Marc Arnela

Effects of higher order propagation modes in vocal tract like geometries

In this paper, a multimodal theory accounting for higher order acoustical propagation modes is presented as an extension to the classical plane wave theory. This theoretical development is validated against experiments on vocal tract replicas, obtained using a 3D printer and finite element simulations. Simplified vocal tract geometries of increasing complexity are used to investigate the influence of some geometrical parameters on the acoustical properties of the vocal tract. It is shown that the higher order modes can produce additional resonances and anti-resonances and can also strongly affect the radiated sound. These effects appear to be dependent on the eccentricity and the cross-sectional shape of the geometries. Finally, the comparison between the simulations and the experiments points out the importance of taking visco-thermal losses into account to increase the accuracy of the resonance bandwidths prediction.

Effects of head geometry simplifications on acoustic radiation of vowel sounds based on time-domain finite-element simulations

One of the key effects to model in voice production is that of acoustic radiation of sound waves emanating from the mouth. The use of three-dimensional numerical simulations allows to naturally account for it, as well as to consider all geometrical head details, by extending the computational domain out of the vocal tract. Despite this advantage, many approximations to the head geometry are often performed for simplicity and impedance load models are still used as well to reduce the computational cost. In this work, the impact of some of these simplifications on radiation effects is examined for vowel production in the frequency range 0-10 kHz, by means of comparison with radiation from a realistic head. As a result, recommendations are given on their validity depending on whether high frequency energy (above 5 kHz) should be taken into account or not.

Finite element computation of elliptical vocal tract impedances using the two-microphone transfer function method

A two-microphone transfer function (TMTF) method is adapted to a numerical framework to compute the radiation and input impedances of three-dimensional vocal tracts of elliptical cross-section. In its simplest version, the TMTF method only requires measuring the acoustic pressure at two points in an impedance duct and the postprocessing of the corresponding transfer function. However, some considerations are to be taken into account when using the TMTF method in the numerical context, which constitute the main objective of this paper. In particular, the importance of including absorption at the impedance duct walls to avoid lengthy numerical simulations is discussed and analytical complex axial wave numbers for elliptical ducts are derived for this purpose. It is also shown how the direct impedance of plane wave propagation can be computed beyond the TMTF maximum threshold frequency by appropriate location of the virtual microphones. Virtual microphone spacing is also discussed on the basis of the so-called singularity factor. Numerical examples include the computation of the radiation impedance of vowels /a/, /i/, and /u/ and the input impedance of vowel /a/, for simplified vocal tracts of circular and elliptical cross-sections.

Two-dimensional vocal tracts with three-dimensional behavior in the numerical generation of vowels.

Two-dimensional (2D) numerical simulations of vocal tract acoustics may provide a good balance between the high quality of three-dimensional (3D) finite element approaches and the low computational cost of one-dimensional (1D) techniques. However, 2D models are usually generated by considering the 2D vocal tract as a midsagittal cut of a 3D version, i.e., using the same radius function, wall impedance, glottal flow, and radiation losses as in 3D, which leads to strong discrepancies in the resulting vocal tract transfer functions. In this work, a four step methodology is proposed to match the behavior of 2D simulations with that of 3D vocal tracts with circular cross-sections. First, the 2D vocal tract profile becomes modified to tune the formant locations. Second, the 2D wall impedance is adjusted to fit the formant bandwidths. Third, the 2D glottal flow gets scaled to recover 3D pressure levels. Fourth and last, the 2D radiation model is tuned to match the 3D model following an optimization process. The procedure is tested for vowels /a/, /i/, and /u/ and the obtained results are compared with those of a full 3D simulation, a conventional 2D approach, and a 1D chain matrix model.

