马铁菊头蝠声道声学特性的数值分析

采用有限元数值计算得到了马铁菊头蝠声道内部的声场分布,给出了马铁菊头蝠声道内几种特殊的腔体结构在蝙蝠发声过程中的作用。通过微型CT扫描并经过三维重构得到了马铁菊头蝠声道的三维立体模型用于有限元数值计算,通过在声门处放置单位声源计算得到了整个声道内部以及鼻孔周围的声压分布。结果表明,马铁菊头蝠声道包含了鼻腔结构后声波在声门上方的声压幅度明显大于不含鼻腔结构的情况,从传输曲线来看,声门上方鼻腔的存在使得系统对声波传输在二次谐波频率处呈现低阻抗效果,同时鼻腔的改变还可影响二次谐波的位置。而声门下方的气管空腔主要影响声波的背向转播,声门下方的气管空腔的存在可明显降低蝙蝠发声时声场在声道声门下方的声压幅度,同时抑制声音背向传播时二次谐波成分的强度。     

马铁菊头蝠声道声学特性的数值分析

采用有限元数值计算得到了马铁菊头蝠声道内部的声场分布,给出了马铁菊头蝠声道内几种特殊的腔体结构在蝙蝠发声过程中的作用。通过微型CT扫描并经过三维重构得到了马铁菊头蝠声道的三维立体模型用于有限元数值计算,通过在声门处放置单位声源计算得到了整个声道内部以及鼻孔周围的声压分布。结果表明,马铁菊头蝠声道包含了鼻腔结构后声波在声门上方的声压幅度明显大于不含鼻腔结构的情况,从传输曲线来看,声门上方鼻腔的存在使得系统对声波传输在二次谐波频率处呈现低阻抗效果,同时鼻腔的改变还可影响二次谐波的位置。而声门下方的气管空腔主要影响声波的背向转播,声门下方的气管空腔的存在可明显降低蝙蝠发声时声场在声道声门下方的声压幅度,同时抑制声音背向传播时二次谐波成分的强度。     

Mathematical model of acoustic speech production with mobile walls of the vocal tract

A mathematical speech production model is considered that describes acoustic oscillation propagation in a vocal tract with mobile walls. The wave field function satisfies the Helmholtz equation with boundary conditions of the third kind (impedance type). The impedance mode corresponds to a threeparameter pendulum oscillation model. The experimental research demonstrates the nonlinear character of how the mobility of the vocal tract walls influence the spectral envelope of a speech signal.     

A comparison of vocal tract perturbation patterns based on statistical and acoustic considerations

The purpose of this study was to investigate the relation between vocal tract deformation patterns obtained from statistical analyses of a set of area functions representative of a vowel repertoire, and the acoustic properties of a neutral vocal tract shape. Acoustic sensitivity functions were calculated for a mean area function based on seven different speakers. Specific linear combinations of the sensitivity functions corresponding to the first two formant frequencies were shown to possess essentially the same amplitude variation along the vocal tract length as the statistically derived deformation patterns reported in previous studies.     

Technique for "tuning" vocal tract area functions based on acoustic sensitivity functions

A technique for modifying vocal tract area functions is developed by using sum and difference combinations of acoustic sensitivity functions to perturb an initial vocal tract configuration. First, sensitivity functions [e.g., Fant and Pauli, Proc. Speech Comm. Sem. 74, 1975] are calculated for a given area function, at its specific formant frequencies. The sensitivity functions are then multiplied by scaling coefficients that are determined from the difference between a desired set of formant frequencies and those supported by the current area function. The scaled sensitivity functions are then summed together to generate a perturbation of the area function. This produces a new area function whose associated formant frequencies are closer to the desired values than the previous one. This process is repeated iteratively until the coefficients are equal to zero or are below a threshold value.     

A methodological and preliminary study on the acoustic effect of a trumpet player's vocal tract

A methodological study is presented to examine the acoustic role of the vocal tract in playing the trumpet. Preliminary results obtained for one professional player are also shown to demonstrate the effectiveness of the method. Images of the vocal tract with a resolution of 0.5 mm (2 mm in thickness) were recorded with magnetic resonance imaging to observe the tongue posture and estimate the vocal-tract area function during actual performance. The input impedance was then calculated for the player's air column including both the supra- and subglottal tracts using an acoustic tube model including the effect of wall losses. Finally, a time-domain blowing simulation by Adachi and Sato [J. Acoust. Soc. Am. 99, 1200-1209 (1996)] was performed with a model of the lips. In this simulation, the oscillating frequency of the lips was slightly affected by using different shapes of the vocal tract measured for the player. In particular, when the natural frequency of the lips was gradually increased, the transition to the higher mode occurred at different frequencies for different vocal-tract shapes. Furthermore, simulation results showed that the minimum blowing pressure required to attain the lip oscillation can be reduced by adjusting the vocal-tract shape properly.     

Longitudinal mode acoustic losses in short nozzles

This paper investigates and reviews the state of knowledge on the damping of combustion instability oscillations by short nozzles. The wave energy flux through the nozzle entrance is determined and the relationship between this energy flux and engine stability is discussed. Nozzle decay coefficients are predicted and compared with available experimental data.     

Investigating acoustic correlates of human vocal fold vibratory phase asymmetry through modeling and laryngeal high-speed videoendoscopy

Vocal fold vibratory asymmetry is often associated with inefficient sound production through its impact on source spectral tilt. This association is investigated in both a computational voice production model and a group of 47 human subjects. The model provides indirect control over the degree of left-right phase asymmetry within a nonlinear source-filter framework, and high-speed videoendoscopy provides in vivo measures of vocal fold vibratory asymmetry. Source spectral tilt measures are estimated from the inverse-filtered spectrum of the simulated and recorded radiated acoustic pressure. As expected, model simulations indicate that increasing left-right phase asymmetry induces steeper spectral tilt. Subject data, however, reveal that none of the vibratory asymmetry measures correlates with spectral tilt measures. Probing further into physiological correlates of spectral tilt that might be affected by asymmetry, the glottal area waveform is parameterized to obtain measures of the open phase (open/plateau quotient) and closing phase (speed/closing quotient). Subjects' left-right phase asymmetry exhibits low, but statistically significant, correlations with speed quotient (r=0.45) and closing quotient (r=-0.39). Results call for future studies into the effect of asymmetric vocal fold vibration on glottal airflow and the associated impact on voice source spectral properties and vocal efficiency.     

Controlling changes in vocal tract resistance.

There is some evidence that speech aerodynamics follows the rules of a regulating system. The purpose of the present study was to assess how the speech system manages perturbations that produce "errors" within the system. Three experimental approaches were used to evaluate the physiological responses to an imposed change in airway resistance. The first involved subjects with varying degrees of velopharyngeal inadequacy. The second and third approaches involved noncleft subjects whose airway was perturbed by bleed valves and bite blocks during consonant productions. The pressure-flow technique was used to measure aerodynamic variables associated with the production of test consonants. The results of this study provide additional evidence that the speech system actively responds to perturbations in ways that tend to minimize a change in consonant speech pressures. The degree of success in stabilizing pressures appears to reflect the capability of the system to use whatever articulatory and respiratory responses are available.     

Resonances of a bent vocal tract

Analyses of wave propagation in a vocal tract invariably assume the tract to be a variable area tube with a straight axis. No estimates appear to be available of the effect of curvature of the tract on its resonances (or, equivalently, on its transfer function). For arbitrarily varying cross section, the computation is difficult. However, one can choose an idealized bent vocal tract with a uniform cross section for which the wave equation is separable. The resonance frequencies for such a bent tract are computed and compared to those of the corresponding straight tract. The comparison shows that for typical dimensions of the tract the shift in the resonance frequencies below 4 kHz is in the range of 2%-8%.     

Analysis of vocal tract parameters in parkinsonian speech

A method for the analysis of vocal tract parameters is developed, aimed to perform quantitative analysis of rigidity from speech signals of Parkinsonian patients. The cross-sectional area function of the vocal tract is calculated using pitch synchronous autoregressive moving average (ARMA) analysis. The changes in Parkinsonian subjects of the cross-sectional area during the utterance of sustained sounds are attributed to both Parkinsonian tremor and rigidity. In order to isolate the effects of the rigidity on the vocal tract from those of the tremor, an adaptive tremor cancellation (ATC) algorithm is developed, based on the correlation of tremor signals extracted from different locations of the speech production system.     

Mammalian laryngseal air sacs add variability to the vocal tract impedance: physical and computational modeling

Cavities branching off the main vocal tract are ubiquitous in nonhumans. Mammalian air sacs exist in human relatives, including all four great apes, but only a substantially reduced version exists in humans. The present paper focuses on acoustical functions of the air sacs. The hypotheses are investigated on whether the air sacs affect amplitude of utterances and/or position of formants. A multilayer synthetic model of the vocal folds coupled with a vocal tract model was utilized. As an air sac model, four configurations were considered: open and closed uniform tube-like side branches, a rigid cavity, and an inflatable cavity. Results suggest that some air sac configurations can enhance the sound level. Furthermore, an air sac model introduces one or more additional resonance frequencies, shifting formants of the main vocal tract to some extent but not as strongly as previously suggested. In addition, dynamic range of vocalization can be extended by the air sacs. A new finding is also an increased variability of the vocal tract impedance, leading to strong nonlinear source-filter interaction effects. The experiments demonstrated that air-sac-like structures can destabilize the sound source. The results were validated by a transmission line computational model.     

Fresnel approximations for acoustic fields of rectangularly symmetric sources

A general approach is presented for determining the acoustic fields of rectangularly symmetric, baffled, time-harmonic sources under the Fresnel approximation. This approach is applicable to a variety of separable source configurations, including uniform, exponential, Gaussian, sinusoidal, and error function surface velocity distributions, with and without focusing in either surface dimension. In each case, the radiated field is given by a formula similar to that for a uniform rectangular source, except for additional scaling of wave number and azimuthal distance parameters. The expressions presented are generalized to three different Fresnel approximations that correspond, respectively, to diffracted plane waves, diffracted spherical waves, or diffracted cylindrical waves. Numerical results, for several source geometries relevant to ultrasonic applications, show that these expressions accurately depict the radiated pressure fields, except for points very near the radiating aperture. Highest accuracy near the source is obtained by choice of the Fresnel approximation most suited to the source geometry, while the highest accuracy far from the source is obtained by the approximation corresponding to diffracted spherical waves. The methods are suitable for volumetric computations of acoustic fields including focusing, apodization, and attenuation effects.     

On the validity of some new acoustic scattering approximations

Two new approximations for predicting the elastic scattering of plane acoustic waves by a weak scatterer are proposed. The approximations have been obtained by drawing an analogy between acoustic and light scattering problems. The validity of these approximations has been examined numerically for the exactly soluble case of scattering by a homogeneous sphere. Results show that for small angle scattering the proposed approximations have a considerably larger domain of validity in comparison to the extensively used Born approximation.     

On the validity of some new acoustic scattering approximations

Abstract

Two new approximations for predicting the elastic scattering of plane acoustic waves by a weak scatterer are proposed. The approximations have been obtained by drawing an analogy between acoustic and light scattering problems. The validity of these approximations has been examined numerically for the exactly soluble case of scattering by a homogeneous sphere. Results show that for small angle scattering the proposed approximations have a considerably larger domain of validity in comparison to the extensively used Born approximation.     

Sound-absorbing media with two types of acoustic losses

Sound-absorbing media with two types of acoustic losses, namely, inertial and deformation are considered. Such a medium is described by a complex density and a complex compressibility. It is shown that a certain relation between the latter quantities imparts some specific properties to the medium: a real wave impedance, complete absorption of sound waves at the boundary with the external medium, the properties of a viscous or “superviscous” medium, etc. The results are generalized to the case of a layered inhomogeneous medium with two types of losses.     

Comparison of vocal tract formants in singing andnonperiodic phonation

The skilled use of nonperiodic phonation techniques in combination with spectrum analysis has been proposed here as a practical method for locating formant frequencies in the singing voice. The study addresses the question of the degree of similarity between sung phonations and their nonperiodic imitations, with respect to both frequency of the first two formants as well as posture of the vocal tract. Using magnetic resonance imaging (MRI), linear predictive coding (LPC), and spectrum analysis, two types of nonperiodic phonation (ingressive and vocal fry) are compared with singing phonations to determine the degree of similarity/difference in acoustic and spatial dimensions of the vocal tract when these phonation types are used to approximate the postures of singing. In comparing phonation types, the close similarity in acoustic data in combination with the relative dissimilarity in spatial data indicates that the accurate imitations are not primarily the result of imitating the singing postures, but have instead an aural basis.     

Effect of artificially lengthened vocal tract on vocal fold oscillation's fundamental frequency.

The fundamental frequency of vocal fold oscillation (F(0)) is controlled by laryngeal mechanics and aerodynamic properties. F(0) change per unit change of transglottal pressure (dF/dP) using a shutter valve has been studied and found to have nonlinear, V-shaped relationship with F(0). On the other hand, the vocal tract is also known to affect vocal fold oscillation. This study examined the effect of artificially lengthened vocal tract length on dF/dP. dF/dP was measured in six men using two mouthpieces of different lengths. Results: The dF/dP graph for the longer vocal tract was shifted leftward relative to the shorter one. Conclusion: Using the one-mass model, the nadir of the "V" on the dF/dP graph was strongly influenced by the resonance around the first formant frequency. However, a more precise model is needed to account for the effects of viscosity and turbulence.