THE IDT WITH HIGH INTERNAL REFLECTION SUPPRESSION

Based on Chen-Haus′theory,an experimental research on the reflection of SAW from ashort-circuited Au-grating on YZ LiNbO_3 and a short-circuited Al-recessed groove grating on ST-Xquartz is first presented in this paper where an anomalous reflection phenomena existing in two gratings areconfirmed.In addition,a residual reflection which couldn′t be completely cancelled by means of changingstrip-thickness is pointed out for a short-circuited Au-grating on YZ LiNbO_3 both experimentally andtheoretically through extending Chen-Haus′theory to anisotropic crystals.Finally.two kinds of IDT composed of single electrodes:Au-electrode on YZ LiNbO_3 and Al-recessed-groove-electrode on ST-X quartz,are proposed for high internal reflection suppression.The experimental results of bandpass filters constructed with the IDTs mentioned above show a great im-provement in passband distortion and a potential usefulness of new structure in high frequency range.     

AN INVERSION METHOD FOR OBTAINING BOTTOM REFLECTION LOSS

A method for the inversion of the transmission losses for the bottom reflection loss is proposed on thebasis of the theory of the smooth-averaged sound field.The procedure of the inversion is based on the cri-terion of the least square error in the transmission losses between calculation and measurement.By using theGauss-Newton iterative approach,the non-linear least square aloqrithm is equivalent to solving a se-quence of lineared least square problems.The physical causes of the instability of the invertion problem arediscussed and the stability is improved by means of the Levenberg-Marquardt method.Both numericalsimulations with noise and experimental results show that the inversion for the bottom reflection loss ofsmall grazing angle has high precision and the certain perturbation in the measured transmission losses doesnot lead to serious deviation in the inversion result of the bottom reflection loss.     

REFLECTION AND REFRACTION OF PLANE SOUND WAVES IN MOVING STRATIFIED MEDIA

This paper deals with the fundamental problems concerning the propagation of plane soundwaves in moving stratified media.Starting from the wave equation in moving media,we haveaccomplished a systematic study of reflection and refraction of the waves at the interface between twomoving homogeneous media under the assumption that the Mach numbers of the motion are smallcompared with unity.The coefficients of reflection and transmission as well as the equation for thetotal reflection cone are obtained. Similar treatment is extended to a slowly varying stratified moving medium,and the W.K.B.solu-tion and the successive modifications to the sound field are worked out.Following the approachdeveloped by Gans,we have investigated the field within the total reflection zone where the geometricalacoustics is no longer valid,and obtained an appropriate solution which can be connected with thegeometrical acoustics solution at the boundaries of that zone.     

NORMAL REFLECTION AND TRANSMISSION OF A LARGE AMPLITUDE PLANE SOUND WAVE

Using the method of successive approximation,the reflection and transmission of a large planesound wave are treated,as the sound wave impings normally on the plane interface of two acousticallydifferent media.     

A NEW CEPSTRUM TECHNIQUE FOR CANCELLING THE EFFECTS OF SOUND REFLECTION

For correct measuring the acoustic signal in situ,it is necessary to eliminate the disturbance caused byreflection.In this paper,a new cepstrum technique for cancelling the effects of reflection is offered.Aftercomputer simulations,a series of experiments in an anechoic chamber were carried out.The results of bothcomputer simulation and experiments show that the offered technique is effective.     

REFLECTION OF SOUND PULSE BY PLANE INTERFACE BETWEEN AIR AND POROUS MEDIUM

This paper presents a theoretical analysis of the reflection of a sound pulse by the plane interfacebetween air and ideal porous medium.A general expression of impulse response for a pulsed pointsource is derived.The detailed analysis is presented for three special cases:1)source and receiverlocated on a line perpendicular to the interface,semi-infinite media;2)a layer of porous medium;3)semi-infinite media,grazing incidence.In the second case,the problem is solved by consideringthe multi-reflections in the layer of porous medium.Experimental results are compared with the-oretical calculations for a kind of porous material.     

A STUDY OF THE SOUND REFLECTION FROM POROUS MATERIAL BY IMPULSE METHOD

This paper presents a study of the reflection of sound pulse from porous material when the im-pulse sound source and receiver are located on a line perpendicular to the surface of the material.The transfer functions of reflected sound waves were calculated from the experimental incident and thereflected sound pulses by use of DFT.The impulse response of reflected sound waves can be calculatedtheoretically under the assumption of idealized porous material.Both local reacting and extendedreacting materials were taken into account in the calculations. comparison of the calculationsfor spherical waves and plane waves provides a measure of the effect of spherical waves on thetransfer function of reflected sound waves.For a layer of porous material the reflections in thematerial are studied.     

THE REFLECTION AND REFRACTION OF A LARGE AMPLITUDE PLANE SOUND WAVE IN TWO DIMENSIONS

By using the successive approximation method,a wave equation,in which the second order ofapproximation is taken into account,has been derived from the basic equations of hydrodynamicsin Lagrangian coordinates.The reflection and refraction of a large amplitude plane sound waveon the plane interface of two non-mixing liquids are studied.It is interesting to point out that thereflection and refraction coefficients of second approximation not only depend on parameters ofmedia,but also relate to the position of sound source.It is found that the amplitudes of the secondorder reflected wave and refracted wave do not keep constant at the wavefront anymore as inlinear acoustics.Hence there are reflected wave and refracted wave which propagate in the directionother than that of waves,travelling along the directions defined by laws of reflection and refractionin linear acoustics.The nonlinear effects have to be taken into account in order to understand thephysical meaning of the results reported by Muir et al..     

INFERENCE OF SEA-BOTTOM IMPEDANCE PROFILE FROM THE REFLECTION OF NORMALLY INCIDENT PLANE SOUND WAVES

The inverse method to infer the profiles of acoustical parameters under the sea-bottom from exactsolutions of the wave equation is generally not applicable to situations where discontinuity exists in theacoustical impedance profile.However,the ray approximation is still effective,and satisfactory results havebeen obtained under some practical circumstances. In this paper a scattering-inverse method is presented based on the equithickness(measured in thepropagation time)division of the subbottom medium.The program is capable of judging the existence ofmultiple reflectio is and elimnating their influence.The problem of the occurence of pseudo boundaries arising from multiple reflections and the computation errors caused by them have been preliminarily solved.     

DETERMINATION OF REFLECTION TIME DELAY AND CRITERION OF CORRECT OVERLAP IN ULTRASONIC WAVE VELOCITY MEASUREMENT WITH PEO METHOD

The Pulse-Echo-Overlap method(PEO method)has now been considered as the most adequatemethod for accurate measurement of the absolute value of ultrasonic velocity.For this accurate timemeasurement,it is essential to find the time delay and implement its corrections,e.g.the correction ofreflection time delay,the correction of diffraction time delay,etc.To overcome the inconvenience of thewidely used in practice method of reflection time delay calculation,and its imperfection in theory,a newcalculation procedure of the reflection time delay correction is worked out in this paper,and an experimentaldetermination method for its value by means of double crystal is suggested.This paper further suggests aparameter of the period increment of zero phase shift Δτ_r,and considers it as an experimental constant,whose value is about 1.7 ns in practical applications. Furthermore,this paper suggests a criterion of correct overlap.If the echoes are correctly overlapped,the slope of the curve of the ultrasonic wave tri