Influence of vocal tract geometry simplifications on the numerical simulation of vowel sounds

For many years, the vocal tract shape has been approximated by one-dimensional (1D) area functions to study the production of voice. More recently, 3D approaches allow one to deal with the complex 3D vocal tract, although area-based 3D geometries of circular cross-section are still in use. However, little is known about the influence of performing such a simplification, and some alternatives may exist between these two extreme options. To this aim, several vocal tract geometry simplifications for vowels [ɑ], [i], and [u] are investigated in this work. Six cases are considered, consisting of realistic, elliptical, and circular cross-sections interpolated through a bent or straight midline. For frequencies below 4-5 kHz, the influence of bending and cross-sectional shape has been found weak, while above these values simplified bent vocal tracts with realistic cross-sections are necessary to correctly emulate higher-order mode propagation. To perform this study, the finite element method (FEM) has been used. FEM results have also been compared to a 3D multimodal method and to a classical 1D frequency domain model.

Using a Biomechanical Model and Articulatory Data for the Numerical Production of Vowels

We introduce a framework to study speech production using a biomechanical model of the human vocal tract, ArtiSynth. Electromagnetic articulography data was used as input to an inverse tracking sim ...

Reconstruction of Vocal Tract Geometries from Biomechanical Simulations: Vocal Tract Geometry

Abstract Medical imaging techniques are usually utilized to acquire the vocal tract geometry in 3D, which may then be used, eg, for acoustic/fluid simulation. As an alternative, such a geometry may also be acquired from a biomechanical simulation, which allows to alter the anatomy and/or articulation to study a variety of configurations. In a biomechanical model, each physical structure is described by its geometry and its properties (such as mass, stiffness, and muscles). In such a model, the vocal tract itself does not have an explicit representation, since it is a cavity rather than a physical structure. Instead, its geometry is defined implicitly by all the structures surrounding the cavity, and such an implicit representation may not be suitable for visualization or for acoustic/fluid simulation. In this work, we propose a method to reconstruct the vocal tract geometry at each time step during the biomechanical simulation. Complexity of the problem, which arises from model alignment artifacts, is addressed by the proposed method. In addition to the main cavity, other small cavities, including the piriform fossa, the sublingual cavity, and the interdental space, can be reconstructed. These cavities may appear or disappear by the position of the larynx, the mandible, and the tongue. To illustrate our method, various static and temporal geometries of the vocal tract are reconstructed and visualized. As a proof of concept, the reconstructed geometries of three cardinal vowels are further used in an acoustic simulation, and the corresponding transfer functions are derived.

Far field directivity of an omnidirectional parametric loudspeaker consisting of piezoelectric ultrasonic transducers set on a sphere

In this work, it is proposed to extend the convolution model for the far-field directivity of a parametric loudspeaker array (PLA) in [C. Shi and Y. Kajikawa, J. Acoust. Soc. Am. 137 (2) (2015)] to predict the far-field sound field generated by an omnidirectional parametric loudspeaker (OPL). The original two-dimensional model, intended for flat PLAs, relies on convolving the directivity of the primary waves with the Westervelt one, rather than on performing their product. This allows one to deal with piezoelectric ultrasonic transducers (PZTs) having large beam widths, typical of PLA applications. The model is herein enhanced to three dimensions, to predict the far-field pressure level of the difference wave at any observation point in space, for a PZT located and pointing anywhere. This makes the model amenable to compute the far-field directivity of PLAs on curved surfaces. In particular, it is shown how it can be applied to compute the far-field pressure of an OPL consisting of a spherical surface wit...

Finite element simulation of diphthongs in three-dimensional realistic vocal tracts with flexible walls

During the production of diphthongs, acoustic waves propagate along a time-varying three-dimensional (3D) vocal tract of complex geometry. The shape of the vocal tract walls does not only change because of the action of the articulators to produce a given sound, but also experience an elastic back reaction to the inner acoustic pressure. In this work the Finite Element Method (FEM) is used to simulate these phenomena. The mixed wave equation for the acoustic pressure and acoustic particle velocity expressed in an Arbitrary Lagrangian-Eulerian (ALE) frame of reference is solved to account for acoustic wave propagation in moving domains. The flexibility of walls is considered by solving a mass-damper-stiffness auxiliary equation for each boundary node. Dynamic vocal tract geometries are generated from the interpolation of static 3D vocal tract geometries of vowels, obtained from Magnetic Resonance Imaging (MRI). Some diphthong sounds are generated as examples