Sound generation in human phonation and the underlying fluid–structure–acoustic interaction that describes the sound production mechanism are not fully understood. A previous experimental study, with a silicone made vocal fold model connected to a straight vocal tract pipe of fixed length, showed that vibroacoustic coupling can cause a deviation in the vocal fold vibration frequency. This occurred when the fundamental frequency of the vocal fold motion was close to the lowest acoustic resonance frequency of the pipe. What is not fully understood is how the vibroacoustic coupling is influenced by a varying vocal tract length. Presuming that this effect is a pure coupling of the acoustical effects, a numerical simulation model is established based on the computation of the mechanical-acoustic eigenvalue. With varying pipe lengths, the lowest acoustic resonance frequency was adjusted in the experiments and so in the simulation setup. In doing so, the evolution of the vocal folds’ coupled eigenvalues and eigenmodes is investigated, which confirms the experimental findings. Finally, it was shown that for normal phonation conditions, the mechanical mode is the most efficient vibration pattern whenever the acoustic resonance of the pipe (lowest formant) is far away from the vocal folds’ vibration frequency. Whenever the lowest formant is slightly lower than the mechanical vocal fold eigenfrequency, the coupled vocal fold motion pattern at the formant frequency dominates.

In the human phonation process, acoustic standing waves in the vocal tract can influence the fluid flow through the glottis as well as vocal fold oscillation. To investigate the amount of acoustic back-coupling, the supraglottal flow field has been recorded via high-speed particle image velocimetry (PIV) in a synthetic larynx model for several configurations with different vocal tract lengths. Based on the obtained velocity fields, acoustic source terms were computed. Additionally, the sound radiation into the far field was recorded via microphone measurements and the vocal fold oscillation via high-speed camera recordings. The PIV measurements revealed that near a vocal tract resonance frequency f_R, the vocal fold oscillation frequency f_o (and therefore also the flow field’s fundamental frequency) jumps onto f_R. This is accompanied by a substantial relative increase in aeroacoustic sound generation efficiency. Furthermore, the measurements show that f_o-f_R-coupling increases vocal efficiency, signal-to-noise ratio, harmonics-to-noise ratio and cepstral peak prominence. At the same time, the glottal volume flow needed for stable vocal fold oscillation decreases strongly. All of this results in an improved voice quality and phonation efficiency so that a person phonating with f_o-f_R-coupling can phonate longer and with better voice quality.

The three-dimensional reconstruction of vocal folds in medicine usually involves endoscopy and an approach to extract depth information like structured light or stereo matching of images. The resulting mesh can accurately represent the superior area of the vocal folds, while new approaches also try to reconstruct the inferior area. We propose a novel approach to extract the time-dependent 3D geometry of the vocal fold from optical measurements on both the superior and inferior side, requiring optical measurements only from the superior side. First, a time-dependent, tri-variate surface velocity vector field is reconstructed using a high-speed camera and a laser Doppler vibrometer in an experimental environment. This vector field serves as target in an inverse finite-element simulation that optimizes the forces applied to a deformable vocal fold model such that the resulting movement after FEM simulation matches the velocity observations on the superior side. The required forces for the finite element method simulation are treated as unknowns and are assembled using multiple scalar fields. We use tensor products in Bézier Bernstein basis for our scalar fields to reduce the degrees of freedom for our optimization. We use gradient descent to optimize the control points of the force field polynomials. Our utilized error metric for gradient descent consists of two terms. The first term is used to match the simulated velocities to the observed measurements, while the second term measures the silhouette difference between observation and simulation.

In this work, we investigate the role of transient pressure waves in human voice production, which closely resemble the water hammer effect in hydraulic systems during valve closure. We study these waves by means of a synthetic larynx model in combination with simplified, straight vocal tract shapes and measure the spatial evolution of the frequency-dependent pressure fluctuations within the vocal tract. Applying a plane-wave fitting approach to the data allows to split the pressure data at the sensor locations into purely fluid-dynamical and purely acoustic components. The data shows that during vocal fold closure, negative pressure waves with amplitudes in the kPa range are generated periodically that propagate through the vocal tract with the speed of sound; multiple reflections at the open end and the vocal folds occur, shaping the sound spectrum in this process.The reflected sound waves interact with the vocal fold vibration, influencing sound generation even when the lowest vocal tract resonance remains well above the vocal fold oscillation frequency.A compressible one-dimensional flow solver, as commonly used in water hammer studies, accurately predicts sound generation during human phonation when the glottal area waveform is prescribed. The present results suggest that water hammer theory provides a useful framework for understanding voice production and for interpreting the origin of the transient pressure waves within the vocal tract. This raises the question of how specific vocal tract shapes alter wave propagation within the vocal tract and, consequently, wave-source interaction.

In this work, a new methodology is proposed for measuring three-dimensional (3D) structural surface vibrations. It is based on employing a combination of a high-speed camera and a Laser-Scanning-Vibrometer on a common optical axis, i.e. in a monoscopic setup. The advantage of this method in comparison to established methods like 3D Digital Image Correlation or 3D Laser-Scanning-Vibrometry is the fact that optical access to the vibrating surface is only needed from one viewing angle instead of two or three. This facilitates 3D vibration measurements in cases where optical access is limited. The high-speed camera in combination with an optical flow algorithm yields in-plane vibration velocities, while the Laser-Scanning-Vibrometer measures the out-of-plane vibration component. Interpolation on a common grid leads to 3D vibrational velocities. To obtain 3D displacements, a particle advection scheme on the measured velocities is applied. We validate the method on the case of a synthetic vocal fold vibration by comparing it to a stereo marker tracking approach. Both methods show generally good agreement in oscillation amplitudes and phase of vibration velocity and displacement (on average 5.2\% deviation in the amplitude and <2\% in the phase), revealing that the monoscopic method can be used to quantify 3D vibration with good accuracy. We apply the method to another case of synthetic vocal fold vibration with an attached vocal tract, where it enables us to perform 3D vibration measurements under strong constraints regarding the optical accessibility of the vibrating surface.