Calculation of acoustic scattering of a nonrigid surface using physical acoustic method

In this paper the physical acoustic method or the Kirchhoff approxima-tion is extended to treat the scattering of a nonrigid surface in order to estimatethe target strength of targets with absorbing coatings.By using the locally planewave approximation,the relationship between the sound pressure and its normalderivative on the surface can be represented by the plane wave reflectioncoefficient and the acoustic impedance of the surface.The resulting modifiedKirchhoff approximation involves the plane wave reflection coefficient.For aimpedance sphere,a comparison between the physical acoustic method and theexact solution shows that the physical acoustic method still is a good approxima-tion at higher κα values.     

The study of underwater acoustic communication technology based-on the acoustic vector sensor

Underwater acoustic (UWA) communication based on an acoustic vector sensor is studied. The method of joint weighted sound pressure and velocity processing is used in phase modulation high-speed UWA communication system combined with coherent demodulation and adaptive equalization algorithm to demodulate and decode. Whereas the sound intensity could be used instead of pressure for frequency decoding in frequency modulation UWA communication system. The results of theory analysis, simulation calculations and lake trials have shown that either in phase modulation or in frequency modulation UWA communication system, the processing gain can be evidently increased, so that the BER (bit error rate) can be effectively reduced and the telemetry distance can be enlarged by using the acoustic vector sensor.     

Blind estimation of shallow water acoustic channel

This paper proposed a method for blind estimation of underwater channels in shallow water environment by using received data at a single hydrophone or from single beam. First,the received signal is used for source signal reconstruction by means of signal-dependent TF(Time-Frequency)distribution,in association with instantaneous frequency estimation and TF inversion.Then the shallow-water channel estimation is achieved via WRELAX technique by use of the received signal and the estimated source signal.Finally,the results of numerical simulation and experimental test from real data taken in South China Sea trial have proved satisfactory.It is shown that the proposed method is useful for underwater channel estimation.     

Computer simulation of acoustic nonlinear parameter tomography

I.IntroductionUltrasonicimaginghasbcenwide1yusedinthcfie1dofclinicaldiagnosis,becauseitcanvisualizethetissuetharacterizationandinternalstructureofbiologicalobjectsbyacousticwave.Usingconventionalultrasonicimagingtechnique,theimagesofacousticlinearparameterssuchassoundve1ocity,acousticimpedenccandattenuationcoefficientmaybeobtained.Thesehavebecometheeffectivemethodsofu1trasonicdiagnosis.How-ever,inordertoobscrvctheearlystageofcanccr,weintendtoobtainmoreaccurateandmorecompleteinformationaboutth…     

Theoretical analysis of acoustic suspended sediment observation

1 IntroductionSince 1980 s the obserVations of suspended sedimellts based on acoustical back-scatteringhave been applied to the studies of sediment dynamics in the estuary and coastal area and tothe monitoring of the marine pollutant and plankton[1--lol. Compared with the clajssical watersampling and the optical obserwtions, the acoustical ObserVation can continuously observe thein-site sediment concentration profilers along the water depth and their variation with timewithout disturbing the …     

Inversion of acoustic mode coupling coefficient matrix

It is found that the normal mode amplitude time series consist of multi-frequency component by analyzing the structure of acoustical signal when internal wave propagation exists, and each frequency is the product of internal wave speed and the normal mode wave number difference between acoustical receivers and source. The amplitude of each component is proportional to the acoustic mode coupling coefficient. The structure of the normal mode coefficient time series is still complex even the internal waves do not reshape when they propagate from the acoustical receivers to the source. A method is presented to compute the AMCCM by the feature of IWs' motion and the relation between the AMCCM and the acoustical signal fluctuation amplitude. The IWs data measured in the 2001 Asia experiment (ASIAEX2001) is used to check the accuracy of this method by numerical simulation. It is show that the method is accurate to compute the AMCCM.     

Torsional waves excited by electromagnetic–acoustic transducers during guided-wave acoustic inspection of pipelines

A theory of propagation of torsional waves excited by an electromagnetic–acoustic transducer in a pipe is proposed. This theory takes into account the excitation parameters, geometry, viscosity, and the elastic characteristics of an object. The main testing parameters (the frequency and geometry of the transducer) that determine the possibilities of guided-wave testing of pipelines of various dimensions using torsional waves are theoretically substantiated.     

Experimental study of the acoustic characteristics of micro-perforated functional absorbers

Micro-perforated panels have been widely used as fiber-free acoustical material for decades in the form of wall or ceiling covering with some air space behind. This paper presents the test study on the acoustical characteristics of two different types of functional absorbers, panel type and tube type, constructed with micro-perforated metal or PVC (polyvinyl chloride) sheets. Acoustical measurements of such functional absorbers in reverberation chamber demonstrate the merit of good absorption as expected. They are not only cost effective in construction and installation, but also appealing esthetically for architectural interior design.     

Design of sandwich acoustic window for sonar domes

Aimed at the low noise design of sonar dome in ships, a method has been presented for calculating the sonar self noise of a simplified sonar dome consisting of sandwich acoustic window and parallel acoustic cavity, which is excited by stationary random pressure fluctuation of turbulence boundary layer, using temporal and spatial double Fourier transform and wavenumber-frequency spectrum analysis. After numerically analyzing the influence of geometrical and physical parameters of acoustic window on the sonar self noise, the design method and reasonable parameters for sandwich acoustic window are proposed. The results show that the property of low noise induced by acoustic window of sandwich is dominated by the cut-off effect of longitudinal wave and transverse wave propagating in the visco-elastic layer of sandwich as well as the mismatch effect of impedance. If the thickness, density, Young‘s modulus and damping factor of plates and visco-elastic layer as well as the sound speed of longitudinal wave and transverse wave in the visco-elastic layer are selected reasonably, the maximum noise reduction of sandwich acoustic window is 6.5 dB greater than that of a single glass fiber reinforced plastic plate.     

Study on the acoustic emission-model of concrete damage

I.lntroductiollBecausetherearethedefects.cracksandnon-homologousmicrostructurewithdifferentpropertiesincollcrete,whenaloadisputonconcretematerialthedamageandbreakagemayresultandsoonafterwardsexpanded.Itisadifficutproblem,howtodecidethedamageofconcretematerialwithcracksanddefects.TheacousticemissionisanassociatedphenomenollwiththeprocessofthenonhomologousdeformationorthebreakaeandexPansiollofmaterial.InthispaPertherelationshipbetweentheacousticemis8ionparametersandthedamageparameters,andthedam…     

Research on modal analysis of structural acoustic radiation using structural vibration modes and acoustic radiation modes

Modal analysis of structural acoustic radiation from a vibrating structure is discussed using structural vibration modes and acoustic radiation modes based on the quadratic form of acoustic power. The finite element method is employed for discretisizing the structure. The boundary element method and Rayleigh integral are used for modeling the acoustic fluid. It is shown that the power radiated by a single vibration mode is to increase the radiated power and the effect of modal interaction can lead to an increase or a decrease or no change in the radiated power, moreover, control of vibration modes is a good way to reduce both vibration and radiated sound as long as the influence of interaction of vibration modes on sound radiation is insignificant. Stiffeners may change mode shapes of a plate and thus change radiation efficiency of the plate‘s modes. The CHIEF method is adopted to obtain an acoustic radiation mode formulation without the nonuniqueness difficulty at critical frequencies for three-dimensional structures by using Moore-Penrose inverse. A pulsating cube is involved to verify the formulation. Good agreement is obtained between the numerical and analytical solutions. The shapes and radiation efficiencies of acoustic radiation modes of the cube are discussed. The structural acoustic control using structural vibration modes and acoustic radiation modes are compared and studied.     

Ultrasonic evaluation of creep damage of Cr-Mo-V steel by acoustic microscopy

The evaluation of creep damages of Cr-Mo-V steel used in the gas turbine of power plant has been carried out using the line-focus-beam (LFB) acoustic microscopy. The velocity of leaky surface acoustic wave (LSAW) propagated near the specimen surface was used for the quantitative evaluations, which was measured by LFB acoustic microscopy using the v(z) curve analysis method. It has been shown that with the increase of creep damage fraction of metal material the LSAW velocity decreases obviously, which suggests the possibility of nondestructive and quantitative evaluations of creed damages in metal materials by acoustic microscopy.     

Evaluation of corrosion damage of aluminum alloy using acoustic emission testing

Current studies are aiming at monitoring corrosion damage of aircraft main structures by using acoustic emission (AE) technique and at supplying useful data for determining calendar life of the aircraft. The characteristics of AE signals produced during accelerating corrosion process are described, and methods for evaluating corrosion damages and determining remaining life of main structures of aircraft using AE testing are outlined. Experimental results have shown that AE technique can detect corrosion damage of aluminum alloy much earlier than conventional non-destructive testing means, such as ultrasonic testing and eddy current testing. Relationship between corrosion damage and AE parameters was obtained through investigating corrosion damage extent and changes of AE signals during accelerating corrosion test, and showing that AE technique can be used to detect early corrosion, investigating corrosion developing trend, and in monitoring and evaluating corrosion damages.     

A study of computations for acoustic velocities of arbitrarily dipping layers

Through deriving expressions relating the dip-angle((?)_m)ofthe lower boundary of a layer to the acoustic velocity (v_m) of the layerand other pre-determinable parameters,v_m and (?)_m can be taken assimultaneously iterative variables while solving Shah‘s equations.Consequently the previous method of computing v_m and (?)_m presented byZHANG S.is improved,and the accuracy of solutions increasedgreatly.     

Effect of the spatial structure of an acoustic field on Bragg’s acoustooptic diffraction under strong acoustic anisotropy conditions

Bragg’s acoustooptic diffraction in an acoustically anisotropic medium is considered taking into account the two-dimensional spatial diffraction structure of the acoustic beam. The conditions are determined under which reverse transfer of optical power from the diffracted to the transmitted beam in the regime of 100% efficiency of diffraction is considerably suppressed. It is shown that this effect is due to diffraction bending of wave fronts of the acoustic beam in the acoustooptic diffraction plane. The problem of optimization of the piezoelectric transducer size and the spatial position of the input light beam is solved using the criterion of the minimal required power of the acoustic field. The results of simulation in a wide range of the acoustooptic interaction parameters for a Gaussian light beam are reported. The correctness of the model is confirmed experimentally. Recommendations for designers of acoustooptic devices are formulated.     

An investigation on the acoustic characteristics of microperforated muffler

I.lntroductionTheac0usticperformanccofmicropcrforatedmumerhasbeengreatlynoticedinre-ccntyears.Especially,itshighsi1encingva1ueandbroadsi1encingfrequencyrangeenableittobeusedwidelyinmanyyiclds,suchasvehicleexhaustsystem,ventilator.Butitisdifficulttodesignagoodsilcnccrbecauseoritscomp1exacousticperformanccwithinPerforatedtubes.Thegoverningwaveequationofmicroperforatedmufflerisnotlinearduetothenonuniformmassflowofgasaswc11asthetcmpcraturegradientalongthePerforatedducts.Inordertoutilizethegoodsi…     

A prediction of the acoustic field of a vibrating box by FEM-BEM

I.Mathematicalmodelofacousticfie1dByapplyingtheSommerfeldradiationcondihonThus,anacousticfic1disdescribcdbyHelmholtzformula.Byconsideringaficldpointinthearea,Helmholtzformulaisgivenbythefollowingthreeintegra1equations:(1)Theexternalintcgra1fOrmu1a'(2)Thcintcrnalintegralformu1af(3)Thesurfaceintegralformu1afwhcreP.isthesoundpressureonthcSsurface;rthedistancefrompointqtothee1ementsurfacedSiqtheficldpoint,tuthefrcqucncyoftheacousticradiation;pthedensityofthesurroundingmedium,vsthenormalvibrati…     

A study of noise reduction by acoustic shock waves

I.IntroductionAcousticshockwavcs(ASW)isanimportantphcnomcnoninnonlinearacoustics.Experimentalrcsultshavcshownthatwhenanaircraftcngincinletopcratesneartheson-iccondition,vcrystrongnoisegcncratedbythcfanscanbcreduccdgreat1yowingtothcformationofASWatthcthroatofthcin1etll].ASWisadiscontinuityofacousticvaria-bles,whichisdifTcrcntfromthcshockwavesoccurringinhighspcedsteadyflowinducts.Theformer'sintensityismuch1cssthanthelattcr's.Furthcrmorc,thepositionandintensityofASWisalwayschangedwithtime.l…     

The specific acoustic impedance of aperture on cylindrical surface

The acoustic impedance caused by sound radiation at the terminals ofthe aperturc on cylindrical surface is analyzed based upon rigorous soundtheory.In low frequency range,the approximate solution of the specific acousticresistance is derived.Special treatments are introduced in order to simplify the so-lution of the specific acoustic reactance.It is verified that the specific acousticreactancc of such an aperture is equal to that of a similar aperture on the infinitelylarge plane when the small quantities of high order are neglected.     

A study of measuring acoustic velocities of thin and hard sea-bed layers

It has been known that the error of measuring acousticveloicities of thin sediment layers by the well-known T~2-X~2 approach isusually untolerable,and that this approach is unavailable in the casewhere sea-bed is hard because no echo from any subsurface belowsea-bottom can be received.Therefore applying the ray-parametermethod to thin layers and the refraction.method to hard layers need tobe considered in an acoustic velocity measurement system composed of asound source and a towed hydrophone streamer.Some problems of practi-cal importance about the applications of the two methods,such asecho-data processing procedures and error estimations in measuring acous-tic veloicities,are discussed,and the effectiveness of theoretical analyseshas been verified through computer simulations.