Acoustic radiation impedance of rectangular pistons on prolate spheroids

The self and mutual radiation impedances for rectangular piston(s) arbitrarily located on a rigid prolate spheroidal baffle are formulated. The pistons are assumed to vibrate with uniform normal velocity and the solution is expressed in terms of a modal series representation in spheroidal eigenfunctions. The prolate spheroidal wave functions are obtained using computer programs that have been recently developed to provide accurate values of the wave functions at high frequencies. Results for the normalized self and mutual radiation resistance and reactance are presented over a wide frequency range for different piston sizes and spheroid shapes.     

Acoustic impedance of rectangular panels

The modal radiation impedance of a rectangular panel simply supported in an infinite baffle in the presence of an inviscid, uniform, subsonic flow is determined. The analysis is based on an expansion in normal modes of the transverse vibration displacement field of the panel and the use of wavenumber-frequency transforms. Integral expressions for the modal radiation impedance and the cross-modal coupling impedance are formulated. A Gaussian quadrature with reusable abscissas is developed to evaluate these modal radiation impedances. Changes with respect to flow speed in modal radiation damping of a rectangular membrane are measured. For the first observable low order mode the change is significant and agrees with the theory. For higher order modes non-significant changes are predicted and measured. The computed modal radiation reactances of the membrane are compared with the best available experiments. Significant discrepancies exist. Their causes are discussed.     

Acoustic characteristics analysis of slender cavity system with arbitrary impedance end conditions

A modeling method for the dynamic characteristics analysis of a slender acoustical cavity with impedance end conditions is established. In order to satisfy the continuity requirement at impedance ends for the first order differential of sound pressure, field function is constructed as the standard Fourier series supplemented by boundary smoothed auxiliary polynomials. System characteristic equation is derived by solving the governing differential equation and impedance acoustic boundary of slend...     

考虑任意阻抗壁面条件管腔结构声场特性分析

针对任意阻抗壁面条件一维管腔声学系统建模,对系统动力学特性进行预报。为了满足阻抗边界条件对声压一阶导数连续性要求,管腔声压函数通过在标准傅里叶级数端点位置引入边界光滑辅助多项式进行构建。结合壁面阻抗声学边界和管腔声学Helmholtz控制微分方程得到强形式标准特征值问题,获得相应的声学模态信息。在数值仿真中,通过算例给出各种边界条件下管腔声学模态频率、声压振型、声压和质点振速频率响应曲线,与现有文献中相关结果进行对比,充分验证了本文求解方法的正确性和有效性,证明该方法可对任意阻抗壁面条件管腔系统声学特性进行准确预报。     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

Acoustic impedance measurements-correction for probe geometry mismatch

The effect of evanescent mode generation, due to geometrical mismatch, in acoustic impedance measurements is investigated. The particular geometry considered is that of a impedance probe with an annular flow port and a central microphone, but the techniques are applicable to other geometries. It is found that the imaginary part of the measured impedance error is proportional to frequency, and that the sign of the error is positive for measurements made on tubes with diameter much larger than that of the inlet port, but negative for tubes with diameter close to that of the inlet. The result is a distortion of the measured frequencies of the impedance minima of the duct while the maxima are largely unaffected. There is, in addition, a real resistive component to the error that varies approximately as the square root of the frequency. Experiment confirms the results of the analysis and calculations, and a calibration procedure is proposed that allows impedance probes that have been calibrated on a semi-infinite tube of one diameter to be employed for measurements on components with an inlet duct of some very different diameter.     

Acoustic impedance microscopy for biological tissue characterization

A new method for two-dimensional acoustic impedance imaging for biological tissue characterization with micro-scale resolution was proposed. A biological tissue was placed on a plastic substrate with a thickness of 0.5 mm. A focused acoustic pulse with a wide frequency band was irradiated from the “rear side” of the substrate. In order to generate the acoustic wave, an electric pulse with two nanoseconds in width was applied to a PVDF-TrFE type transducer. The component of echo intensity at an appropriate frequency was extracted from the signal received at the same transducer, by performing a time–frequency domain analysis. The spectrum intensity was interpreted into local acoustic impedance of the target tissue. The acoustic impedance of the substrate was carefully assessed prior to the measurement, since it strongly affects the echo intensity. In addition, a calibration was performed using a reference material of which acoustic impedance was known. The reference material was attached on the same substrate at different position in the field of view. An acoustic impedance microscopy with 200 × 200 pixels, its typical field of view being 2 × 2 mm, was obtained by scanning the transducer. The development of parallel fiber in cerebella cultures was clearly observed as the contrast in acoustic impedance, without staining the specimen. The technique is believed to be a powerful tool for biological tissue characterization, as no staining nor slicing is required.     

Reduction of acoustic radiation by impedance control with a perforated absorber system

A model of sound radiation from an infinite plate with an absorptive facing is proposed and investigated theoretically from the viewpoint of acoustic power. Acoustic characteristics on the plate surface are represented by impedance derived from iso-absorption curves. A parametric study is carried out to clarify the effect of the impedance on the acoustic power. Results derived from this model show that acoustic radiation depends on change in impedance as well as the absorption coefficient, and there is a possibility of reducing the radiation from vibrating surface by introducing an appropriate impedance surface. In order to realize this effect, a model using a perforated board with a back cavity attached to the vibrating surface is proposed, in which the motion of the perforated board is made equal to that of the vibrating surface. To obtain fundamental data, a theoretical study is performed under a simplified condition, assuming an infinite plane piston. The calculated results are compared to experimental data measured by using an acoustic tube. The results, which are in good agreement in the reduction effect, show that this system can achieve the reduction of radiated sound power at arbitrary frequencies.     

Acoustic impedance of perforated panels covered by membranes

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掠过流作用下穿孔板的声阻抗

应用三维时域数值方法研究掠过流对穿孔板声阻抗的影响。建立了掠过流作用下穿孔板声阻抗计算的计算流体动力学(CFD)模型,通过时域计算得到掠过流作用下穿孔板的声阻抗,分析结构参数和掠过流马赫数对穿孔板声阻抗的影响。根据计算结果拟合掠过流作用下穿孔板声阻抗的近似表达式,利用获得的穿孔声阻抗新公式预测穿孔管消声器的传递损失,数值预测和实验结果吻合良好。计算结果表明,掠过流对穿孔板的声阻抗和穿孔管消声器的消声性能有明显影响。     

通过流作用下穿孔板的声阻抗

穿孔元件在进排气消声器中广泛使用,气体流动对穿孔元件声阻抗具有较大的影响。为了获得更加精确的穿孔声阻抗模型,使用三维时域CFD方法计算通过流作用下穿孔的声阻抗。探究了通过流作用下穿孔声阻抗的获取方法,并且将无量纲小孔声阻抗的预测值与已发表的实验测量值进行了对比,两者吻合较好。分析了小孔中的通过流马赫数M_o (0.05~0.20)、穿孔的分布形式、小孔的直径d_h (2~5 mm),穿孔板的厚度t (0.8~2 mm)和穿孔率φ(4.51%~24.93%)对无量纲声阻抗的影响规律,并且通过不同参数的非线性回归分析得到了通过流作用下声阻抗的模型。作为工程计算的应用,利用Jing&Sun的声阻抗模型和本文声阻抗模型计算了横流式穿孔管消声器的传递损失,与实验测量结果比较表明,本文模型具有较高的准确性。     

Acoustic impedance measurements with a sound intensity meter

Acoustic impedance measurements have been made by evaluating, with a sound intensity meter, sound pressure and normal velocity close to absorbing material samples.Measurements have been made both in a Kundt interferometer and in the free field. With the Kundt interferometer the same results are achieved as with the classical Kundt interferometer but the present method is much faster and easier to implement.Free field measurements are accurate for frequencies high enough to allow one to neglect the finite dimension effect of the absorbing material sample.     

The radiation impedance of a rectangular piston

The radiation impedance of a rectangular piston is expressed as the Fourier transform of its impulse response, which is obtained from the recent work of Lindermann [1]. The analytical evaluation of the transform is performed and new integral expressions are presented for both the radiation resistance and reactance. The integrals are readily evaluated in terms of elementary functions at both the low and high frequency limits. The integrals are also expressed as series of Bessel functions which are valid for all frequencies and aspect ratios. Numerical results are presented to illustrate the behavior of the radiation resistance and reactance as a function of the aspect ratio of the piston and a normalized frequency parameter. Additional numerical results are then presented to illustrate the accuracy of the analytical expressions for the radiation resistance and reactance at low and high frequencies. Finally, numerical results are presented to illustrate the application and accuracy of using standard FFT algorithms to evaluate the radiation resistance and reactance directly from the impulse responses.     

Acoustic radiation on heating deformed aluminum

Acoustic emission is recorded in the process of heating aluminum which was deformed at room temperature. Data are cited indicating that acoustic radiation is caused by the processes of recovery and recrystallization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 8–11, April, 1986.     

Acoustic impedance of an artificially lengthened and constricted vocal tract

Voice training techniques often make use of exercises involving partial occlusion of the vocal tract, typically at the anterior part of the oral cavity or at the lips. In this study two techniques are investigated: a bilabial fricative and a small diameter hard-walled tube placed between the lips. Because the input acoustic impedance of the vocal tract is known to affect both the shaping of the glottal flow pulse and the vibrational pattern of the vocal folds, a study of the input impedance is an essential step in understanding the benefits of these two techniques. The input acoustic impedance of the vocal tract was investigated theoretically for cases of a vowel, bilabial occlusion (fully closed lips), a bilabial fricative, and artificially lengthening the tract with small diameter tubes. The results indicate that the tubes increase the input impedance in the range of the fundamental frequency of phonation by lowering the first formant frequency to nearly that of the bilabial occlusion (the lower bound on the first formant) while still allowing a continuous airflow. The bilabial fricative also has the effect of lowering the first formant frequency and increasing the low-frequency impedance, but not as effectively as the extension tubes.     

On the radiation impedance of a rectangular piston

Single integral representations for the resistive and reactive components of the radiation impedance appropriate to a rectangular piston are established, thereby enabling a systematic refinement of estimates at both short and long wavelengths. Comparisons with previous analyses are made explicit as well as extensions and corrections thereto.     

The radiation impedance of a baffled piston source

The problem of a vibrating disk set in a finite, rigid, concentric baffle is reduced to the solution of a Fredholm integral equation of the second kind. The acoustic radiation impedance is computed from the numerical solution of this equation and results tabulated for values of (wavenumber × disk radius) between 0·2 and 2·0.     

Acoustic analysis of a rectangular cavity with general impedance boundary conditions

A Fourier series method is proposed for the acoustic analysis of a rectangular cavity with impedance boundary conditions arbitrarily specified on any of the walls. The sound pressure is expressed as the combination of a three-dimensional Fourier cosine series and six supplementary two-dimensional expansions introduced to ensure (accelerate) the uniform and absolute convergence (rate) of the series representation in the cavity including the boundary surfaces. The expansion coefficients are determined using the Rayleigh-Ritz method. Since the pressure field is constructed adequately smooth throughout the entire solution domain, the Rayleigh-Ritz solution is mathematically equivalent to what is obtained from a strong formulation based on directly solving the governing equations and the boundary conditions. To unify the treatments of arbitrary nonuniform impedance boundary conditions, the impedance distribution function on each specified surface is invariantly expressed as a double Fourier series expansion so that all the relevant integrals can be calculated analytically. The modal parameters for the acoustic cavity can be simultaneously obtained from solving a standard matrix eigenvalue problem instead of iteratively solving a nonlinear transcendental equation as in the existing methods. Several numerical examples are presented to demonstrate the effectiveness and reliability of the current method for various impedance boundary conditions, including nonuniform impedance distributions.