Estimation of Source-Filter Interaction Regions Based on Electroglottography

Female Classical Singers Apply Different Resonatory Strategies Than Male: Insights from an MRI-Based Analysis

INTRODUCTION

The morphology of the vocal tract plays a crucial role in singing. Adjustments of the lower part of the vocal tract are essential for voice quality and timbre. Structured investigations of this region are challenging due to the small extent of the morphological modifications.

MATERIAL AND METHODS

This study analyzed the morphology of the endolaryngeal tube and parts of the surrounding hypopharynx for the sustained vowels /a/, /e/, /i/, //, and // by 11 female singer subjects who were at the beginning of their academic singing studies. As in a previous study with male subjects, analysis was based on two phonatory conditions: a natural, speech-like phonation and a singing phonation, like in classical singing. By means of 3D image processing from magnetic resonance imaging, multiple measures were derived. The data were statistically analyzed using linear mixed effects models and ANOVA.

RESULTS

The female singers enlarged the ratio of the endolaryngeal tube to the adjacent hypopharynx for singing.

DISCUSSION

In contrast to the findings of the male singers in a previous study, the ratios of the laryngeal volumes to the neighboring hypopharynx increased while singing. The female singer subjects of the present study increased those ratios, mediated by an over-proportional enlargement of the endolaryngeal tube. Existing acoustic differences to male singers are thus given a morphological basis.

CONCLUSION

The study gives insight into the characteristic adjustments of the vocal tract of female classical singers. The resonatory strategies for females in classical singing differ from those found in males.

Estimating Formant Frequencies of Vowels Sung by Sopranos Using Weighted Linear Prediction

This study introduces the weighted linear prediction adapted to high-pitched singing voices (WLP-HPSV) method for accurately estimating formant frequencies of vowels sung by lyric sopranos. The WLP-HPSV method employs a variant of the WLP analysis combined with the zero-frequency filtering (ZFF) technique to address specific challenges in formant estimation from singing signals. Evaluation of the WLP-HPSV method compared to the LPC method demonstrated its superior performance in accurately capturing the spectral characteristics of synthetic /u/ vowels and the /a/ and /u/ natural singing vowels. The QCP parameters used in the WLP-HPSV method varied with pitch, revealing insights into the interplay between the vocal tract and glottal characteristics during vowel production. The comparison between the LPC and WLP-HPSV methods highlighted the robustness of the WLP-HPSV method in accurately estimating formant frequencies across different pitches.

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.

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.

Experimental study on nonlinear source-filter interaction using synthetic vocal fold models

Under certain conditions, e.g., singing voice, the fundamental frequency of the vocal folds can go up and interfere with the formant frequencies. Acoustic feedback from the vocal tract filter to the vocal fold source then becomes strong and non-negligible. An experimental study was presented on such source-filter interaction using three types of synthetic vocal fold models. Asymmetry was also created between the left and right vocal folds. The experiment reproduced various nonlinear phenomena, such as frequency jump and quenching, as reported in humans. Increase in phonation threshold pressure was also observed when resonant frequency of the vocal tract and fundamental frequency of the vocal folds crossed each other. As a combined effect, the phonation threshold pressure was further increased by the left-right asymmetry. Simulation of the asymmetric two-mass model reproduced the experiments to some extent. One of the intriguing findings of this study is the variable strength of the source-filter interaction over different model types. Among the three models, two models were strongly influenced by the vocal tract, while no clear effect of the vocal tract was observed in the other model. This implies that the level of source-filter interaction may vary considerably from one subject to another in humans.

The evolution of the syrinx: An acoustic theory

The unique avian vocal organ, the syrinx, is located at the caudal end of the trachea. Although a larynx is also present at the opposite end, birds phonate only with the syrinx. Why only birds evolved a novel sound source at this location remains unknown, and hypotheses about its origin are largely untested. Here, we test the hypothesis that the syrinx constitutes a biomechanical advantage for sound production over the larynx with combined theoretical and experimental approaches. We investigated whether the position of a sound source within the respiratory tract affects acoustic features of the vocal output, including fundamental frequency and efficiency of conversion from aerodynamic energy to sound. Theoretical data and measurements in three bird species suggest that sound frequency is influenced by the interaction between sound source and vocal tract. A physical model and a computational simulation also indicate that a sound source in a syringeal position produces sound with greater efficiency. Interestingly, the interactions between sound source and vocal tract differed between species, suggesting that the syringeal sound source is optimized for its position in the respiratory tract. These results provide compelling evidence that strong selective pressures for high vocal efficiency may have been a major driving force in the evolution of the syrinx. The longer trachea of birds compared to other tetrapods made them likely predisposed for the evolution of a syrinx. A long vocal tract downstream from the sound source improves efficiency by facilitating the tuning between fundamental frequency and the first vocal tract resonance.