摘要:

Low‐frequency coherent scatter from a rough surface may be conveniently investigated using a linear boundary condition applied to a smoothed surface, of the form ∂φs/∂z=ηφ, where φ, φsare solutions of the wave equation representing, respectively, the total and scattered field potentials. The validity of the theories discussed here is restricted tokh≲1, wherekis the wavenumber andhthe mean spacing between roughness elements. The constant η is a function of frequency, angle of incidence, and type of roughness; in the general case of scatterers distributed isotropically over an interface between two fluids it is sensitive to eight physical parameters. Methods of calculating η for various types of rough boundary are discussed, and comparisons are made—notably between the boss and the stochastic perturbation models. Also examined are interesting implications of the smoothed boundary conditions; e.g., theboundarywavewhich is a true propagating mode corresponding to energy trapped in the vicinity of a rough surface, and which is only generated by a source near this surface (it thus differs fundamentally from the evanescent modes of a diffraction grating which may be excited by plane waves and are therefore not true boundary modes). For source and receiver near the boundary, and for negligible incoherent scatter, the farfield amplitude of the boundary wave may exceed that of the direct (normal) acoustic arrival—a fact which has been verified experimentally in model work. Incoherent scatter introduces an attenuation factor exp(−δr), whereris the source–receiver distance and δ is proportional to the sixth power of the frequency.

ISSN:0001-4966    

DOI:10.1121/1.390395

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The study presented here introduces a novel methodology to predict the (frequency‐dependent) effective material parameters characterizing the dynamic behavior of viscoelastic substances containing many randomly located air‐filled perforations. These composite materials have uses as underwater sound absorbers. The methodology described here is an extension of our earlier work which pertains to the case of gas‐filled perforations in nonabsorbing matrices. [G. Gaunaurd and H. Überall, J. Acoust. Soc. Am.71, 282–295 (1982)]. That prior work was extended here to the case of absorbing matrices containing ensembles of cavities of various sizes following several arbitrary size‐distribution functions. The method accounts for the effect of resonances, for arbitrary levels of viscosity, for arbitrary cavity‐size distributions, and it is fundamental insofar as it generates direct predictions accounting for all these effects starting straight from the basic principles of Continuum Mechanics. Computer codes to implement the model predictions were generated, and a large number of pertinent plots of the frequency dependence (at fixed concentrations) or of the concentration dependence (at fixed frequencies) of the various effective moduli and other material descriptors have been computed and displayed in many graphs. Under various conditions the present results reduce to many of the earlier results available in the literature which serve as checkpoints. The various plots generated here pertain to various chosen cavity‐size distribution functions and to various selected levels of (dilatational and shear) absorption in the matrix. The generation and display of graphs such as these permit the present analysis of these matrix‐viscosity and size‐distribution effects mentioned in the title.

ISSN:0001-4966    

DOI:10.1121/1.390401

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A spheroidal‐coordinate‐based transition matrix is derived for acoustic and elastic wave scattering. The formalism is based on Betti’s third identity and an appropriately chosen set of vector spheroidal basis functions. Transition matrices are obtained for the scattering from an elastic inclusion in an elastic medium and in an inviscid fluid.

ISSN:0001-4966    

DOI:10.1121/1.390297

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Predictions based on the geometrical theory of diffraction for sound radiation from a source within an open rigid rectangular box are compared with a set of experimental and numerical results obtained by Furue, Terai, and Matsu’ura (9th International Congress of Acoustics, Madrid, Spain, July 1977). The comparison with the experimental results shows a substantial verification of the theory.

ISSN:0001-4966    

DOI:10.1121/1.390298

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Magnetoelastodynamic stress intensity factors are computed for diffraction of normally incident longitudinal waves by two coplanar Griffith cracks in a soft ferromagnetic elastic solid. The solid is permeated by a uniform magnetostatic field normal to the crack surface. The problem is formulated by means of integral transforms, and reduced to the solution of a singular integral equation of the first kind. Numerical calculations are carried out and stress intensity factors are obtained for several values of frequency, magnetic field, and geometrical parameter.

ISSN:0001-4966    

DOI:10.1121/1.390299

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Approximate expressions are given for the characteristic impedance and propagation wavenumber for linear acoustic transmission through a gas enclosed in a rigid cylindrical duct. These expressions are most complicated in the transition zone where the thermoviscous boundary layers are on the order of the tube radius. The approximations are accurate to within 1% for all frequencies and tube diameters except within the transition zone where the approximations are accurate to within 10%. A simple modification of the transmission line parameters is presented for the case where the tube walls are nonisothermal.

ISSN:0001-4966    

DOI:10.1121/1.390300

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

It is often useful to know the radiation impedance of an unflanged but thick‐walled circular duct exhausting a hot gas into relatively cold surroundings. The reactive component is shown to be insensitive to temperature, but the resistive component is shown to be temperature dependent. A temperature correlation is developed permitting prediction of the radiation resistance from a knowledge of the temperature difference between the ambient air and the gas flowing from the duct, and a physical basis for this correlation is presented.

ISSN:0001-4966    

DOI:10.1121/1.390301

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A theoretical model is presented which can be used to calculate the pressure field patterns of pulsed, focused ultrasonic radiators in attenuating and nonattenuating media. Pressure pulses are calculated by superimposing continuous wave solutions at discrete frequencies. Due to the speed of the method, time signals can be calculated at many positions in the transducer beam in a reasonable amount of time. To test the model, theoretical predictions for the pressure signals are compared to hydrophone measurements for a conventional diagnostic transducer. In addition, signal envelopes are studied in order to determine the effects of attenuation and dispersion on the imaging characteristics of a focused radiator. This work may have significant application to the design of transducers for specific imaging purposes or to the analysis of the imaging process.

ISSN:0001-4966    

DOI:10.1121/1.390302

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A simple method to predict the noise generated by cylindrical‐shaped machine elements in axial vibration is presented. An approximation of the Helmholtz integral equation valid when the receiver–source distance is much greater than either the cylinder’s diameter or length is used to determine the acoustic pressure generated by axial oscillations of cylinders at any aspect ratio or frequency. The results are used in developing free‐field and reverberent field design contours. Experimental evidence points to the validity of the prediction model. Included are two design problems and solutions that show the method can be used to reduce noise generated by cylindrical shaped bodies.

ISSN:0001-4966    

DOI:10.1121/1.390303

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A finite amplitude sound wave can exert a radiation force on an object due to second‐order effect of the wave field [L. V. King, Proc. R. Soc. London, Ser. A147, 212–240 (1934)]. In this work we study the radiation force on a rigid small sphere (i.e., in the long wavelength limit), which has a temperature different from that of the environment. This investigation assumes no thermally induced convection and is relevant to material processing in the absence of gravity. Both isotropic and nonisotropic temperature profiles are considered. In this calculation the acoustic effect and heat transfer process are essentially decoupled because of the long wavelength limit. The heat transfer information required for determining the force is contained in the parameters which are integrals over the temperature distribution.

ISSN:0001-4966    

DOI:10.1121/1.390304

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP