摘要:

Smoothed boundary conditions have heretofore been used for describing acoustic scatter from hard (infinitely rigid) rough walls or from rough interfaces between fluids in the domainkb ≂ kh ≲ 1, wherekis the wavenumber,bandhare, respectively, measures of the roughness height and spacing between roughness elements. The present paper extends the technique to the case of acoustic scatter in a fluid off the rough surface of an elastic solid. The boundary condition defining the perturbation of the displacement potential φsin the fluid is isomorphic to the equivalent condition for two fluids, the main difference residing in the calculation of the physical constants. These calculations are straightforward; with continuity of pressure (stress) at the boundary, one can solve the scatter problem for an acoustic source in the fluid. Of particular interest is the boundary wave problem, corresponding to modes with potential φ1(in the fluid) and φ2, ψ2(in the solid) falling off exponentially away from the interface. The characteristic equation here has always at least one solution—the Stoneley wave mode as affected by roughness (which makes it dispersive and introduces attenuation due to incoherent scatter). When the fluid sound velocityc1is less than the shear velocitycsin the solid, there is, in addition, a surface roughness boundary mode solution—essentially the surface roughness boundary wave of the hard wall solution described elsewhere [I. Tolstoy, J. Acoust. Soc. Am.66, 1135–1144 (1979)] and measured by Medwin [H. Medwinetal., J. Acoust. Soc. Am.66, 1131–1134 (1979)], but modified by the elasticity of the solid. Stoneley wave attenuation due to incoherent scatter is greater than that of the surface roughness boundary mode. The boundary wave pressure field excited by a point source near the boundary will therefore have greater farfield amplitudes than the Stoneley mode (even though its nearfield amplitudes are less).

ISSN:0001-4966    

DOI:10.1121/1.392758

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Two recently developed related spectral theories [Kamel and Felsen, J. Acoust. Soc. Am.73, 1120–2230 (1983); Arnold and Felsen, J. Acoust. Soc. Am.73, 1105–1119 (1983)] have provided potentially new options for calculating source‐excited sound fields in a weakly range dependent ocean environment. So far, these theories have been applied to a homogeneous two‐dimensional ocean and bottom separated by a plane sloping interface. It has been recognized that their common building blocks are what have been referred to as ‘‘intrinsic modes’’ [Arnold and Felsen, J. Acoust. Soc. Am.76, 850–860 (1984)]. Intrinsic modes have spectral integral representations that reduce in a lowest order approximation to adiabatic modes, where these can be defined, but which remain uniformly valid in their integral form through the cutoff transition in upslope propagation. An efficient numerical algorithm has now been developed for calculating the intrinsic modes and the Green’s function in the ocean and in the bottom. Numerical results are compared with those from the parabolic equation [Jensen and Kuperman, J. Acoust. Soc. Am.67, 1564–1566 (1980)] and the augmented adiabatic mode theory [Pierce, J. Acoust. Soc. Am.74, 1837–1847 (1983)]. Also discussed and compared are asymptotic approximations of the spectral integral, which are found to be accurate and computation intensive everywhere in the water and also at small depths in the bottom.

ISSN:0001-4966    

DOI:10.1121/1.392759

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

At long ranges in an underwater duct, in a widening transition region around the transmitter depth, strong focusing effects due to a pileup of caustics occur which invalidate the use of ray acoustics even if the ray field is augmented with conventional caustic corrections. This deficiency can be repaired with a hybrid ray‐mode representation, wherein the caustic forming troublesome rays are replaced by a group of surface ducted modes, while the troublefree rays are kept intact. Such a formulation is demonstrated here in a model duct environment, wherein a line source emits a high‐frequency Gaussian pulse. Comparison of the hybrid solution with a reference solution obtained by summation of normal modes reveals the accuracy of the hybrid scheme and its numerical efficiency at high frequencies. The hybrid solution automatically and self‐consistently blends the raylike behavior of the received waveform at early times with the modelike behavior at late times, thereby providing a physically incisive interpretation of the propagation process.

ISSN:0001-4966    

DOI:10.1121/1.392760

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The attenuation of sound through a wide variety of sediments is proportional to the first power of frequency. In order that a plane wave remain causal after suffering such an attenuation, it must also be dispersed. The dispersion necessary to maintain causality in typical sediment is shown to have a logarithmic dependence on frequency and to be measurably large. A truncated parametric array is used to provide a wideband pulse whose travel through a typical medium sand is observed. The measured dispersion is in agreement with the theoretical prediction, exhibiting a variation in phase velocity of 0.5% across a frequency range of 50–350 kHz.

ISSN:0001-4966    

DOI:10.1121/1.392761

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An explicit expression is derived for the phase shifts of the partial waves scattered by a solid elastic sphere, incorporating the effects of the viscosity of the ambient fluid. The solution is applicable to frequencies and scatterer sizes for which the viscous boundary layer thickness is much less than the radius of the sphere. For particle sizes in the sand‐size range (30 μm

ISSN:0001-4966    

DOI:10.1121/1.392762

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Many measurements of sound backscattered by fish have been made, but most of these are farfield measurements from whole fish. Some of the acoustic properties of a fish can be inferred from the dependence of scattering on aspect and removal of organs such as the swim bladder. However, farfield measurements do not give the detailed structure needed to construct a Huygen’s scatterer acoustic model of a fish. We built a 220‐kHz focused sonic scanner. The focal distance was 10 cm and the axis of the fish was placed at 10 cm from the transducer for measurements. We measured the peak amplitudes of backscatter sound along the length of yellow perch (Percaflavescens) (12–14 cm) and hog sucker (Hypenteeiumnigricans) (30 cm). Repeatability of results was examined in three different yellow perch and gave a good agreement between the resulting backscattering curves. Preservation (freezing) of the fish resulted in approximately 30% reduction of the overall backscatter but did not affect the shape of the backscattering curve. The measurements showed that the swim bladder gave about 80% of the scattered energy for the yellow perch. The swim bladder gave only about 20% of the scattered energy for the hog sucker. Vertebrae, head, and flesh were distributed scatterers. We observed a gross dependence of the distribution of scatterers on the species of fish. These differences should be observable in measurements of the probability density functions of fish echoes.

ISSN:0001-4966    

DOI:10.1121/1.392763

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The facet‐ensemble method [W. A. Kinneyetal., J. Acoust. Soc. Am.73, 183–194 (1983)] is used to determine if the spatial wavenumber power spectrum of a rough surface is sufficient to predict the amplitude and spatial coherence of acoustic energy scattered from that surface. Two sets of water surfaces are studied. Within each set, the surfaces have the same rms roughness (0.2 cm) and spatial power spectrum. For each surface, the acoustic field in the water is computed at two omnidirectional receivers for both backscatter and forward scatter at a grazing angle of 35°. The source emits a 100‐kHz signal and has a beamwidth of 13°. The total travel distance is 506 cm. Results indicate that the backscattered energy is more sensitive to detailed surface shape than is the forward scattered energy. This implies that the surface spatial power spectrum may not be sufficient to predict backscatter accurately.

ISSN:0001-4966    

DOI:10.1121/1.392764

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An experimental investigation has been conducted into the production of high‐intensity tones by deep cylindrical cavities in a flat surface at relatively low Mach numbers (i.e., 0.12–0.24). The application of such a mechanism is to the acoustic coding of moving objects containing drilled cylindrical cavities. The sound intensity and frequency have been determined as functions of flow velocity, diameter, and depth of the cavities resulting in correlations to be used for the design of a whistle in the range off=5–17 kHz. As a practical matter, it is possible to produce a whistle producing 106 dB at a 30.5‐cm distance from a cylindrical cavity of 0.508 cm in diameter and 1.32 cm in depth with an airflow of 57.7 m/s past the hole. An eddy‐driven oscillator is represented by a one‐dimensional oscillator with a forcing term dependent on a time‐delayed displacement. This simple model shows how the natural frequencies of the shear layer are pulled to the acoustic frequencies at resonance, describes the shape of resonance, and shows how the natural frequencies of a pipe with no flow correspond to actual frequencies of the whistle which depend on the flow through the factor (1−M)−1.

ISSN:0001-4966    

DOI:10.1121/1.392765

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A modulated acoustic radiation pressure technique to produce quadrupole shape oscillations of drops ranging in diameter from 50–220 μm has been used by us. These drops have been suspended by acoustic levitation in a small chamber mounted on a stage of an optical microscope, which allowed easy viewing. The fission of drops and the deformation of sea urchin eggs were also observed.

ISSN:0001-4966    

DOI:10.1121/1.392766

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A description is given of a precise technique for measuring the threshold for acoustic cavitation inception. The system, which is automated so as to remove operator involvement, utilizes a slow ramping of the acoustic pressure amplitude until cavitation occurs. The detection criterion is the generation of a sufficiently intense sonoluminescent signal. Measurements made in filtered water show a well‐defined, reproducible, and stable cavitation threshold. Measurements of the dependence of the threshold on filter size, on time, and on the concentration of dissolved ions for various salts are also presented. Many of these results appear anomalous.

ISSN:0001-4966    

DOI:10.1121/1.392767

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP