A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials

A method for evaluating the acoustical properties of homogeneous and isotropic porous materials that may be modeled as fluids having complex properties is described here. To implement the procedure, a conventional, two-microphone standing wave tube was modified to include: a new sample holder; a section downstream of the sample holder that accommodated a second pair of microphone holders and an approximately anechoic termination. Sound-pressure measurements at two upstream and two downstream locations were then used to estimate the two-by-two transfer matrix of porous material samples. The experimental transfer matrix method has been most widely used in the past to measure the acoustical properties of silencer system components. That procedure was made more efficient here by taking advantage of the reciprocal nature of sound transmission through homogeneous and isotropic porous layers. The transfer matrix of a homogeneous and isotropic, rigid or limp porous layer can easily be used to identify the material's characteristic impedance and wave number, from which other acoustical quantities of interest can be calculated. The procedure has been used to estimate the acoustical properties of a glass fiber material: good agreement was found between the estimated acoustical properties and those predicted by using the formulas of Delany and Bazley.     

Complement to standard method for measuring normal incidence sound transmission loss with three microphones

Complement to standard E2611-09 of the American Society for Testing and Materials [Standard Test Method for Measurement of Normal Incidence Sound Transmission of Acoustical Materials Based on the Transfer Matrix Method (American Society for Testing and Materials, New York, 2009)] is proposed in order to measure normal incidence sound transmission loss of materials in a modified impedance tube using a three-microphone two-load or one-load method. The modified tube is a standard two-microphone impedance tube, where a third microphone is mounted on a movable hard termination. This method is conceptually identical to the four-microphone two-load or one-load method described in the standard; however, it requires fewer transfer functions and one microphone less. The method is validated on (1) symmetrical homogeneous and (2) non-symmetrical non-homogeneous specimens.     

An impulse method of measuring normal impedance at oblique incidence

An experimental method of determining the oblique incidence behaviour of sound absorbing materials is given. The method involves the measurement of acoustic impulses at the surface of an absorbing material and comparing the complex frequency components with those of a reference signal recorded remote from the surface. Amplitude and phase characteristics of the reflected signal are thus obtained and hence the normal impedance of the material is found. Comparison between measurements made by the “surface” method and standing wave tube measurements shows good agreement for normal incidence. However, at very oblique angles of incidence the method is in obvious error, the accuracy being limited by a sphericity effect.     

Measurement of sound absorption by underwater acoustic material using pulse-separation method

An alternative pulse-separation method is presented for measuring the sound absorption at normal incidence of an underwater acoustic material in a water-filled impedance tube. A damped sine pulse was generated in the water-filled impedance tube with a regular waveform and a short duration time of approximately 1 ms. During the generation of the pulse, the inverse filter principle was used to compensate for the transducer response. In addition, the effects of the characteristics of the tube termination can be eliminated during the generation of the pulse to obtain a single plane pulse wave in the impedance tube, which is a necessary condition for this technique. Measurements of the sound absorption coefficient of the rubber material and the reflection coefficient from a water/air interface were used to verify the pulse-separation method.     

An investigation into the acoustic insulation of triple-layered panels containing Newtonian fluids: Theory and experiment

Sound insulation of triple-layered panels consisting of two impervious layers with the middle layer being a Newtonian fluid is studied here both theoretically and experimentally. The progressive impedance model is used to predict the transmission loss (TL) provided by the panel in a normal incidence field. Corrections are then made to obtain the TL values of such panels in random incidence field. A modified B&K impedance tube was constructed for experimental evaluation. Results are presented for a Pyrex glass cylindrical tube containing motor oil, a ferromagnetic nanoparticles fluid (in the absence of a magnetic field) and air. Good agreement is obtained between the measured and analytical results for a wide range of frequencies. Also, a significant difference in TL values, particularly at low frequencies (f ? 4 kHz), is observed once the air is replaced by the fluid layer.     

Oblique incidence sound absorption of parallel arrangement of multiple micro-perforated panel absorbers in a periodic pattern

The sound absorption performance of a micro-perforated panel (MPP) absorber array at oblique incidence and in diffuse field is investigated both numerically and experimentally. The basic module of the MPP absorber array consists of four parallel-arranged MPP absorbers with different cavity depths, and the whole MPP absorber array is created by arranging the basic modules in a periodically repeating pattern. Results show that the influence of incidence angle mainly lies in two aspects. First, the parallel absorption mechanism breaks down at lower frequencies at oblique incidence than at normal incidence due to the non-compactness of the resonating MPP absorber, which becomes non-compact if the time delay of incident wave across it is comparable to or larger than π/2. Second, the equivalent acoustic impedance of the MPP varies with respect to incidence angle which in turn changes the sound absorption performance of the MPP absorber array. Influence of the azimuthal angle is insignificant. Because of mutual influence among the member MPP absorbers, the normal incidence sound absorption of the MPP absorber array can be noticeably different from that of the basic module tested in impedance tube. The measured sound absorption coefficients of a prototype specimen in reverberation room compare well with the numerical predictions. The extra sound absorption due to diffraction of sound at the free edges of test specimen is the most efficient around 500 Hz.     

Experimental observations of the dependence of impedance tube behavior upon gas phase losses and propellant self-noise

Some of the unexpected behavior observed during admittance measurement of burning solid propellants in a modified impedance tube set-up is discussed. Specifically, repeated tests conducted with the same solid propellant resulted, unexpectedly, in different standing wave structures in the impedance tube when the exhaust configuration was changed. This resulted in the calculation of different admittances at the propellant surface. It is shown in this paper that the observed experimental trends can be explained when the presence of gas phase damping and a propellant self-noise are taken into consideration in the development of a simplified analytical model describing the behavior of the impedance tube.     

Transmission loss measurement of acoustic material using time-domain pulse-separation method (L)

An alternative method for measuring the normal incidence sound transmission loss (nSTL) is presented in this paper based on the time-domain separation of so-called Butterworth pulse with a short-duration time about 1 ms in a standing wave tube. During the generation process of the pulse, inverse filter principle was adopted to compensate the loudspeaker response, besides this, the effect of the characteristics of tube termination can be eliminated through the generation process of the pulse so as to obtain a single plane pulse wave in the standing wave tube which makes the nSTL measurement very simple. A polyurethane foam material with low transmission loss and a kind of rubber material with relatively high transmission loss are used to verify the proposed method. When compared with the traditional two-load method, a relatively good agreement between these two methods can be observed. The main error of this method results from the measuring accuracy of the amplitude of transmission coefficient.     

Rayleigh wave scattering from a vertical edge of isotropic substrates

The paper numerically studies the harmonic Rayleigh wave scattering at the 90-degree corner of isotropic substrate. The finite element method is used. The main attention is paid to two cases. The first one is the apex of the substrate corner is rounded off. The second one consists in that a layer of foreign material is deposited on the face which scatters the Rayleigh wave. The dependence of the reflection and the transmission coefficients on the Poisson ratio, the angle of incidence, the fillet radius, and the layer thickness are obtained. It is found that if the Rayleigh wave is incident perpendicularly to the substrate border, then the fillet of small radius as compared to the wavelength increases the reflection coefficient and decreases the transmission coefficient by factors 1.3–1.8. At normal incidence, the Poisson ratio does not change qualitatively the dependence of the reflection and transmission coefficients on the fillet radius. But the Poisson ratio can substantially affect the angle dependence of these coefficients if the wave is incident obliquely on the corner rounded off. It is also found out that a layer can modify the conditions of scattering such that the incident wave is totally reflected without transmission and conversion into bulk waves in a wide interval of angle of incidence, although, in principle, the bulk wave generation is allowed within a part of this angle interval.     

Comparison of resonant tunneling in AlGaAs/GaAs parabolic and diffusion modified quantum wells

Double barrier resonant tunneling diode using annealing induced diffusion modified quantum well is proposed as a viable alternative to that using parabolic quantum well which requires complex techniques to fabricate it. The transmission coefficients are calculated using the hybrid incremental airy function plane wave approach. The room temperature current-voltage characteristics have been calculated using transmission coefficients. The current-voltage characteristics are found to be similar in both diodes.     

螺旋线型三导体脉冲形成线快波抵消方法

针对螺旋线型三导体脉冲形成线的快波振荡问题,提出一种阻抗分两段阶梯变化的螺旋线结构,分析了此类螺旋线快波反射与慢波反射相互抵消快波的条件,给出两段变阻抗螺旋线慢波系数之间的关系式以及匝数密度之比计算方法,通过仿真计算证明该方法能有效消除快波反射对螺旋线输出脉冲的影响。     

The effect of grazing mean flow on acoustical characteristics of the micro-perforated panel absorber

There are three effects of grazing mean flow on acoustical characteristics of the micro-perforated panel absorber(1) on radiation impedance of the orifice,(2) on acoustical impedance of the construction at sound wave angle incidence,(3) on sound propagation property in a duct lined by absorber.Based on the acoustical fundamental principle,these effects were analyzed respectively,and relative formulas were derived. Some qualitative tendencies were shown that radiation impedance of an orifice,value ofξin function coth(ξ) which estimates cavity impedance,and transmission loss in a lined duct all will decrease with flow speed increases as well as the resonant frequency will move to lower frequency caused by Doppler Effect,when sound wave propagation direction is the same with flow direction. The discussion was also supported by a relative experimental study.     

Groundwave propagation over horizontally layered anisotropic media

Expressions for the surface impedance and admittance, and the corresponding Fresnel reflection coefficients are derived for uniform plane TM and TE waves which are obliquely incident upon plane layered anisotropic media. The permittivity and permabi-lity tensors of each layer are assumed to be symmetric, and the propagation vector of the incident wave lies in one of the planes of anisotropy. The results can be used directly in well known formulas for groundwave propagation over isotropic media, and so there is no need to rederive these formulas for the various horizontal or vertical, electrical or magnetic dipole sources. The effect of the anisotropy upon the wave tilt is also discussed.     

Investigations on the acoustic transmission characteristics of underwater perforated panel structures

Perforated panel structures have a wide potential in underwater applications. However, up to now there has been little related research. The acoustic impedance of an underwater perforated panel is obtained based on the theories for air perforated panel sound absorption. In this paper sound transmission characteristics of underwater perforated panel structures are theoretically analyzed by the transfer matrix method. A formula for normal incidence sound transmission coefficients is given. The main factors that have effects on the acoustic transmission coefficient are analyzed by numerical simulations. The perforated panel structures made by ourselves are tested in a standing-wave tube by the four-sensor transfer-function method. The experimental results are well in accord with the results obtained by the numerical method, which proves that the theoretical analysis is correct. This paper has provided theoretical and experimental bases for the design of underwater perforated panel structures.     

三导体螺旋脉冲形成线的输出特性

 采用横电磁波传输理论,分析了三导体螺旋脉冲形成线（PFL）（包含内筒、螺旋中筒和外筒）的脉冲形成过程,给出了螺旋PFL的波阻抗、慢波系数、输出脉冲的平顶波动幅度等特征参数的计算公式。研究表明:三导体螺旋PFL具有与螺旋传输线同样的波阻抗和慢波系数;螺旋PFL的输出脉冲平顶波动,随着慢波系数或负载阻抗的增大而减小。     

Traveling wave tube measurements for low-frequency properties of underwater acoustic materials

A traveling wave tube measurement technique for measuring acoustic properties of underwater acoustic materials was developed.Water temperature and pressure environments of the ocean can be simulated in a water-filled tube,and the acoustic parameters of samples of underwater acoustic materials are measured in the range of low-frequency.A tested sample is located at central position of the tube.A pair of projectors is separately located at both ends of the tube.Using an active anechoic technique,the sound wave transmitting the tested sample is hardly reflected by the surface of secondary transducer.So the traveling sound field is built up in the tube.By separately calculating the transfer functions of every pair of double hydrophones in the sound fields from the both sides of the sample,its reflection coefficients and transmission coefficients are obtained.In the measurement system,the inside diameter of the tube isΦ208 mm,the working frequency range is from 100 to 4000 Hz,the maximum pressure is 5 MPa.The reflection coefficients and transmission coefficients of a water layer and a stainless steel layer samples are measured actually and calculated theoretically.The results show that the measured values are in good agreement with the values calculated,and the measurement uncertainty is not greater than 1.5 dB.     

A practical acoustical absorption analysis of coir fiber based on rigid frame modeling

An analytical study based on rigid frame model is demonstrated to evaluate the acoustic absorption of coir fiber. Effects of different conditions such as combination of air gap and perforated plate (PP) are studied in this work. Materials used here are treated as rigid rather than elastic, since the flow resistivity of coir fiber is very low. The well-known rigid frame Johnson-Allard equivalent-fluid model is applied to obtain the acoustic impedance of single layer coir fiber. Atalla and Sgard model is employed to estimate the surface impedance of PP. Acoustic transmission approach (ATA) is utilized for adding various consecutive layers in multilayer structure. Models are examined in different conditions such as single layer coir fiber, coir fiber backed with air gap, single layer PP in combination with coir fiber and air gap. Experiments are conducted in impedance tube on normal incidence sound absorption to validate the results. Results from the measurement are found to be in well agreement with the theoretical absorption coefficients. The performance of the rigid frame modeling method is checked more specifically in all conditions, by the mean prediction error rate of normal incidence sound absorption coefficients. Comparison between the measured absorption coefficients and predicted by rigid frame method shows discrepancy lower than 20 and 15% for most of the conditions in the frequency range of 0.2?C1.5 and 1.5?C5 kHz, respectively. Moreover, acoustic absorption of various single and multilayer structures is compared with the simpler empirical methods such as Delany-Bazley and Miki model; and complicated method such as Biot-Allard Model and Allard Transfer Function (TF) method. Comparisons show that the presented method offers a better accuracy of the results than the empirical models. Subsequently, it can provide almost same absorption plot with Biot-Allard model (single layer combination) and TF method (multilayer combination) proving it to be a comprehensively easy and general analytical tool. Therefore, the rigid frame model can be implemented relatively easier than other similar models to analyze the acoustic absorption of coir fiber in most of the conditions.     

Reproducibility experiments on measuring acoustical properties of rigid-frame porous media (round-robin tests)

This paper reports the results of reproducibility experiments on the interlaboratory characterization of the acoustical properties of three types of consolidated porous media: granulated porous rubber, reticulated foam, and fiberglass. The measurements are conducted in several independent laboratories in Europe and North America. The studied acoustical characteristics are the surface complex acoustic impedance at normal incidence and plane wave absorption coefficient which are determined using the standard impedance tube method. The paper provides detailed procedures related to sample preparation and installation and it discusses the dispersion in the acoustical material property observed between individual material samples and laboratories. The importance of the boundary conditions, homogeneity of the porous material structure, and stability of the adopted signal processing method are highlighted.     

一维压电Fibonacci类准周期声子晶体传输特性

基于传输矩阵法研究了一维压电Fibonacci类准周期声子晶体的传输特性, 比较了一维Fibonacci序列压电准周期声子晶体与非压电准周期声子晶体以及压电周期性声子晶体的透射性. 计算结果表明:弹性波通过一维准周期结构压电声子晶体时与周期性声子晶体一样会有带隙的出现, 且发现具有压电性的Fibonacci序列准周期声子晶体禁带宽度发生了展宽. 进一步讨论了入射角度对固定频率下声子透射系数的影响,结果表明一维压电Fibonacci序列准周期结构声子透射性依赖于入射角度的选取.