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.     

一种间接测量惯性管阻抗的方法

惯性管是脉冲管制冷机的一个重要部件,需要保证其在脉冲管热端提供合适的阻抗幅值和相位。如何准确确定惯性管阻抗一直是个难点。本文利用直线压缩机背腔内压力波动计算体积流率,从而间接得到惯性管入口体积流率,进而得到惯性管阻抗特性,并与DeltaEC计算做了详细对比,为进一步优化脉冲管制冷机打下基础。本文首先介绍了惯性管的热声计算模型及其湍流修正方法,并详细介绍利用压缩机了测量体积流率的方法。实验中重点考察了频率、惯性管入口压力波动、气库体积大小对阻抗的影响,并对实验结果进行了分析。实验结果表明,阻抗幅值及相角跟湍流模型计算结果总体而言相当吻合,但在某些区域还需进一步修正。     

Recent approaches to the measurement of acoustic impedance and materials characterization

Three methods are discussed: an automated pulse tube system; a direct, point measurement technique; and the application of a parametric array for oblique angle measurement.The first of these extends the capability of a proven impedance measurement technique using a waterborne acoustic waveguide (pulse tube). Data obtained in a frequency range 3 to 100 kHz, determined from complex reflection coefficients, are presented, via a transfer function analyser interfacing with a computer and plotter, to produce impedance diagrams.A direct, point impedance technique based on sensing particle velocity, or displacement of a surface and associated acoustic pressure is next discussed. Use is made of laser interferometry to measure the vector quantity, while scalar values are determined from a pressure sensor. This data affords a direct measurement of point impedance and can be applied in obtaining complex response information from heterogeneous materials or structures.The last method employs a non-linear acoustic device to obtain a requisite acoustic beam-width allowing characterization of materials at oblique angles, with samples of limited size, at low ultrasonic frequencies.     

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.     

Flat-response sound source technique for using the two-microphone method in an impedance tube

In this letter, to improve the acoustic impedance measurement in a tube using the two-microphone method, a sound source technique is developed that is capable of generating incident sound waves of uniform intensity over the entire frequency range of interest. The basic principle is to use a digitally synthesized compensation input signal to achieve a flat response of the sound source provided by one or more loudspeakers. The present sound source technique has been demonstrated to be very effective and easily implemented with the current impedance tube testing systems.     

基于四水听器的充水弹性管声速测量方法

实验室中水声材料声学参数的测量主要在水声声管中进行。管内平面波声速是正确测量这些参数的基础。该文提出一种基于四水听器结合不同边界的测量充水弹性管中声速的新方法。该方法利用4个固定位置处的水听器,采用最小二乘的方法,使得两组水听器分别得到的声管末端入射波声压差值的平方最小的声速即为管内平面波声速。该方法利用单频信号,在每一频率点均可测得声速,可以在任一种声管末端边界下进行测量,同时无需知道各水听器到边界的精确距离,在文中的3种边界下声速测量结果具有很好的一致性,实验操作简单、误差很小。该方法的仿真结果与管内声速的理论值吻合得很好,同时实验测量结果与仿真值之间的误差很小,证明了方法的准确性以及鲁棒性,为声管声速测量提供一个很好的思路。     

A criterion for the use of linear regression analysis in the determination of low values of absorption coefficient using the impedance tube method

This article describes a quick and convenient method for estimating the percentage error associated with any measurement of absorption coefficient using the impedance tube method. A parameter Z is calculated, using constants appropriate to the tube and the decibel difference between a pressure maximum and minimum. The value of this parameter can be used as a criterion as to whether a more accurate determination, using linear regression, should be employed. The technique is valuable as an aid when low values of absorption coefficient, particularly at low frequencies, are to be measured.     

多阶声模态分解的改进阻抗管法测量材料的高频吸声系数

为了测量高频材料吸声系数,采用声模态分解的方法,基于阻抗管构建测试设备,在阻抗管内测量超过平面波截止频率的的高频吸声系数.测量过程中,通过在阻抗管的周向和轴向分别布置传声器阵列,分离管道内前3阶周向声模态以及各阶声模态的轴向传播入射波和反射波,从而得到最高频率达10000 Hz的材料吸声系数,并通过对比常规阻抗管测试方...     

充水阻抗管中测量材料声学性能的校准方法研究

提出了一种在充水阻抗管中,应用传递函数测量材料声学性能的三参数校准方法。在双传感器传递函数法的基础上,考虑充水阻抗管中水听器间不一致性和管内结构对声场局部扰动等因素的校准,导出了三个综合的无量纲校准参数。通过改变充水阻抗管中测量面的三个已知反射系数,测量出对应的三个传递函数,可以求出这三个校准参数。在充水阻抗管中分别对水/空气界面和一种水声材料进行了测量试验,应用该方法对测量结果的进行了校准,并对校准前后的结果进行了分析比较。      

Inverse estimation of the acoustic impedance of a porous woven hose from measured transmission coefficients

A porous tube, comprised of a resin-coated woven fabric has recently been used as an effective component for use in intake systems of internal combustion engines to reduce the intake noise. For the prediction of the acoustic performance of an engine intake system with a porous woven hose, the acoustic wall impedance of the hose must be known. However, the accurate measurement of the wall impedance of a porous woven hose is not easy because of its peculiar acoustical and structural characteristics. A new measurement technique is proposed herein, that is valid over the low to mid frequency ranges. The acoustics impedance is inversely estimated from an overdetermined set of measured pressure transmission coefficients for specimens of different lengths and the reflection coefficient of end termination. The method involves only one measurement setup, and, as a result, it is very simple. A variation of the proposed method, an inverse estimation method using one of the four-pole parameters is also proposed. An error sensitivity analysis was performed to investigate the effect of measurement error on the accuracy of the final result. The measured TL for samples with arbitrary lengths and arbitrary porous frequency are in reasonably good agreement with values predicted from curve-fitted impedance data.     

充水粘弹性管道的频散曲线计算分析*

针对谱方法分析计算充水粘弹性管道的广义特征值问题,根据Chebyshev多项式及微分矩阵、位移和应力连续条件,将波动方程离散为相应的线性方程。利用MATLAB数值编程计算充水弹性和粘弹性管道对应频率下的轴对称纵向导波频散曲线和衰减曲线。分析表明,波传播在粘弹性管道中不仅具有衰减特性,而且由于水和粘弹性壳体交叉耦合作用,在一定频率范围内产生两种截断模态。     

Inversion for acoustic impedance of a wall by using artificial neural network

A new approach for measuring acoustic impedance is developed by using artificial neural network (ANN) algorithm. Instead of using impedance tube, a rectangular room or a box is simulated with known boundary conditions at some boundaries and an unknown acoustic impedance at one side of the wall. A training data basis for the ANN algorithm is evaluated by similar source method which was developed earlier by Too and Su [Too G-PJ, Su T-K. Estimation of scattering sound field via nearfield measurement by source methods. Appl Acoust. 1999;58:261-81 (SCI) (EI)] for the estimation of interior and exterior sound field. The training data basis is constructed by evaluating of acoustic pressure at a field point with various acoustic impedance conditions at one side of the wall. Then, the inversion for unknown acoustic impedance of a wall is performed by measuring several field data and substituting these data into ANN algorithm. The simulation result indicates that the prediction of acoustic impedance is very accurate with error percentage under 1%. In addition, one field point measurement in the present approach for acoustic impedance provides more straightforward and easier evaluation than that in the two point measurement of impedance tube.     

Variable volume piezometer for measurement of volumetric properties of materials under high pressure

A piezometer was designed to measure volume change of foods and other biomaterials as a function of pressure, and calibrated and tested up to 700 MPa at 25°C. The sensor was constructed from a polycarbonate tube, sealed with a plug on one end and a removable piston on the other. Samples were placed inside the tube. Upon pressurization, the piston displaced inside the tube, transmitting pressure to the sample; impedance in a magnet wire coil on the outside of the tube was measured to characterize volume change. Sensor calibration with water yielded a polynomial that characterized volume as a function of impedance. Sensor precision is 0.3% of measured volume; its resolution allows measurement of sub-microliter volume changes. Testing with ethanol and sucrose solutions yielded data within 1% of previously reported values. The sensor's applications include measurement of volumetric properties of liquid and solid biomaterials, and chemical reaction volumes.     

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.     

Acoustic load on the ear caused by headphones

The standardized method for measurement of complex impedances according to ISO 10534 Part 2 is applied to the acoustic impedance of the ear with an "open-pinna" condition and with different types of headphones. The method is based on measurement of the transfer function of two microphone locations in an impedance tube and subsequent signal processing of the complex signal spectra. The termination of the tube is interpreted as ear canal entrance, while the measurement direction is, apparently, from "inside" the head towards outside. A tube which was specifically designed for this purpose works well, even though extremely small impedances must be measured. The impedances of the free pinna are similar to the "soft" end condition in the open tube, approximately following the radiation impedance of a piston into free space. The headphone impedances can be separated according to the type of headphone. In addition, the absolute impedances as the differences to the open ear compared with a number of headphones are interesting and may be starting point for further investigations. One possibility is, of course, quality control of headphones. The results are also expected to be useful for psychoacoustic research, for better understanding of sound perception, and for use in development of audio equipment.     

Measurement of the eardrum impedance of human ears

The determination of an acoustical impedance requires measurements of pressure and volume velocity. As no direct method is available for measuring velocity in an ear canal, a technique was developed which is based on pure pressure measurements. The ear canal is used as a measuring tube, the area function of which is also deduced from the pressure measurements. High-frequency measurements in living subjects involve many sources of errors. A criterion for deciding if a good measurement has been made is given. The technique of measurements is described, regarding both the use of probe tube microphones and the computer aided data recording. Finally, the results are presented, and some comments are given. A reliable interpretation of the results seems to be impossible because of lack of our knowledge of the middle ear function at high frequencies.     

Electromechanical impedance measurement from large structures using a dual piezoelectric transducer

This study develops a new electromechanical (EM) impedance measurement technique specifically for continuous monitoring of large structures. Conventionally, a single surface-mounted lead zirconate titanate (PZT) transducer is often used to measure the EM impedance of a coupled PZT-structure system for damage diagnosis. However, when the target structure is massive, the impedance measurement becomes challenging. In this study, a dual piezoelectric transducer, composed of two separate but concentric PZT segments, is used for effective measurement of the EM impedance from large-scale structures. The impedance measurement using the dual PZT is theoretically formulated and numerically verified. Then, a series of experiments are carried out on a laboratory-size specimen and full-scale bridge and building structures. The experimental results reveal that the proposed technique successfully measures the EM impedance signals from massive structures with a high signal-to-noise ratio (SNR) and good repeatability even when the conventional techniques fail to do so. At the same time, the proposed technique allows low-cost and fast measurement of impedance signals.     

Modeling and validation of polyurethane based passive underwater acoustic absorber

The acoustic behavior of an acoustically transparent polyurethane and an interpenetrating polymer network of polyurethane with polydimethyl siloxane were studied using dynamic mechanical analysis, finite element modeling, and experimental evaluation of acoustic properties in a water-filled pulse tube setup. Dynamic mechanical measurements in the temperature range -50?°C to +70?°C were carried out, and the data were used for time temperature superposition to generate material behavior at high frequencies. These inputs were used for modeling the acoustic behavior of these materials using ATILA, which is a commercial finite element code, capable of computing transmission and reflection characteristics of materials. From this data, absorption characteristics were computed. The results were compared with the experimental results obtained using a water-filled pulse tube facility.     

A comparison of impedance models for the inverse estimation of the non-acoustical parameters of granular absorbers

An impedance model relates acoustic properties of porous materials with non-acoustical parameters. Although such parameters can be measured, specific equipment is required for each of them, so that numerical methods have been proposed for estimating their value from a more manageable measurement of the normal incidence absorption coefficient in an impedance tube. This inverse procedure requires both an impedance model and an inversion technique. This paper compares four impedance models, Miki, Hamet–Berengier, Johnson–Allard–Champoux and Champoux–Stinson, when Simulated Annealing is used for the inverse estimation of the non-acoustical parameters of three granular materials, consisting of packings of small spherical glass beads of different diameters. Some of these parameters have also been measured, so that they can be compared with these estimated by the proposed inverse method.     

Far-infrared embedding impedance measurements

A technique which allows the measurement of detector embedding impedance has been developed. By using a bismuth microbolometer as a variable resistance load the impedance of one element in a bow-tie antenna array operating at 94GHz was inferred. The technique is frequency insensitive, and could be used throughout the far-infrared.Supported by the Department of Energy contract DE-AM03-765F-00010 Task IIA; Army Research Office contract DAAG29-82-K-0165; and the Jet Propulsion Laboratory through Dr. M. Litvak.