摘要:

This paper studies sound propagation in a layered atmosphere bounded by a ground, whose impedance is described by the Delany–Bazley–Chessell’s empirical model. The problem is formulated in terms of a Green’s function integral in the spectral domain, and is numerically evaluated by a Fast Field Program (FFP). Numerical results are included to show that (i) in simple test cases, the FFP solution is in excellent agreement with existing asymptotic solutions; (ii) numerical overflow arises when the number of layers is large and/or the frequency is high, and a method to circumvent this difficulty is described; and (iii) the FFP is a most powerful tool in solving propagation problems in layered media bounded by complex impedances.

ISSN:0001-4966    

DOI:10.1121/1.391906

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A theory is presented for the calculation of the acoustic material signature of a multilayered elastic half‐space overlain by a fluid. The solid layers are composed of homogeneous isotropic linearly elastic materials and are firmly bonded at the interfaces. The calculation procedure is valid at an arbitrarily high frequency of excitation. Results are presented for a uniform, a single layered and a four layered model of the half‐space at two frequencies of excitation; one moderate (35 MHz) and the other relatively high (370 MHz). Several new features of the material signatures and their possible use in the material characterization of layered specimen are indicated.

ISSN:0001-4966    

DOI:10.1121/1.391907

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The method proposed in this paper provides a computational method for implementing the Helmholtz integral formula for acoustic radiation and scattering problems associated with arbitrary shaped three‐dimensional bodies. In particular an isoparametric element formulation is used in which both the surface geometry and the acoustic variables on the surface of the body are represented by second‐order shape functions within the local coordinate system. A general formula for the surface velocity potential and the exterior field is derived. This result is applicable to nonsmooth bodies, i.e., it includes the case where the surface may have a nonunique normal (e.g., at the edge of a cube). Test cases are shown involving spherical, cylindrical, and cubical geometry.

ISSN:0001-4966    

DOI:10.1121/1.391908

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Calculations of the scattering of acoustic waves by rigid prolate spheroids of various aspect ratios were performed with both theT‐matrix method (TMM) and the Helmholtz integral equation method (HIEM) on the same computer to compare the numerical stability of the two methods. For spherical targets the TMM converged more rapidly than the HIEM, but with increasing aspect ratio the rate of convergence of the TMM deteriorated rapidly while that of the HIEM was only slightly reduced. Moreover, roundoff error quickly became a serious problem for the TMM with increasing aspect ratio while for the HIEM it posed no problem at all in the cases considered. The numerical difficulties of the TMM were exacerbated by the fact that the matrices which had to be inverted became increasingly more ill conditioned as the number of partial waves was increased. For the HIEM, on the other hand, the matrices to be inverted became increasingly better‐conditioned as their dimension increased with the order of the calculation.

ISSN:0001-4966    

DOI:10.1121/1.391909

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A multiple scattering theory for elastic wave propagation in a discrete random medium is presented. A self‐consistent multiple scattering formalism using theTmatrix of a single scatterer in conjunction with the quasicrystalline approximation (QCA) and a self‐consistent pair correlation function is employed to study the phase velocity and coherent attenuation of elastic waves by a random distribution of cavities and elastic inclusions embedded in an elastic matrix. Both uniform and Gaussian size distributions are assumed. The theoretical results obtained in this study are shown to be in excellent agreement with experimental observations.

ISSN:0001-4966    

DOI:10.1121/1.391910

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The sound pressure in a one‐dimensional duct contains amplitude and phase components, both of which vary with position. The spatial variation of phase is examined in this paper and is shown to be closely related to the relative flow of incident and reflected acoustic energy. A technique that determines the acoustic reflection coefficient of the duct termination by measuring the maximum rate change of phase with position is discussed. For the measurements of phase variation a single moveable probe microphone is used with a phasemeter. Good agreement has been obtained between this method and the more usual impedance tube method, which considers the amplitude component of sound pressure. The phase measurements, though, can be applied equally well to ducts of conical, rather than uniform, cross section, and require a relatively small length of duct in which to translate the probe (as little as 1/20 wavelength, for moderate reflectivities). Some preliminary measurements have been made in human ear canals. The eardrum reflection coefficients obtained in the 10‐ to 15‐kHz range are between 0.6 and 0.85, somewhat higher than those obtained by other methods.

ISSN:0001-4966    

DOI:10.1121/1.392379

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Using the fact that low‐frequency sound waves are diffracted by any kind of obstacle, a ‘‘sound velocity reducer’’ waveguide has been designed so as to create a 180° phase lag between the diffracted part of the incident noise and that transmitted through the duct of the guide. Further, this waveguide, being a low‐pass filter, insures that the acoustic pressure amplitudes diffracted and transmitted are of the same order of magnitude. For a low‐frequency band of noise, this results in the formation of coherent dipole line sources and, thus, of destructive interferences, at a distance from the acoustically transparent obstacle. The goal of this paper is to show how we can calculate this phase lag and the relative amplitudes of the components, and how they compare to the actual measurements. The results for the model investigated show good agreement between calculation and measurement, within the precision expected.

ISSN:0001-4966    

DOI:10.1121/1.391911

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

This paper describes a precise and direct acoustical method for experimental verification of the variation of γ in air with humidity. The experimental results are within 100 to 250 ppm of the theoretical predictions. The experimental arrangement may be used to assess the specific heat ratio of other gases or gas mixtures by a comparison method.

ISSN:0001-4966    

DOI:10.1121/1.391912

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The sensitivity of total‐field receptions to sound‐speed profile choice is analyzed using ray theory. The profiles are depth dependent, and may be used to describe a deep‐ocean sound channel. A variety of locations of a fixed source and receiver, separated by less than about 50 km, is considered. Given a specified profile and a second, simpler profile constructed by procedures previously described [J. Acoust. Soc. Am.75, 112–124 (1984)], it is demonstrated that the acoustic fields associated with the profiles are negligibly different. Approximations for total‐field phase and amplitude differences are presented, which facilitate the determination of those ranges where the total fields match closely. In addition, the sensitivity of performance measures for horizontal linear receiving arrays to profile selection is studied. Expressions for normalized power pattern are developed which incorporate certain nonplane‐wave effects associated with an assumed dominant ray arrival. Conditions are presented for which a simpler profile may replace a specified profile and still maintain nearly equivalent array performance.

ISSN:0001-4966    

DOI:10.1121/1.391913

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Two integrals of Cron and Sherman which arise in modeling of ocean‐generated surface noise are given several representations, the most notable of which is obtained from the Laplace transformation of a mild generalization of the given integrals. It is also found that the complex sum, with weights 1 andi, of the integrals reduces to a sum of simpler integrals plus an expression which is representable as a multiple partial derivative of a spherical Hankel function of the first kind, with respect to the normalized vertically projected baseline length of the two relevant sensors. This latter expression is the homolog for the correlation function, of the pressure field of a distant vertical axial multipole of ordermbeing proportional to them‐fold derivative, in a vertical direction, of the Green’s function of a monopole, which is proportional to a spherical Hankel function of the first kind of order zero.

ISSN:0001-4966    

DOI:10.1121/1.391860

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP