Schlieren photography to study sound interaction with highly absorbing materials

Strong absorption of sound is often caused by the conversion of sound energy into heat. When this happens, it is not possible to study the interaction of sound with the absorbing material by means of reflected sound characteristics, because there is no reflected sound. Detecting for example the distance that sound travels in a strongly absorbing material, can be done by heat detection systems. However, the presence of temperature detectors in such materials interferes with the sound field and is therefore not really suitable. Infrared measurements are a possible option. Another option is the use of Schlieren photography for simultaneous visualization of sound and heat. This technique is briefly outlined with a 3 MHz sound beam incident on a highly absorbing sponge.     

Absorption of finite amplitude focused ultrasound

Predictions of the absorption of focused finite amplitude waves based on weak shock theory have been tested experimentally. The characteristics of this absorption are qualitatively different from those associated with small signal losses. Under appropriate conditions, the absorption of finite amplitude ultrasound is determined largely by source amplitude, field geometry, and the nonlinear properties of the medium and is only weakly dependent upon the small signal absorption coefficient of the material. These effects are seen most dramatically in sharply focused sound fields. To emphasize nonlinear absorption in an experimental test of these predictions, measurements of heating were made in agar which has a very small linear absorption coefficient. Under appropriate conditions, nonlinear losses can make the effective absorption coefficient of this poorly absorbing material somewhat greater than the soft tissues of the body.     

Prediction of sound absorption of a periodic groove structure with rectangular profile

Sound absorption of a periodic groove structure with rectangular profile is studied in this paper. On the basis of De Bruijn’s theory, by conducting matrix manipulation in MATLAB, a good prediction of the sound absorption of a periodic groove structure is obtained. Experiments in the Kundt’s tube are carried out and the results match the calculation well. This series of experiments and computation show that with the same period and the same ratio of absorber, groove structures have a much better sound absorbing ability than flat surface structures. Besides, a parametric survey has been done on the performance of the periodic groove structure and porous absorber as well. The parameters are the period of the surface, the ratio of the absorber, the depth of the groove, and the thickness, the flow resistivity, the open volume porosity and the structure factor of the porous absorber. Based on this survey, a periodic groove structure, as well as the porous absorber, with required sound absorbing capability can be created by arranging these parameters suitably.     

In situ measurements of surface impedance and absorption coefficients of porous materials using two microphones and ambient noise

An in situ measurement method is proposed for obtaining the normal surface impedance and absorption coefficient of porous materials using two microphones located close to the material without a specific sound source such as a loudspeaker. Ambient environmental noise that does not excite distinct modes in the sound field is employed as the sound source. Measurements of the normal surface impedance of glass wool and rockwool have been made using this method in various sound fields. The repeatability and wide applicability of the method are demonstrated by comparing results of measurements in one room with different noise conditions and in three other environments (corridor, cafeteria and terrace). The assumed diffuse nature of the sound field on the material is validated by using absorption characteristics obtained experimentally at oblique incidence. This method allows simple and efficient in situ measurements of absorption characteristics of materials in a diffuse field.     

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.     

Prediction of reverberation time using the residual minimization method

In many practical situations the assumption of sound field dispersion needed for the application of the Sabine’s theory is not fulfilled. In general, sound field is sufficiently dispersed if there are no large differences in the dimensions of the room, limiting partitions are not parallel, or the sound absorbing material is uniformly distributed. In practice, very few of these requirements are satisfied. As a result, a number of other formulas describing reverberation time have been created, for example Fitzroy’s or Neubauer’s formulas. However, these methods in many cases differ significantly from the actual measurements. The paper presents a method used to estimate reverberation time as well as its applicability potential involving laboratory models and auditorium rooms. The proposed method can be classified into a group of learning methods and involves the use of statistical methods which allow for approximation with the use of the least squares method.     

An axisymmetric poroelastic finite element formulation.

In the past, various two- and three-dimensional Cartesian, poroelastic finite element formulations have been proposed and demonstrated. Here an axisymmetric formulation of a poroelastic finite element is presented. The intention of this work was to develop a finite element formulation that could easily and efficiently model axisymmetric sound propagation in circular structures having arbitrary, axially dependent radii, and that are lined or filled with elastic porous sound absorbing materials such as foams. The formulation starts from the Biot equations for an elastic porous material expressed explicitly in axisymmetric form. By following a standard finite element development, a u-U formulation results. Procedures for coupling the axisymmetric elements to an adjacent acoustical domain are described, as are the boundary conditions appropriate for unfaced foams. Calculations described here show that the present formulation yields predictions as accurate as a Cartesian, three-dimensional model in much reduced time. Predictions made using the present model are also compared with measurements of sound transmission through cylindrical foam plugs, and the predicted results are shown to agree well with the measurements. Good agreement was also found in the case of sound transmission through a conical foam plug.     

Sound reduction by barriers on the ground

An existing theory on the propagation of spherical sound waves over ground with a finite acoustic impedance is to a large extent verified by field measurements over different types of ground surfaces. It is then shown that it is reasonable to combine this theory with ordinary diffraction theory to get a solution of the mixed problem with a barrier on the ground. Full-scale measurements have been carried out with a 3 m high wall and the results show good agreement with the predicted values. It is also shown that it is generally of minor importance whether a thin screen is sound absorbing or not.     

一种现场测量材料吸声特性的新方法

现有的材料吸声系数测量方法主要有混响室法和驻波管法,都属于实验室测量方法,不适合现场测量。使用普通扬声器的反射法可以对材料的吸声特性进行现场测量,但是对材料尺寸和测试环境有较高的要求。本文利用参量阵非线性自解调可听声的高指向性和在阵长距离内的平面波特性,结合传递函数法,测量材料的吸声系数,并与传统驻波管测量结果进行了对比。结果表明在普通房间条件下,不需要驻波管,混响室等实验环境,即可对小尺寸的材料进行吸声系数的现场快速测量,具有较大的实用性。      

Acoustical properties of double porosity granular materials

Granular materials have been conventionally used for acoustic treatment due to their sound absorptive and sound insulating properties. An emerging field is the study of the acoustical properties of multiscale porous materials. An example of these is a granular material in which the particles are porous. In this paper, analytical and hybrid analytical-numerical models describing the acoustical properties of these materials are introduced. Image processing techniques have been employed to estimate characteristic dimensions of the materials. The model predictions are compared with measurements on expanded perlite and activated carbon showing satisfactory agreement. It is concluded that a double porosity granular material exhibits greater low-frequency sound absorption at reduced weight compared to a solid-grain granular material with similar mesoscopic characteristics.     

The diffraction of sound by an impedance sphere in the vicinity of a ground surface

The problem of sound diffraction by an absorbing sphere due to a monopole point source was investigated. The theoretical models were extended to consider the case of sound diffraction by an absorbing sphere with a locally reacting boundary or an extended reaction boundary placed above an outdoor ground surface of finite impedance. The separation of variables techniques and appropriate wave field expansions were used to derive the analytical solutions. By adopting an image method, the solutions could be formulated to account for the multiple scattering of sound between the sphere and its image near a flat acoustically hard or an impedance ground. The effect of ground on the reflected sound fields was incorporated in the theoretical model by employing an approximate analytical solution known as the Weyl-van der Pol formula. An approximation solution was suggested to determine the scattering coefficients from a set of linearly coupled complex equations for an absorbing sphere not too close to the ground. The approximate method substantially reduced the computational time for calculating the sound field. Preliminary measurements were conducted to characterize the acoustical properties of an absorbing sphere made of open cell polyurethane foam. Subsequent experiments were carried out to demonstrate the validity of the proposed theoretical models for various source/receiver configurations around the sphere above an acoustically hard ground and an impedance ground. Satisfactory comparative results were obtained between the theoretical predictions and experimental data. It was found that the theoretical predictions derived from the approximate solution agreed well with the results obtained by using the exact solutions.     

Measurement of sound field in cavitating media by an optical fibre-tip hydrophone

A fibre-optic technique was applied to measure the sound field in an ultrasonic cleaning vessel under practical conditions. A metal-coated fibre-tip is used as a sensor and a heterodyne interferometer detects the change in the optical path resulting from the movement of the fibre-tip in the sound field. Spectrally resolved sound field parameters such as the fundamental, the subharmonic or cavitation noise are extracted from the measurements and compared with results obtained by a piezo-electric hydrophone. It was found that the fibre sensor provides a signal related to the velocity in the sound field, but the information about cavitation-related parameters is similar to the information for pressure sensing techniques. The fibre-optic sensors have a uniquely high spatial resolution and the sound detection process is strongly influenced by single cavitation events close to the small fibre-tip. This paper shows that fibre-tip sensors are an alternative to common hydrophone techniques. They can open up new possibilities for measurement problems for which so far no solution exists, in particular when a high spatial resolution is required or when the measurement site is small.     

高分子颗粒孔隙结构材料的吸声特性研究

在噪声控制与治理工程中，许多场合了消除由物体表面产生的反射声波，往往需要在物体表面敷设具有吸声性能的材料或结构，以吸收入射声波的声能，本文对一种新型的以高分子材料为基底的颗粒型微孔结构进行了吸声特性研究，首先对颗粒微孔结构的吸声机理进行分析，认为由于受材料自身化学性质与结构特征所决定，这种颗粒微孔结构除了具一般多孔吸声材料的空气粘滞阻力作用吸声外，还存在颗粒材料的内部的弹性弛豫效应吸声，因而吸怕系     

The use of thermistor probes to measure energy distribution in ultrasound fields

The use of small thermistors coated with absorbing material for the measurement of ultrasound fields is discussed. Equilibrium temperature rise gives a rough measure of intensity, but is affected by convective cooling due to streaming. The initial rate of temperature rise provides an accurate parameter for use in assessing energy density in all types of ultrasound field. Measurements made in standing wave fields show that the thermistor response is related to the pressure component of the sound wave, and is determined by absorption throughout the coating material.     

Analogous circuits for plastic foams

Presented here are simple analogous circuits derived from Biot theory which allow predictions of the acoustical properties of low flow resistance and very high flow resistance foams.For different foams with low resistance absorption coefficients measured in a Kundt tube have been compared with the predicted ones.For foams with very high flow resistance only the frequency of the first normal incidence absorption peak for a large panel backed with a rigid layer has been measured. Kundt tube measurements being inaccurate because of sample frame vibrations which appear with this kind of foam, the experiment was done in an anechoic room by studying the stationary waves close to the foam.For the two kinds of foams studied, comparison between experimental results and predictions show that simple analogous circuits may supply useful estimations of the trend of absorption.     

奥古斯特·孔脱：第一流的实验物理学家

奥古斯特·孔脱是 19世纪德国第一流的实验物理学家 .他首创测量声速的方法 ;首测单原子气体的热容比 ,并最早发现了气体的法拉第效应等 .在声学、光学和气体动力学实验研究方面作出了诸多重要贡献 ,创造了许多卓有成效的实验方法 .文章就孔脱的生平、业绩、学术思想等作了较为全面的评介 .     

Radiation transfer in absorbing-scattering liquids—II. Comparisons of measurements with predictions

Comparisons have been made between measured and predicted results for the radiation field in both highly absorbing and scattering aqueous suspensions with a highly reflecting or absorbing bottom. Predictions were based on a discrete ordinate solution to the equation of transfer, with the requisite properties (extinction coefficient, absorption coefficient and scattering phase function) obtained from measurements performed on samples extracted from the suspensions. All trends in the data were predicted by the model, although poor numerical agreement was obtained for suspensions of large albedo and extinction coefficient. The disagreement is attributed to the effect of multiple scattering on the property measurements.     

An experimental study on the acoustic absorption of sand panels

Potentially sand panels could be used as novel sound absorbing materials that are fire resistant, environmentally friendly, mechanically strong and have good durability. However, the performance of sand panels as sound absorbers has not yet been studied. Results of measurements in a reverberation chamber of the random-incidence absorption coefficients of 13 different sand panel compositions and configurations with air gaps are reported. Also the flow resistivities and bulk densities have been measured. The results prove that sand panels could offer effective and wide-band acoustic absorption. As is the case with conventional sound absorbing materials, adding an air space is found to be the most effective way to widen the absorption bands and improve the overall absorption. Comparisons of the measured sand panel absorption data with predictions of the Delany and Bazley and Voronina models reveal that, while neither model is very accurate, the former gives more accurate predictions especially for sand panels with lower flow resistivity and smaller thickness.     

Sound absorption characteristics of a double-leaf structure with an MPP and a permeable membrane

As for the sound absorbing system using an MPP (microperforated panel), a double-leaf MPP sound absorber has been studied so far. However, this structure uses two MPPs, which are still expensive, and is disadvantageous when its cost is concerned. Therefore, it is considered that it can be advantageous if one of the leaves can be replaced with a less expensive material keeping high sound absorption performance. In this study, the possibility of producing a useful sound absorbing structure with an MPP and a permeable membrane as an alternative less expensive material is examined. The acoustic properties of this MPP and permeable membrane combination absorber are analysed theoretically with a Helmholtz integral formulation. The absorption performance and mechanism are discussed through the numerical examples. Also, the effect of a honeycomb in the air cavity, which is to be used for reinforcing the structure, is also discussed through a theoretical analysis.     

The prediction of speech intelligibility in underground stations of rectangular cross section

Long enclosures are spaces with nondiffuse sound fields, for which the classical theory of acoustics is not appropriate. Thus, the modeling of the sound field in a long enclosure is very different from the prediction of the behavior of sound in a diffuse space. Ray-tracing computer models have been developed for the prediction of the sound field in long enclosures, with particular reference to spaces such as underground stations which are generally long spaces of rectangular or curved cross section. This paper describes the development of a model for use in underground stations of rectangular cross section. The model predicts the sound-pressure level, early decay time, clarity index, and definition at receiver points along the enclosure. The model also calculates the value of the speech transmission index at individual points. Measurements of all parameters have been made in a station of rectangular cross section, and compared with the predicted values. The predictions of all parameters show good agreement with measurements at all frequencies, particularly in the far field of the sound source, and the trends in the behavior of the parameters along the enclosure have been correctly predicted.