阻尼受击板的时域建模及声音合成

为满足声源辨识中对合成冲击声的迫切需求,建立了球-板撞击的时域模型,提出一种时域快速求解方法,并进行了实验验证.首先给出一种将时域有限差分法(FDTD)和模态展开法(MEM)相结合的时域混合方法,求解板的振动方程,并解决了混合方法中MEM的模态截断和初值问题,及两种方法中阻尼的一致性问题;随后,给出了简支矩形板的冲击声计算结果,通过与FDTD方法的运算量进行对比,验证了混合方法的高效性;最后,针对自由边界下的L形板进行了实验验证.结果表明,与传统FDTD方法相比,时域混合方法在保证合成冲击声精度的前提下可将计算效率提高100至1200倍。     

Time-domain simulations of sound propagation through screen-induced turbulence

A flow model in combination with a statistical-dynamical turbulence generator and a linearised Euler time-domain model for sound waves were used to simulate the effect of screen-induced turbulence on the noise level in the acoustical shadow of a screen in wind. Instead of simulating a great number of different frozen turbulence realisations, the concept of transient turbulence was successfully tested and applied. This concept is adequate to the time-domain model and reduces the computational demands. Several two-dimensional simulations allowed to isolate the individual effects of wind and screen on the propagation of 500 Hz sound waves over a 4-m high noise barrier. At a distance of 250 m from the source (240 m behind the screen) the sheltering effect of the screen and the refraction effect of the wind are in the order of 6 and 4 dB, respectively. The screen-induced turbulence leads to fluctuations in the noise level with a standard deviation of 1.2 dB and a maximum amplitude of 3 dB. However, the time averaged effect turned out to be in the order of merely 0.2 dB. The effect of the screen-induced turbulence on the average noise level behind the screen is therefore negligible.     

Time-domain calculations of sound interactions with outdoor ground surfaces

A time-domain formulation for sound propagation in rigid-frame porous media, including waveform attenuation and dispersion, is developed. The new formulation is based on inversion of the relaxation functions from a previous model [Wilson DK, Ostashev VE, Collier SL. J Acoust Soc Am 2004;116:1889-92], thereby casting the convolution integrals in a form amenable to numerical implementation. Numerical techniques are developed that accurately implement the relaxational equations and transparently reduce to previous results in low- and high-frequency limits. The techniques are demonstrated on calculations of outdoor sound propagation involving hills, barriers, and ground surfaces with various material properties. We also compare the relaxation formulation to a widely applied phenomenological model developed by Zwikker and Kosten. The two models can be made equivalent if the resistance constant, structure constant, and compression modulus in the ZK model are allowed to be weakly frequency dependent. But if the ZK parameters are taken to be constant, as is typically the case, the relaxation model provides more accurate calculations of attenuation by acoustically soft porous materials such as snow, gravel, and forest litter.     

有限振幅波的三维时域建模

通过对二维算法(Texas code)做扩展,建立了求解扩展版KZK(Khokhlov-Zabolotskaya-Kuznetsov)方程的三维时域有限差分算法。该算法先将KZK方程变换成TBE(Transformed Beam Equation),然后依次求解衍射(抛物线近似条件下)、热黏滞吸收、弛豫和非线性作用。数值仿真了圆形、矩形和方形阵列的非线性声场,与前人的结果相符,证明了算法的有效性,并分析了把弛豫吸收系数当成热黏滞吸收系数引入空气中的参量阵的模拟会带来的误差。     

Time-domain modeling of finite-amplitude sound beams in three-dimensional Cartesian coordinate system

A three-dimensional time-domain algorithm,which is based on the augmented KZK (Khokhlov-Zabolotskaya-Kuznetsov) equation,is proposed to simulate the nonlinear field of the parametric array.First,KZK equation is transformed into TBE(Transformed beam equation). Then,the effects of diffraction(in parabolic approximation),thermoviscous absorption,relaxation, and nonlinearity are solved with finite difference methods.The numerical results of this code agree well with the theoretical and experimental results presented in previous studies, which demonstrates the validity of the three-dimensional algorithm.Using this code to calculate the nonlinear field of the parametric array in air,it is found that the small time interval is important to the accuracy of the simulation results of the difference frequency wave in the case of high sound pressure level,and the errors caused by taking relaxation absorption for thermoviscous absorption are influenced by the characteristic frequency.     

Time-domain modeling of nonlinear distortion of pulsed finite amplitude sound beams

This work aims to validate a time domain numerical model for the nonlinear propagation of a short pulse of finite amplitude sound beam propagation in a tissue-mimicking liquid. The complete evolution equation is simply derived by a superposition of elementary operators corresponding to the 'one effect equation'. Diffraction LD, absorption and dispersion LAD, and nonlinear distortion LNL effects are treated independently using a first order operator-splitting algorithm. Using the method of fractional steps, the normal particle velocity and the acoustical pressure are calculated plane by plane, at each point of a two-dimensional spatial grid, from the surface of the plane circular transducer to a specified distance. The LA operator is a time convolution between the particle velocity and the causal attenuation filter built after the Kramers-Kroning relations. The LNL operator is a time-based transformation obtained by following an implicit Poisson analytic solution. The LD operator is the usual Rayleigh integral. We present a comparison between theoretical and experimental temporal pressure waveform and axial pressure curves for fundamental (2.25 MHz), second, third and fourth harmonics, obtained after spectral analysis.     

水下任意形旋转薄壳振动和辐射声的时域研究

本文通过采用DAA2方法,有限元模态展开和直接积分混合方法以及Kirchhoff推迟势的边界元解法在时域上对水下任意形旋转薄壳振动和辐射声进行研究。以水下椭球壳的端部轴对称激励问题为例,对力源的时间函数分别为单频填充脉冲和δ脉冲两种情况给出了一些典型的数值计算结果和实验结果,并进行了讨论。     

Time-domain impedance formulation for transmission line matrix modelling of outdoor sound propagation

Outdoor sound propagation modelling has attracted considerable attention in the past and has lead to many analytical and numerical models. More recently, the increase of computational power has led to consider time-domain methods that enable to consider transient phenomena. Among these models, the transmission line matrix (TLM) method has been proposed, but the sound absorption at boundaries appears to be a somewhat underdeveloped aspect of this approach. In this paper, a specific implementation of impedance boundary condition is proposed. The method is based on the approximation of the impedance as a sum of linear systems, which allows the formulation of an equivalent impedance model in the time-domain. The proposed implementation is applied for two common impedance models of porous material. Numerical simulations have been carried out in the case of sound propagation over a flat ground with and without an impedance discontinuity, and for several values of specific airflow resistivity. TLM numerical predictions expressed in terms of excess attenuation relative to free field show a good agreement with analytical solutions.     

水下弹性球壳被激振动和辐射声的时域特性研究

本文在时域上对水下球壳在球冠形均匀单频填充脉冲力源作用下的振动和辐射声问题进行分析和计算。结果表明:在长脉冲力作用下,根据填充频率不同,会激起不同模态振动和产生长的共振辐射声或拍频辐射声,但有一部分模态对远场声辐射的贡献甚微;在短脉冲力作用下,脉冲波在球壳上传播一般会发生展宽和变形。文中还应用DAA2方法简化水——壳体耦合关系和边界元方法计算辐射声场。与精确解比较可知,DAA2方法的结果是令人满意的。     

Time-domain simulation of damped impacted plates. I. Theory and experiments

A time-domain formulation for the flexural vibrations in damped rectangular isotropic and orthotropic plates is developed, in order to investigate transient excitation of plates by means of sound synthesis. The model includes three basic mechanisms of damping (thermoelasticity, viscoelasticity and radiation) using a general differential operator. The four rigidity factors of the plate are modified by perturbation terms, each term corresponding to one specific damping mechanism. The first damping term is derived from the coupling between the thermoelastic stress-strain relations and the heat diffusion equation. The second term is obtained from the general differential formulation of viscoelasticity. The third term is obtained through a Pade approximation of the damping factor which governs the coupling of the plate with the surrounding air. The decay factors predicted by the model reproduce adequately the dependence on both dimensions and frequency of the decay factors measured on rectangular plates of various sizes and thicknesses made of four different materials (aluminum, glass, carbon fiber, and wood). The numerical resolution of the complete problem, including initial and boundary conditions, and the comparison between real and simulated sounds are presented in a companion paper.     

Numerical simulation of parametric sound generation and its application to length-limited sound beam

In this study we propose a simulation model for predicting the nonlinear sound propagation of ultrasound beams over a distance of a few hundred wavelengths, and we estimate the beam profile of a parametric array. Using the finite-difference time-domain method based on the Yee algorithm with operator splitting, axisymmetric nonlinear propagation was simulated on the basis of equations for a compressible viscous fluid. The simulation of harmonic generation agreed with the solutions of the Khokhlov–Zabolotskaya–Kuznetsov equation around the sound axis except near the sound source. As an application of the model, we estimated the profiles of length-limited parametric sound beams, which are generated by a pair of parametric sound sources with controlled amplitudes and phases. The simulation indicated a sound beam with a narrow truncated array length and a width of about one-quarter to half that of regular a parametric beam. This result confirms that the control of sound source conditions changes the shape of a parametric beam and can be used to form a torch like low-frequency sound beam.     

Time-domain simulation of damped impacted plates. II. Numerical model and results

A time-domain model for the flexural vibrations of damped plates was presented in a companion paper [Part I, J. Acoust. Soc. Am. 109, 1422-1432 (2001)]. In this paper (Part II), the damped-plate model is extended to impact excitation, using Hertz's law of contact, and is solved numerically in order to synthesize sounds. The numerical method is based on the use of a finite-difference scheme of second order in time and fourth order in space. As a consequence of the damping terms, the stability and dispersion properties of this scheme are modified, compared to the undamped case. The numerical model is used for the time-domain simulation of vibrations and sounds produced by impact on isotropic and orthotropic plates made of various materials (aluminum, glass, carbon fiber and wood). The efficiency of the method is validated by comparisons with analytical and experimental data. The sounds produced show a high degree of similarity with real sounds and allow a clear recognition of each constitutive material of the plate without ambiguity.     

Time-domain simulation of channel crosstalk and inter-modulation distortion in gain-clamped semiconductor optical amplifiers

A time-domain model is implemented for gain-clamped semiconductor optical amplifiers (GC-SOAs) based on a combination of the separated traveling-wave equations and effective Bloch equations. The key feature of this model lies in its capability of handling the lasing-signal, signal-signal, and signal-noise interactions over a broad wavelength band. Therefore, various nonlinear phenomena such as the cross-gain saturation (XGS) and nondegenerate four-wave mixing (ND-FWM) can readily be captured. After being implemented and validated, this model is applied to the simulation of GC-SOA dynamic behaviors such as the channel crosstalk and intermodulation distortion (IMD). Simulation results show that the third-order IMD can be effectively suppressed by a gain-clamping lasing mode in GC-SOAs in comparison with that in conventional SOAs. The channel crosstalk can also be suppressed to some extent in GC-SOAs, but not as effectively. Other than a homogeneous reduction, the gain-clamping in GC-SOAs does not change the dependence of the channel crosstalk and IMD on the input signal power and channel spacing. It is also shown that the channel crosstalk, unlike the IMD, cannot be efficiently reduced by enlarging the channel spacing even in GC-SOAs.     

Time domain simulation and sound synthesis for the snare drum

The snare drum is a complex system, relying on the interaction of multiple components: the drumheads, or membranes, a set of snares, the surrounding acoustic field and an internal cavity. Because these components are multidimensional, and due to a strong distributed non-linearity (the snare interaction), many techniques used frequently in physical modeling synthesis applications, such as digital waveguides and modal methods are difficult to apply. In this article, finite difference time domain techniques are applied to a full 3D system, and various features of interest, such as the coupling between membranes, and the interaction between the membranes and the snares, are examined in detail. Also discussed are various numerical features, such as spurious splitting of degenerate modes and bandwidth limitation, and estimates of computational complexity are provided. Sound examples are presented.     

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.     

激波与SF6球形气泡相互作用的数值研究

本文基于大涡模拟方法, 采用高阶精度格式对平面入射激波以及不同反射距离条件下的反射激波与SF₆重气泡相互作用过程进行了三维数值模拟. 数值结果清晰地显示了SF₆重气泡在激波作用下诱导Richtmyer-Meshkov不稳定性过程, 揭示了入射激波以及反射激波在气泡界面聚焦诱导射流的过程, 详细分析了不同反射距离条件下反射激波与SF₆重气泡作用过程及流场结构.     

Aerodynamic sound production in low speed flow ducts

Measurements of spoiler aerodynamic noise, generated in a low velocity flow duct and radiated from an open exhaust termination, have been made in the form of sound power spectra. The individual $\text{1}\text{3}$ octave power measurements are satisfactorily collapsed (within ± 3 dB) with the aid of derived theoretical scaling laws. Non-dimensional spectra are presented which permit generalized predictions of flow noise for bluff bodies, including splitter attenuators, mounted in low speed flow ducts.     

Time-domain simulation of acoustic wave propagation and interaction with flexible structures using Chebyshev collocation method

A time-domain Chebyshev collocation (ChC) method is used to simulate acoustic wave propagation and its interaction with flexible structures in ducts. The numerical formulation is described using a two-dimensional duct noise control system, which consists of an expansion chamber and a tensioned membrane covering the side-branch cavity. Full coupling between the acoustic wave and the structural vibration of the tensioned membrane is considered in the modelling. A systematic method of solution is developed for the discretized differential equations over multiple physical domains. The time-domain ChC model is tested against analytical solutions under two conditions: one with an initial state of wave motion; the other with a time-dependent acoustic source. Comparisons with the finite-difference time-domain (FDTD) method are also made. Results show that the time-domain ChC method is highly accurate and computationally efficient for the time-dependent solution of duct acoustic problems. For illustrative purposes, the time-domain ChC method is applied to investigate the acoustic performance of three typical duct noise control devices: the expansion chamber, the quarter wavelength resonator and the drum silencer. The time-dependent simulation of the sound-structure interaction in the drum silencer reveals the delicate role of the membrane mass and tension in its sound reflection capability.     

新型声速测定仪的研制

论述了新型声速测定仪的结构、工作原理和实验内容，阐述了几种测媒质中超声波波速的方法。