摘要:

A dispersion relation has been obtained for the propagation of harmonic waves in an elastic solid containing a three‐dimensional array of regularly spaced spherical cavities of equal radius. The general pattern of the frequency spectrum is one of passing and stopping bands. Results are presented for the lowest acoustic mode, the transition to the first optical mode, and for a segment of the first optical mode.

ISSN:0001-4966    

DOI:10.1121/1.394825

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The results of numerical calculations are presented for the reflection coefficient of the shear horizontal (SH) plane waves incident on an elastic solid–viscoelastic liquid interface. The linear viscoelastic Maxwell liquid with one relaxation time is considered. The modulus and phase of the reflection coefficient as a function of incidence angle and frequency are determined numerically for typical parameters of the considered media. The validity of an approximate expression for the reflection coefficient is analyzed. Some new properties of the reflectedSHwave are considered and discussed.

ISSN:0001-4966    

DOI:10.1121/1.394826

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

It is observed that, when the acoustic impulse from an electric spark is reflected normally from a rigid plate and detected after passage through the remains of the spark region, a precursor component appears in the received waveform. To explain this phenomenon in terms of refraction, a two‐spark experiment is used to examine the acoustic properties of the post‐discharge region near a spark gap. The measured times of arrival of the precursor signal are fitted to a simple lens model consisting of two parameters: lens radius and acoustic index of refraction. The temporal evolution of these lens parameters is also measured and interpreted in terms of a diffusion model. The diffusion coefficient characterizing the process is found to be much larger than that for thermal diffusion in air at room temperature. This is consistent with the much higher temperatures corresponding to the low acoustic index of refraction of the measured lens models.

ISSN:0001-4966    

DOI:10.1121/1.394827

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

It is shown that the effect of a slightly curved surface on the behavior of acoustic ‘‘head waves’’ is: (a) to make the head‐wave arrival time weakly dispersive with frequency; (b) to cause a curvature in the head‐wave arrival time with offset, roughly as (offset)3/2; (c) to cause the head wave to arrive earlier (later) than that corresponding to a flat surface for a synclinal (anticlinal) shape. The amplitude behavior of the head wave is also found to be distorted with (d) a lower (higher) high‐frequency/low‐frequency ratio for reflection from a syncline (anticline) and (e) an enhancement (diminution) relative to that for a head wave from a flat surface for a synclinal (anticlinal) surface with the enhancement (diminution) exponentially dependent on offset, roughly as (offset)3/2in the exponent. These calculations have been done in order to point out that it may be possible to use these features of head waves in order to unravel subtle structural traps in the subsurface, which vary slowly and would not be noticed in conventional seismic processing.

ISSN:0001-4966    

DOI:10.1121/1.394828

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Data obtained by Medwin and D’Spain [J. Acoust. Soc. Am.79, 657 (1986)] for low‐frequency propagation over gravel‐covered rigid planes are inverted by applying the procedure [J. Acoust. Soc. Am. Suppl. 179, S68 (1986)] developed for regularly shaped protuberances. The development is based on analytical results for a point source irradiating an embossed plane, and on the uniform asymptotic form of the coherent field as a Sommerfeld type wave system in terms of the complex error function complement (Q). Initial estimates of unknown parameters are obtained by working with elementary approximations of theQintegral, and then used in the complete integral for refinements and final computations. The procedure delineates the roles of interference and damping on the elementary wave components of theQsystem, and the resulting graphs display the primary data trends.

ISSN:0001-4966    

DOI:10.1121/1.395115

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The minimization of acoustic potential energy has been proposed as a method of actively controlling a harmonic reverberant sound field. In this paper an analysis of plane waves in a finite length duct provides insight into the physical mechanisms of this method and allows comparison with two other methods, the acoustical virtual earth and the absorbing termination. A practical technique of achieving the reduction of acoustic potential energy is presented and discussed.

ISSN:0001-4966    

DOI:10.1121/1.394829

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The nonlinear oscillations of a spherical gas bubble in a compressible viscous liquid subject to the action of a sound field are investigated by the multiscale perturbation method. Approximate analytical solutions for the transient oscillations in the regions of the main resonance, first and second harmonic, and first and second subharmonic, are obtained to second order in the expansion.

ISSN:0001-4966    

DOI:10.1121/1.394830

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Measurements of the variance and scale of the index of refraction fluctuations in the turbulent atmosphere performed in conjunction with a sound propagation study are described. The theory of turbulence generation in the atmosphere is examined and used to develop a model for the functional dependence of variance and scale with altitude from the data. This model is used to calculate the expected turbulence effects for a nonrefracting atmosphere and the results are compared with data from the sound propagation study. This study demonstrates that turbulence effects are smaller for elevated sources than for sources close to the ground. The calculations necessary for a complete description of the turbulence effects are described.

ISSN:0001-4966    

DOI:10.1121/1.394831

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

There is a well‐developed theory to account for the effect of random fluctuations in the sound speed on the underwater scattering of acoustic radiation. Here it is shown that the theory may be readily extended to include the effect of random fluctuations in fluid velocity. It is assumed that the characteristic time and length scales associated with the velocity correlation functions are long when compared to the corresponding scales (period and wavelength) of the acoustic radiation. The application of the theory to ocean studies is discussed with emphasis on the utility of using an inverse method to find the lateral correlation function.

ISSN:0001-4966    

DOI:10.1121/1.394832

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Effects of horizontal stochastic bottom structure on acoustic intensity in an isospeed ocean have been studied previously by the authors [J. Acoust. Soc. Am.76, 1445–1455 (1984)]. The bottom density and sound speed were taken to be random and ray theory was used. In this study we add a rough water–bottom interface, consisting of large‐scale, two‐dimensional random facets upon which small‐scale roughness may be superimposed. Each facet is assumed to possess small random depth deviation, slope, and curvature. Initially, we take acoustic rays to be specularly reflected from the facet bottom, and derive expressions for the mean and variance of incoherent intensity at a point receiver for a transmitted cw signal. The results are sufficiently general to permit their use with different bottom‐acoustic models, but we employ Mackenzie theory here. Relative effects of structure and topography are compared. Subsequently, small‐scale roughness is superimposed on the facets, and the added consequences of scattering are considered.

ISSN:0001-4966    

DOI:10.1121/1.394833

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP