摘要:

This lumped‐parameter analysis models the compliant‐tube array as a homogeneous fluid layer, thin in terms of wavelengths, whose bulk modulus and thickness match those of the tube array. For closely packed arrays, the results are in good agreement with those of the rigorous, numerical solution. For sparse arrays, the lumped‐parameter model fails near and above the fundamental tube resonance frequency because it does not account for accession to inertia.

ISSN:0001-4966    

DOI:10.1121/1.393017

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Laboratory measurements relating to the acoustic detection of marine sediment transport are reported. A series of underwater measurements have been conducted on agitated artificial sediments, and the acoustic noise generated by interparticle collisions monitored. Measurements of the total acoustic pressure and spectral pressure levels were made on a wide range of particle diameters for different values of the mass of agitated material, the collision velocities, and the swept bandwidth of the spectrum analyzer. The objective was to relate the total pressure level to the mass of mobile material and ascertain if there was an acoustic spectral signature unique to particle size. This information could then be used to guide the interpretation of acoustic measurements of bedload transport taken in the marine environment and provide specifications for the development of an acoustic system to monitor remotely the material moving along the seabed.

ISSN:0001-4966    

DOI:10.1121/1.393018

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

In order to perform calculations which require many evaluations of surface‐scattering cross sections, an approximation has been developed for integrals appearing in the Kirchhoff theory of rough surface scattering. This approximation replaces a two‐dimensional integral evaluation by a simple function evaluation. In the radially symmetric case, evaluation of Kirchhoff cross sections is speeded by a factor of about 20. The basis of the approximation, called exponential substitution, is to fit Kirchhoff integrals by a two‐parameter scaled spectrum of surface heights. M. V. Berry [J. Phys. A: Gen. Phys.8,566–684 (1975)] chose parameters to fit zeroth and second moments of Kirchhoff integrals whereas for this study parameters were chosen to fit the zeroth moment and peak values of the integrals. Numerical results are presented which show that the present version of exponential substitution is more accurate than previous versions in describing scattering near the forward direction. PACS numbers: 43.28.Fp

ISSN:0001-4966    

DOI:10.1121/1.393019

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Measurements are presented of the acoustic waveforms obtained over grassland at distances out to 300 m from an impulsive source. When propagating downwind, the pulses exhibit only slight attenuation over that predicted by geometric and atmospheric absorption effects. However, in the presence of a dominant temperature gradient and when propagating into a wind, a marked excess attenuation commences at a region corresponding to a shadow boundary. Inside the meteorological shadow zone, the recorded impulses have quite a different shape, being almost symmetric with a duration increasing with distance from the source. Attempts to fit the pulse behavior by using only the ground wave term were partially successful. A more promising approach is to model the shadow boundary as a frequency filter whose characteristics depend on the temperature and wind gradients.

ISSN:0001-4966    

DOI:10.1121/1.393020

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Anm‐sequence correlation method was used for measuring sound levels 1‐km distant from a point source 1.25 m above the ground, a crop field and a snow‐covered ground, respectively. Meteorological parameters were monitored simultaneously. Wind speed and temperature were measured at several elevations, together with wind direction, relative humidity, and atmospheric pressure at a single height. The effect of refraction on the sound level variation is interpreted in terms of the curvature for near‐horizontal sound rays. It is found that the sound level increases as the curvature goes from negative to positive values. Comparisons with other investigations were made, and qualitative agreement is found. Calculations with acoustic theory were made, the results of which show large discrepancies with those measured.

ISSN:0001-4966    

DOI:10.1121/1.393021

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The conventional Kirchhoff approximation for rough surface scattering does not reduce to perturbation results in the limit of small surface heights. It is shown here how to modify the conventional Kirchhoff approximation by using appropriate half‐space Green’s functions. The resulting expressions for the differential cross section for scattering from rough surfaces reduce to the conventional Kirchhoff results and the perturbation results in the appropriate limits.

ISSN:0001-4966    

DOI:10.1121/1.393022

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The inverse scattering problem for an acoustic medium is considered within the approximate direct inversion framework. As opposed to iterative methods, the direct inversion approach gives an estimate of the medium velocities by operating on the observed scattered field without repeated solutions of the forward problem. Previous solutions to the multidimensional Born velocity inversion problem require either coincident source and receiver arrays or plane‐wave sources. In both cases an array of point sources is required to collect the proper data for inversion. In this paper, the solution to the single point source problem is derived. It is shown that by extrapolating and imaging the observed scattered field appropriately, the projections of the velocity function at all angles can be obtained. The velocities are, then, reconstructed by the inverse Radon transform method of tomography.

ISSN:0001-4966    

DOI:10.1121/1.393023

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

In a previous paper [Roger H. Hackman, J. Acoust. Soc. Am.75, 35–45 (1984)], a spheroidal‐coordinate‐based transition matrix formalism was established for acoustic and elastic wave scattering. In this paper, the acoustic scattering by a solid elastic cylinder with hemispherical endcaps and a length‐to‐diameter ratio of 10 is considered. Numerical results are presented for the backscattered form function as a function of frequency for various angles of incidence. These results are compared with experimental measurements taken at the Naval Coastal Systems Center and given a physical interpretation.

ISSN:0001-4966    

DOI:10.1121/1.393024

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Our earlier study [J. Pott and J. G. Harris, J. Acoust. Soc. Am.76, 1829–1838 (1984)] of the reflection and refraction, at a plane fluid–solid interface, of a Gaussian beam is continued. When the angle of incidence approaches the angle at which the compressional wave is critically refracted, a lateral wave is excited both in the fluid and in the solid. In the fluid, this wave interferes with the reflected beam, making it appear shifted and distorted. A second lateral wave is excited in the fluid when the angle of incidence nears that at which the shear wave is critically refracted. However, this angle is usually quite close to the Rayleigh angle. Thus the nearly coincident excitation of a leaky Rayleigh wave (examined in our earlier paper) masks any effect the lateral wave has. But even for angles of incidence not near these three critical angles, the axis of the reflected beam is slightly rotated and displaced. The refracted beams (examined in our earlier paper) spread quite rapidly in the solid, but, by using a thin lens to focus the incident beam, this spreading can be reduced somewhat.

ISSN:0001-4966    

DOI:10.1121/1.393025

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

It is shown that the technique of mean‐field renormalization permits solution of the problem of perfect reflection from a rough surface. The results for both the statistically sharp reflected wave (the mean wave) and the fluctuations around the mean wave are precise provided that the rms height of the rough surface is small compared to the incident wavelength. In particular, the results are valid for any arbitrary ratio of the spatial scale of the roughness variations compared to the incident wavelength. It is shown that power is conserved to the order considered and that certain basic integrals do not depend for their convergence on the vanishing of the surface height autocorrelation function at large lags. As a consequence, a sinusoidally varying surface can be treated as a limiting case using the mean‐field renormalization approach.

ISSN:0001-4966    

DOI:10.1121/1.393026

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP