On the role of glottis-interior sources in the production of voiced sound

The voice source is dominated by aeroacoustic sources downstream of the glottis. In this paper an investigation is made of the contribution to voiced speech of secondary sources within the glottis. The acoustic waveform is ultimately determined by the volume velocity of air at the glottis, which is controlled by vocal fold vibration, pressure forcing from the lungs, and unsteady backreactions from the sound and from the supraglottal air jet. The theory of aerodynamic sound is applied to study the influence on the fine details of the acoustic waveform of "potential flow" added-mass-type glottal sources, glottis friction, and vorticity either in the glottis-wall boundary layer or in the portion of the free jet shear layer within the glottis. These sources govern predominantly the high frequency content of the sound when the glottis is near closure. A detailed analysis performed for a canonical, cylindrical glottis of rectangular cross section indicates that glottis-interior boundary/shear layer vortex sources and the surface frictional source are of comparable importance; the influence of the potential flow source is about an order of magnitude smaller.     

Experimental verification of the quasi-steady approximation for aerodynamic sound generation by pulsating jets in tubes

Voice production involves sound generation by a confined jet flow through an orifice (the glottis) with a time-varying area. Predictive models of speech production are usually based on the so-called quasi-steady approximation. The flow rate through the time-varying orifice is assumed to be the same as a sequence of steady flows through stationary orifices for wall geometries and flow boundary conditions that instantaneously match those of the dynamic, nonstationary problem. Either the flow rate or the pressure drop can then be used to calculate the radiated sound using conventional acoustic radiation models. The quasi-steady approximation allows complex unsteady flows to be modeled as steady flows, which is more cost effective. It has been verified for pulsating open jet flows. The quasi-steady approximation, however, has not yet been rigorously validated for the full range of flows encountered in voice production. To further investigate the range of validity of the quasi-steady approximation for voice production applications, a dynamic mechanical model of the larynx was designed and built. The model dimensions approximated those of human vocal folds. Airflow was supplied by a pressurized, quiet air storage facility and modulated by a driven rubber orifice. The acoustic pressure of waves radiated upstream and downstream of the orifice was measured, along with the orifice area and other time-averaged flow variables. Calculated and measured radiated acoustic pressures were compared. A good agreement was obtained over a range of operating frequencies, flow rates, and orifice shapes, confirming the validity of the quasi-steady approximation for a class of relevant pulsating jet flows.     

Broadband sound generation by confined turbulent jets

Sound generation by confined stationary jets is of interest to the study of voice and speech production, among other applications. The generation of sound by low Mach number, confined, stationary circular jets was investigated. Experiments were performed using a quiet flow supply, muffler-terminated rigid uniform tubes, and acrylic orifice plates. A spectral decomposition method based on a linear source-filter model was used to decompose radiated nondimensional sound pressure spectra measured for various gas mixtures and mean flow velocities into the product of (1) a source spectral distribution function; (2) a function accounting for near field effects and radiation efficiency; and (3) an acoustic frequency response function. The acoustic frequency response function agreed, as expected, with the transfer function between the radiated acoustic pressure at one fixed location and the strength of an equivalent velocity source located at the orifice. The radiation efficiency function indicated a radiation efficiency of the order (kD)2 over the planar wave frequency range and (kD)4 at higher frequencies, where k is the wavenumber and D is the tube cross sectional dimension. This is consistent with theoretical predictions for the planar wave radiation efficiency of quadrupole sources in uniform rigid anechoic tubes. The effects of the Reynolds number, Re, on the source spectral distribution function were found to be insignificant over the range 20002.5. The influence of a reflective open tube termination on the source function spectral distribution was found to be insignificant, confirming the absence of a feedback mechanism.     

Broadband sound generation by confined pulsating jets in a mechanical model of the human larynx

Experiments were performed to study the production of broadband sound in confined pulsating jets through orifices with a time-varying area. The goal was to better understand broadband sound generation at the human glottis during voicing. The broadband component was extracted from measured sound signals by the elimination of the periodic component through ensemble averaging. Comparisons were made between the probability density functions of the broadband sound in pulsating jets and of comparable stationary jets. The results indicate that the quasi-steady approximation may be valid for the broadband component when the turbulence is well established and the turbulence kinetic energy is comparatively large. A wavelet analysis of the broadband sound showed that random sound production was modulated at the driving frequency. Two distinct sound production peaks were observed during one cycle, presumably associated firstly with jet formation and secondly with flow deceleration during orifice closing. Most high-frequency sound was produced during the closing phase. Deviations from quasi-steady behavior were observed. As the driving frequency increased, sound production during the opening phase was reduced, possibly due to the shorter time available for turbulence to develop. These results may be useful for better quality voice synthesis.     

Vocal tract resonances and the sound of the Australian didjeridu (yidaki) I. experiment

The didjeridu, or yidaki, is a simple tube about 1.5 m long, played with the lips, as in a tuba, but mostly producing just a tonal, rhythmic drone sound. The acoustic impedance spectra of performers' vocal tracts were measured while they played and compared with the radiated sound spectra. When the tongue is close to the hard palate, the vocal tract impedance has several maxima in the range 1-3 kHz. These maxima, if sufficiently large, produce minima in the spectral envelope of the sound because the corresponding frequency components of acoustic current in the flow entering the instrument are small. In the ranges between the impedance maxima, the lower impedance of the tract allows relatively large acoustic current components that correspond to strong formants in the radiated sound. Broad, weak formants can also be observed when groups of even or odd harmonics coincide with bore resonances. Schlieren photographs of the jet entering the instrument and high speed video images of the player's lips show that the lips are closed for about half of each cycle, thus generating high levels of upper harmonics of the lip frequency. Examples of the spectra of "circular breathing" and combined playing and vocalization are shown.     

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。      

An aeroacoustic approach to phonation

A fluid mechanical, or aeroacoustic, point of view is followed to study possible sources of sound during phonation. Concentration is on two features of the vocal tract during phonation: abrupt area change from the glottis to the vocal tract and the finite length of the vocal tract. With these features, a source of sound distinct from the volume velocity source can be identified and a preliminary account of its effect on the acoustic field given. This source of sound is an oscillating force resulting from an interaction of rotational fluid motion with itself. Because of the schematic nature of the geometry of the model used here, this source may be considerably modified in actual phonation. It is concluded that specification of volume velocity is not enough to specify the source during phonation, even neglecting source-tract interaction.     

Experimental investigation of the influence of a posterior gap on glottal flow and sound

The influence of a posterior gap on the airflow through the human glottis was investigated using a driven synthetic model. Instantaneous orifice discharge coefficient of a glottal shaped orifice was obtained from the time-varying orifice area and the velocity distribution of the pulsated jet measured on the axial plane using a single hot-wire probe. Instantaneous orifice discharge coefficient values were found to undergo a cyclic hysteresis loop when plotted versus Reynolds number and time, indicating a pressure head increase and a net energy transfer from the air flow to the orifice wall. The net energy transferred was estimated to be around 10% of the value presumably required to achieve self-sustained oscillation. The radiated sound pressure was measured to characterize the influence of the minimum flow through the posterior gap on the broadband component of the radiated sound. The presence of a posterior gap was found to significantly increase the broadband sound level produced over the frequency range in which human hearing is most sensitive.     

A numerical study on the flow and sound fields of centrifugal impeller located near a wedge

Centrifugal fans are widely used and the noise generated by these machines causes one of the serious problems. In general, the centrifugal fan noise is often dominated by tones at blade passage frequency and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cut-off in the casing. However, only a few researches have been carried out on predicting the noise because of the difficulty in obtaining detailed information about the flow field and considering the scattering effect of the casing. The objective of this study is to understand the generation mechanism of sound and to develop a prediction method for the unsteady flow field and the acoustic pressure field of the centrifugal impeller. A discrete vortex method is used to model the centrifugal impeller and a wedge and to calculate the flow field. The force of each element on the blade is calculated by the unsteady Bernoulli equation. Lowson's method is used to predict the acoustic source. In order to consider the scattering and diffraction effects of the casing, Kirchhoff-Helmholtz boundary element method (BEM) is developed. The source of Kirchhoff-Helmholtz BEM is newly developed, so the sound field of the centrifugal fan can be obtained. A centrifugal impeller and wedge are used in the numerical calculation and the results are compared with the experimental data. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. The radiated acoustic field shows the diffraction and scattering effect of the wedge.     

Prediction of intake noise of an automotive engine in run-up condition

It is very important to predict the radiated noise from the engine intake system for the effective noise control and virtual prototyping of in-cavity and outdoor noise of a vehicle. To this end, one should precisely measure the in-duct acoustic source parameters of the intake system, viz., source strength and source impedance. Usually, the noise radiation characteristics need to be expressed as a function of engine speed. In this study, acoustic source parameters of an engine intake system under engine run-up condition were measured by using the direct method. Direct method employed two external loudspeakers, turned on simultaneously, and three microphones for the separation of upstream and downstream wave components. It was noted that the frequency spectra of source impedance hardly changes with the increase of engine speed. Utilizing this fact, source strength under the engine run-up condition was calculated by assuming invariant source impedance. Predicted insertion loss and radiated sound pressure level using the measured source parameters were compared with those of measured data and predicted data using several idealized source models, which have been adopted for the calculations. A reasonably good agreement was observed between measured sound spectra at the intake orifice and predicted one using the measured source data. It was shown that the source data obtained by the present method yielded a far better prediction accuracy than those by the idealized source models.     

Sound field separation with a double layer velocity transducer array (L)

In near-field acoustic holography sound field separation techniques make it possible to distinguish between sound coming from the two sides of the array. This is useful in cases where the sources are not confined to only one side of the array, e.g., in the presence of additional sources or reflections from the other side. This paper examines a separation technique based on measurement of the particle velocity in two closely spaced parallel planes. The purpose of the technique is to recover the particle velocity radiated by a source in the presence of disturbing sound from the opposite side of the array. The technique has been examined and compared with direct velocity based reconstruction, as well as with a technique based on the measurement of the sound pressure and particle velocity. The double layer velocity method circumvents some of the drawbacks of the pressure-velocity based reconstruction, and it can successfully recover the normal velocity radiated by the source, even in the presence of strong disturbing sound.     

On the Mach number and temperature dependence of jet noise: Results from a simplified numerical model

Numerical simulations of sound radiation from perturbed round jets are used, firstly to explore the structure of the sound sources and then to carry out a parametric study of the effect of jet Mach number and jet temperature. The simplified model problem includes a steady base jet flow, maintained in the absence of disturbances, superimposed with instability waves that are free to interact nonlinearly. Simulations over a range of subsonic jet Mach numbers show that a nonlinear mechanism dominates over a linear mechanism for low-frequency sound radiation, while for supersonic Mach numbers the linear mechanism is dominant. Additional insight is gained from a frequency-wavenumber analysis, including a transformation in the radial direction. With this decomposition, the acoustic field is located by the arc of a circle in plots of radial against streamwise wavenumber for discrete frequencies. The transformation is applied to both the pressure field, showing the sound directivity, and to selected source terms, showing characteristic directivity patterns for the streamwise and radial quadrupole terms. Decreasing the Mach number leads to a reduction in amplitude of the sources and of the sound radiation. Simulations with broadband forcing show that the qualitative effects of Mach number and jet heating are captured by this approach, which requires less resolution than a direct numerical simulation. A significant increase in the strength of the acoustic radiation for cold jets is observed, which is worthy of further investigation.     

Noise generation by instabilities in low Reynolds number supersonic jets

An experimental investigation of noise generation by instabilities in low Reynolds number supersonic air jets has been performed. Sound pressure levels, spectra and acoustic phase fronts were measured with a traversing condenser microphone in the acoustic field of axisymmetric, perfectly expanded, cold jets of Mach numbers 1·4, 2·1 and 2·5. Low Reynolds numbers in the range from Re = 3700 to Re = 8700 were obtained by exhausting the jets into an anechoic vacuum chamber test facility. This contrasts with Reynolds numbers of over 10⁶ for similar jets exhausting into atmospheric pressure. The flow fluctuations of the instability in all three jets have been measured with a hot-wire and the results are documented in a previous paper by Morrison and McLaughlin. Acoustic measurements show that the major portion of the sound radiated by all three jets is produced by the instability's rapid growth and decay that occurs near the end of the potential core. This takes place over a relatively short distance (less than two wavelengths of the instability) in the jet. In the lower two Mach number jets the instability has a phase velocity less than the ambient acoustic velocity. In the Mach number 2·5 jet the instability phase speed is 1·11 times the ambient acoustic velocity. In this case the acoustic phase fronts indicate the possibility of a Mach wave component. It was also determined that low level excitation at the dominant frequency of the instability actually decreased the radiated noise by suppressing the broad band component.     

线阵声强缩放方法在水下声源辐射功率估算中的应用

为了解决水下声源辐射声功率难以计算的问题,利用线阵声强缩放方法在波束形成声源识别的基础上,根据波束输出结果与声源辐射声功率之间的换算关系来获得相应的声功率。为了提高线阵声强缩放方法的水下声功率估算精度,给出了一定动态范围限制的主瓣区域积分方法,并通过仿真验证了该方法的有效性。在消声水池中开展了水下声功率估算的实验研究。在不同的测试距离下,对双声源条件下的单频以及宽带声源在阵列侧的辐射声功率进行了估算,以混响法的测量结果为参考值,研究了估算误差随声源频率、测试距离等影响因素的变化规律。实验结果表明,无论是单频还是宽带声源,声功率的最大估算误差不超过2.6 dB,在高频时不超过1.6 dB。验证了线阵声强缩放方法应用于水下声源辐射声功率估算的正确性与可行性。      

The radiating part of circular sources

An analysis is developed linking the form of the sound field from a circular source to the radial structure of the source, without recourse to far-field or other approximations. It is found that the information radiated into the field is limited, with the limit fixed by the wavenumber of the source multiplied by the source radius (Helmholtz number). The acoustic field is found in terms of the elementary fields generated by a set of line sources whose form is given by Chebyshev polynomials of the second kind and whose amplitude is found to be given by weighted integrals of the radial source term. The analysis is developed for tonal sources, such as rotors, and for Helmholtz number less than two, for random disk sources. In this case, the analysis yields the cross-spectrum between two points in the acoustic field. The analysis is applied to the problems of tonal radiation, random source radiation as a model problem for jet noise, and to noise cancellation, as in active control of noise from rotors. It is found that the approach gives an accurate model for the radiation problem and explicitly identifies those parts of a source which radiate.     

Numerical simulation of turbulence transition and sound radiation for flow through a rigid glottal model

Large eddy simulation (LES)-based computational aeroacoustics techniques were applied to a static model of the human glottis, idealized here as a planar channel with an orifice, to study flow-acoustic interactions related to speech. Rigid models of both converging and diverging glottal passages, each featuring a 20 deg included angle and a minimal glottal diameter of 0.04 cm, with an imposed transglottal pressure of 15 cm H2O, were studied. The Favre-filtered compressible Navier-Stokes equations were integrated for this low-Mach-number flow using an additive semi-implicit Runge-Kutta method and a high-order compact finite-difference scheme with characteristic-based nonreflecting boundary conditions and a multiblock approach. Flow asymmetries related to the Coanda effect and transition to turbulence, as well as the far-field sound, were captured. Acoustic-analogy-based far-field sound predictions were compared with direct simulations and showed that dipole sources, arising from unsteady flow forces exerted on the glottal walls, are primarily responsible for the tonal sound observed in the divergent glottis case.     

Integrated aerodynamic measurements

The myoelastic-aerodynamic model of phonation implies that aerodynamic factors are crucial to the evaluation of voice function. Subglottal pressure and mean flow rate represent the vocal power source. If they can be related to the magnitude of the radiated sound power, they may provide an index of vocal efficiency. Methods of evaluating the aerodynamic characteristics associated with the ventilatory and laryngeal systems are critically discussed, and normative aerodynamic values for use in diagnosis and physiologic investigations are presented. Measurements having excellent time resolution of the glottal flow wave and of pressure changes in the vicinity of the larynx itself demonstrate the importance of study vocal tract acoustics.     

Frequency-domain acoustic pressure formulation for rotating source in uniform subsonic inflow with arbitrary direction

This paper presents a frequency-domain formulation for predicting noise radiated from the rotating thickness and loading sources in uniform subsonic inflow with arbitrary direction. The proposed frequency-domain formulation is an extension of the recently published frequency-domain formulation for the stationary medium. It avoids the singular integral and numerical interpolation problems encountered in the time-domain numerical method. Three test cases, i.e., noise radiation from the rotating monopole and dipole point sources and the Isom thickness noise of a transonic rotor in the subsonic uniform flow, have been carried out to validate the proposed formulation. Both the acoustic pressure spectrum and directivity pattern computed with the present frequency-domain method are in good agreement with those obtained from the time-domain method, thus validating the correctness of the present formulation. Furthermore, the numerical results indicate that the frequency-domain formulation is suitable for tonal noise prediction, while it is inefficient for broadband noise prediction.     

Automotive panel noise contribution modeling based on finite element and measured structural-acoustic spectra

The radiated noise contributions of automotive body panels to the interior sound pressure levels are modeled using an approximate spectral formulation that applies the theoretical interior acoustic sensitivity terms derived from a finite element model and measured spatial-averaged structural-acoustic spectra. The finite element calculation is validated by comparison to a set of experimental acoustic transfer functions. A measurement set-up for the sound intensity and structural-acoustic response is applied to acquire the cross and auto power spectra needed to predict the relative mean-squared velocity term of each control plane near the panel surface, and to obtain the individual panel contribution function. The proposed approach also computes the noise spectra in 1/12 octave band form at selected positions in the passenger compartment, which matches well with the overall experimental results. Through an actual passenger car application, the approximate computational scheme is proven to be generally quite robust and effective for analyzing higher frequency interior noise problems.     

Effect of blade wrap angle on efficiency and noise of small radial fan impellers—A computational and experimental study

Radial impellers have several technical applications. Regarding their aerodynamic performance, they are well optimized nowadays, but this is in general not true regarding acoustics. This work was therefore concerned with analyzing the flow structures inside isolated radial impellers together with the far-field sound radiated from them in order to optimize the aerodynamic and acoustic performance. Both numerical and experimental techniques were applied in order to study the effect of varying wrap angle and otherwise identical geometric configuration on aerodynamics and acoustics of the radial impellers. The results give a detailed insight into the processes leading to sound generation in radial impellers. Measurements were performed using laser Doppler anemometry for the flow field and microphone measurements to analyze the radiated noise. In addition, unsteady aerodynamic simulations were carried out to calculate the compressible flow field. An acoustic analogy was employed to compute far-field noise. Finally, the phenomena responsible for tonal noise and the role of the wrap angle could be identified. Using this knowledge, design guidelines are given to optimize the impeller with respect to the radiated noise. This work shows that improved aerodynamic efficiency for isolated impellers does not automatically lead to a smaller flow-induced sound radiation.