing demonstrates a statistically significant difference in (1) the distance between the lips, (2) the antero-posterior tongue diameter, (3) the distance between the lips and the tip of the tongue, (4) the distance between the epiglottis and the posterior pharyngeal wall, (5) the narrowing of the subglottic space, and (6) the oropharyngeal and the hypopharyngeal areas. Acoustical analysis reveals slightly more noise and irregularity during reverse phonation. The central frequency of F0 and F1 is identical, whereas that of F2 and F3 increases, and that of F4 varies.

CONCLUSIONS

A smaller mouth opening, a narrowing of the subglottic space, a larger supralaryngeal inlet, and a smaller antero-posterior tongue diameter can be considered as morphological characteristics for reverse phonation. Acoustically, reverse phonation discretely contains more noise and perturbation. The formant frequency distribution concurs with a mouth narrowing and pharyngeal widening during inhaling.

Laryngeal evidence for the first and second passaggio in professionally trained sopranos

Introduction

Due to a lack of empirical data, the current understanding of the laryngeal mechanics in the passaggio regions (i.e., the fundamental frequency ranges where vocal registration events usually occur) of the female singing voice is still limited.

Material and methods

In this study the first and second passaggio regions of 10 professionally trained female classical soprano singers were analyzed. The sopranos performed pitch glides from A3 (ƒ_o = 220 Hz) to A4 (ƒ_o = 440 Hz) and from A4 (ƒ_o = 440 Hz) to A5 (ƒ_o = 880 Hz) on the vowel [iː]. Vocal fold vibration was assessed with trans-nasal high speed videoendoscopy at 20,000 fps, complemented by simultaneous electroglottographic (EGG) and acoustic recordings. Register breaks were perceptually rated by 12 voice experts. Voice stability was documented with the EGG-based sample entropy. Glottal opening and closing patterns during the passaggi were analyzed, supplemented with open quotient data extracted from the glottal area waveform.

Results

In both the first and the second passaggio, variations of vocal fold vibration patterns were found. Four distinct patterns emerged: smooth transitions with either increasing or decreasing durations of glottal closure, abrupt register transitions, and intermediate loss of vocal fold contact. Audible register transitions (in both the first and second passaggi) generally coincided with higher sample entropy values and higher open quotient variance through the respective passaggi.

Conclusions

Noteworthy vocal fold oscillatory registration events occur in both the first and the second passaggio even in professional sopranos. The respective transitions are hypothesized to be caused by either (a) a change of laryngeal biomechanical properties; or by (b) vocal tract resonance effects, constituting level 2 source-filter interactions.

One-second MRI of a three-dimensional vocal tract to measure dynamic articulator modifications

PURPOSE

To enable three-dimensional (3D) vocal tract imaging of dynamic singing or speech tasks at voxel sizes of 1.6 × 1.6 × 1.3 mm³ at 1.3 s per image.

MATERIALS AND METHODS

A Stack-of-Stars method was implemented and enhanced to allow for fast and efficient k-space sampling of the box-shaped vocal tract using a 3 Tesla MRI system. Images were reconstructed using an off-line image reconstruction using compressed sensing theory, leading to the abovementioned spatial and temporal resolutions. To validate spatial resolution, a phantom with holes of defined sizes was measured. The applicability of the imaging method was validated in an eight-subject study of amateur singers that were required to sustain phonation at a constant pitch, past their comfortable expiratory level. A segmentation of the vocal tract over all phonation time steps was done for one subject. Anatomical distances (larynx position and pharynx width) were calculated and compared for all subjects.

RESULTS

Analysis of the phantom study revealed that the imaging method could provide at least 1.6 mm isotropic resolution. Visual inspection of the segmented vocal tract during phonation showed modifications of the lips, tongue, and larynx position in all three dimensions. The mean larynx position per subject amounted to 52-85 mm, deviating up to 5% over phonation time. Parameter pharynx width was 32-181 mm² on average per subject, deviating up to 16% over phonation time. Visual inspection of the parameter course revealed no common compensation strategy for long sustained phonation.

CONCLUSION

The results of both phantom and in vivo measurements show the applicability of the fast 3D imaging method for voice research and indicate that modifications in all three dimensions can be observed and quantified.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:94-101.

Morphometric Differences of Vocal Tract Articulators in Different Loudness Conditions in Singing

Introduction

Dynamic MRI analysis of phonation has gathered interest in voice and speech physiology. However, there are limited data addressing the extent to which articulation is dependent on loudness.

Material and Methods

12 professional singer subjects of different voice classifications were analysed concerning the vocal tract profiles recorded with dynamic real-time MRI with 25fps in different pitch and loudness conditions. The subjects were asked to sing ascending scales on the vowel /a/ in three loudness conditions (comfortable = mf, very soft = pp, very loud = ff, respectively). Furthermore, fundamental frequency and sound pressure level were analysed from the simultaneously recorded optical audio signal after noise cancellation.

Results

The data show articulatory differences with respect to changes of both pitch and loudness. Here, lip opening and pharynx width were increased. While the vertical larynx position was rising with pitch it was lower for greater loudness. Especially, the lip opening and pharynx width were more strongly correlated with the sound pressure level than with pitch.

Conclusion

For the vowel /a/ loudness has an effect on articulation during singing which should be considered when articulatory vocal tract data are interpreted.