摘要:

We present a new method to calculate the diffraction of elasticSHwaves by irregular surfaces. The technique developed is applicable to boundaries of arbitrary shape and steepness as well as to periodically corrugated surfaces; it is valid at all frequencies. The approach consists of determining the surface forces which cancel the incident stress along the surface. The method relies on the introduction of a periodicity in the surface shape and on a discretization of the boundary at regular spacing. The surface sources which radiate the scattered wave field are obtained by iteration. Examples of calculations and comparisons with other methods are presented.

ISSN:0001-4966    

DOI:10.1121/1.392258

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The complex eigenfrequencies of impenetrable or penetrable target objects form a pattern which is characteristic for a given target, as far as its shape and/or composition is concerned; they manifest themselves as poles (resonances) in the amplitude of waves scattered from the object. We here obtain the eigenfrequency patterns of acoustically rigid and soft spheroids in the complex frequency plane, and study their displacements when the eccentricity of the spheroids is varied. The eigenfrequencies were obtained numerically by subjecting spheroidal wavefunctions to the Neumann or Dirichlet boundary conditions, respectively.

ISSN:0001-4966    

DOI:10.1121/1.391903

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A time‐domain energy theorem for the scattering of plane acoustic waves in fluids by an obstacle of bounded extent is derived. It is the counterpart in the time domain of the ‘‘optical theorem’’ or the ‘‘extinction cross section theorem’’ in the frequency domain. No assumptions as to the acoustic behavior of the obstacle need to be made; so, the obstacle may be fluid or solid, acoustically nonlinear, and/or time variant (a kind of behavior that is excluded in the frequency‐domain result). As to the wave motion, three different kinds of time behavior are distinguished: (a) transient, (b) periodic, and (c) perpetuating, but with finite mean power flow density. For all three cases the total energy [case (a)] or the time‐averaged power [cases (b) and (c)]that is both absorbed and scattered by the obstacle is related to a certain time interaction integral of the incident plane‐wave and the spherical‐wave amplitude of the scattered wave in the farfield region, when observed in the direction of propagation of the incident wave.

ISSN:0001-4966    

DOI:10.1121/1.392275

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

This paper is mainly concerned with an analytical study of multiple scattering of a plane sound wave by a group of circular, rigid cylinders oscillating in an ideal fluid. The objectives of the study are to investigate the phenomenon of multiple scattering due to the presence and the movement of many scatterers in the passage of sound waves and to determine the effects of multiple scattering and scatterers’ oscillations on the associated scattering functions. The fluid medium is assumed inviscid and compressible and the solid cylinders are assumed to move freely under the action of the sound waves. The acoustical radiation field caused by the translatory oscillations of the cylinders was taken into account for the correction to the main scattering caused by the mere presence of the rigid cylinders.

ISSN:0001-4966    

DOI:10.1121/1.392281

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Earlier results for coherent propagation of sound in correlated random distributions of two‐parameter particles of radiusa(with minimum separationb≥2asmall compared to wavelength λ=2π/k) are generalized to obtain the refractive and absorptive terms and the corresponding bulk parameters to order (ka)2. The present development includes higher order terms of the earlier multiple scattering by monopoles and dipoles, as well as scattering and multipole‐coupling effects through quadrupole terms. The correlation aspects are determined by the statistical mechanics radial distribution functionf(R) for impenetrable particles of diameterb. The new terms for slab scatterers and spheres involve the integral off R(first moment), or ofR f ln Rfor cylinders. The new packing factor is evaluated exactly for slabs as a simple algebraic function of the volume fractionw, and it is shown that the bulk index of refraction reduces to that of one particle in the limitw=1. Similar results are obtained for spheres in terms of the Percus–Yevick approximation and the unrealizable limitw=1.

ISSN:0001-4966    

DOI:10.1121/1.392271

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

This paper describes spectral analyses of echoes scattered by a random medium and a plane reflector by using mathematical expressions of echoes which have been proposed by us before [M. Ueda and H. Ichikawa, J. Acoust. Soc. Am.70, 1768–1775 (1981):75, 1012 (1984)]. It becomes possible to express the spectrum of echoes scattered by the random medium in terms of a backscattering coefficient of the random medium and a velocity potential of an ultrasonic transducer. The expressions hold for any sound field if a correlation length of the random medium is much smaller than a beamwidth and a radius of the transducer. Then the spectrum of the echo scattered by the plane reflector is expressed in terms of a directivity of the transducer. By using a ratio of the two spectrums an expression which gives the backscattering coefficient of the random medium is formulated. It works in a farfield of a flat circular transducer and at a focal plane of a concave one. It is shown that our formula contains those cases which have been proposed so far as the extreme cases of the conditions. Finally, the backscattering coefficients of sponges are measured with several transducers. The results show that our formula gives the coefficients regardless of the transducer geometry.

ISSN:0001-4966    

DOI:10.1121/1.391898

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An experimental investigation of Shaw’s theory of sound propagation in acoustic horns with spatially varying density and elasticity is reported. The density‐variation technique was used to simulate a conical horn in a tube of uniform cross section using perforated sheet metal plates. The performances of the density variation horn and the equivalent conical horn were evaluated by comparison of the acoustic impedance and the acoustic pressure within the horns. It was observed that the density‐variation conical horn had sound‐absorbing properties as a result of the use of the perforated plates. Due to the absorption properties of the density‐variation cone it can be used as a muffler. A muffler was designed and its performance evaluated using both a loudspeaker and an internal combustion engine as sound sources.

ISSN:0001-4966    

DOI:10.1121/1.391899

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The propagation of a cylindrical sound wave generated by a sinusoidal source is studied in a nonlinear case by means of a generalized Burgers’ equation with two dimensionless parameters. The amplitude of the wave at a large distance from the source, where all higher harmonics are absorbed, is determined in a region for the two parameters where the amplitude is given by a single number not depending on the parameters. The method is to find a series expansion of the solution of the generalized Burgers’ equation. The series converges both far from the source and in the Taylor shock region nearer to the source. Despite the slow convergence of the series in the latter region it is possible to find the desired amplitude by means of an asymptotic expression for the terms in the series and some numerical work.

ISSN:0001-4966    

DOI:10.1121/1.391900

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The application of the averaging method to the one‐dimensional inhomogeneous, nonlinear acoustic wave equation with dissipative term makes it possible to give asymptotic solutions for any kind of external resonance excitation. It shows that the lowest‐order solution consists of the superposition of two modulated counterpropagating waves, where the amplitude of each is a solution of Burgers equation. The method is extended to the treatment of oscillating boundaries; in that case it also leads to Burgers equations. Explicit stationary solutions are given for the particularly important forms of the external excitation, harmonic distributed forces, and harmonic oscillating boundaries. The application of several other computational methods to this problem leads to the same results.

ISSN:0001-4966    

DOI:10.1121/1.391901

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Measurements of the sound field from a point source above a relatively soft surface layer backed by an acoustically hard material are reported. These were made outdoors over layers of snow from 5 to 50 cm thick and to propagation distances of up to 15 m. The conditions of the snow layer varied from newly fallen to aged and windblown snow. Experiments were also conducted indoors at shorter distances above reticulated foam of similar thicknesses. The theory of Pyett [Acustica3, 375–382 (1953)] for the acoustic impedance of a porous layer is used to extend the solution for reflection of spherical waves from a porous half‐plane to reflection from a porous layer. This simple theory shows good agreement with the indoor measurements at all frequencies. Similarly good agreement between this theory and experiments over snow layers outdoors is obtained above about 200 to 500 Hz depending on the geometrical configuration. However below these frequencies the measurements suggest that the layer of snow behaves as semi‐infinite porous ground.

ISSN:0001-4966    

DOI:10.1121/1.391902

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP