摘要:

Asymptotic expansions as ε→0 that are uniformly valid intare obtained for the membrane’s motion and the scattered acoustic pressure field. The small parameter ε is the density ratio of the acoustic fluid and the membrane. For simplicity of presentation, only plane, compact incident pulses are considered. The scattered field depends on the pulse’s structure. If it is a sufficiently narrow bandwidth pulse which contains none of theinvacuonatural frequencies of the membrane, then it is essentially reflected as though the baffled plane is completely rigid. However, if the pulse spectrum is sufficiently broad so that it contains one or more of theinvacuonatural frequencies of the membrane, an additional scattered field is produced. This scattered field insonifies distant observation points after the rigidly reflected pulse has arrived. It is the sum of slightly damped and oscillating outgoing spherical waves that represents the ‘‘decayed ringing’’ of the membrane. Application is given to the baffled circular membrane which is insonified by a normally incident pulse. Graphs of the membrane’s motion and the farfield acoustic pressure are given. They demonstrate the importance of the incident pulse width on the qualitative features of the response.

ISSN:0001-4966    

DOI:10.1121/1.393615

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A new method for solving the scattering problem of acoustic pulses by baffled membranes is described. It is an adaptation of the finite difference technique which uses an ‘‘artificial’’ boundary condition to mimic an infinite region. The numerical method is used on several problems involving a plane compact pulse. It is also used on a time‐harmonic pulse of infinite extent. This pulse generates a time‐harmonic response ast→∞; this is proved in the Appendix. The numerical scheme is marched out in time until the transients have decayed away and a time‐harmonic solution has been established. This ‘‘relaxation’’ scheme yields accurate solutions in a reasonable amount of time without requiring heavy or light fluid loading assumptions.

ISSN:0001-4966    

DOI:10.1121/1.393560

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

This paper describes the acoustic behavior of polyurethane base, microballoon filler composites for echo‐reduction applications in water‐filled tanks. The approach is similar to that of Corsaroetal. [J. Acoust. Soc. Am.68, 655 (1980)] for silicon rubber, three‐component anechoic systems with the additional study of off‐normal incidence. Depending on the way the mixtures are cured, the surface echo reduction of the resulting composites can be improved by 13 dB at normal incidence. For the case of oblique incidence, the sound speed in these compounds is relatively close to that in water; this optimizes their surface echo reduction once the condition of impedance match is reached. Measurements reveal a front echo reduction better than 35 dB at 45° of incidence. A formula to estimate the ‘‘Net Echo Reduction’’ of an anechoic water‐filled tank is also given.

ISSN:0001-4966    

DOI:10.1121/1.393649

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Finite amplitude acoustic wave propagation through bubbly liquids is studied by using the Burgers–Korteweg–DeVries model equation. Numerical solutions are obtained using a pseudospectral method. Spectral analysis of the numerical solutions is performed to study the spectral energy transfer due to nonlinearity. Numerical solutions of the two limiting cases obtained by neglecting either the dissipation or dispersion term of the Burgers–Korteweg–DeVries equation are also analyzed. In nonlinear wave propagation, short waves are generated by long waves due to nonlinearity. In bubbly media, these short waves may undergo strong resonance absorption due to the presence of bubbles, even if other mechanisms of dissipation are negligible. The effect of such an absorption is simulated by applying a low‐pass filter on the results obtained with the dissipation term neglected. The filter eliminates the short waves generated after each time step. It is shown that such resonance absorption corrections may be necessary for any quantitative comparisons of computed results with experiment.

ISSN:0001-4966    

DOI:10.1121/1.393602

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Recent theoretical studies of acoustic wave propagation in a model waveguide consisting of a homogeneous wedge with one reflecting and one penetrable boundary have established the utility of the concept of an intrinsic mode, which generalizes for nonseparable problems the normal mode of separable configurations. In this paper, it is shown by explicit calculations on the same model problem that direct orthogonal expansion (in local normal modes) of the intrinsic mode produces the same expansion coefficients as those obtained by a perturbation analysis of the coupled mode equations. The perturbation method used is that of renormalization, in which the mode coupling operator is iteratively diagonalized up to a certain order in the nonseparability parameter, which, in this case, is the wedge angle α.

ISSN:0001-4966    

DOI:10.1121/1.393603

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The contribution to underwater ambient noise from the persistent winds which blow at high latitude is discussed and estimated. Energy is ducted into low‐loss paths in the deep sound channel by favorable horizontal sound‐speed gradients. A concept of ‘‘wind‐noise lanes’’ is developed into a simple model which is used to predict the resulting underwater ambient noise levels as a function of season and latitude. Three main oceans are considered—the Pacific, Atlantic, and Indian Oceans. In general terms, predicted omnidirectional levels at 50 Hz lie between 65 and 75 dBre: 1 μPa2/Hz and are comparable to those generally associated with light‐to‐moderate ship traffic. The spectral shape of high‐latitude wind noise is also similar to that associated with ships. High‐latitude wind noise arrives within ±15° of horizontal, and the predicted magnitude of this low‐angle component also matches measurements well. It is concluded that the component of noise associated with high‐latitude winds can be significant at low frequencies. Properties of omnidirectional noise, as well as vertical and horizontal directivity, may be dominated by this component under some conditions. It appears, however, to have been often overlooked in the interpretation of data.

ISSN:0001-4966    

DOI:10.1121/1.393604

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

An experiment has been performed to investigate the coupling of surface–ship noise to the deep ocean sound channel. These calibrated measurements of sound propagation from the 18‐m, 135‐Hz source were obtained at a deep ocean receiver while the source tow proceeded from deep water up the Sable Island Bank at ranges between 730 and 910 km. The sound propagation path was from the cold slope waters over the bank through the Gulf Stream and to the edge of the Sargasso Sea. The mean value of the transmission loss was 110 dB with a down‐slope enhancement estimated to be 4 dB resulting from the combined effects of trapping in the strong shallow sound channel and reflections from the slope. Comparisons with calculated results using the parabolic equation method were good and demonstrate coupling to the deep ocean sound channel. The acoustic field, sampled by 16 transverse hydrophone groups with a 4.75‐m center spacing over six consecutive (12.5 s, 0.08 Hz) samples, yielded a mean standard deviation of 1.7 dB (6≤S/N≤20 dB) corresponding to a coefficient of variation of 0.6. Coherent summation of slope‐reflected and deep‐refracted hydrophone signals yielded estimated mean values of the spatial coherence of 0.89 and an estimated spatial coherence length of 460 m, when multipath effects were not dominant; however, these estimates were found to range as low as 0.63 and 150 m. These results facilitate the interpretation of the spatial coherence of the ship‐induced, ‘‘slope‐enhanced’’ contribution to deep ocean ambient noise.

ISSN:0001-4966    

DOI:10.1121/1.393761

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Expressions derived for the acoustic intensity decay with range in shallow water by Marsh and Schulkin [J. Acoust. Soc. Am.34, 863–864 (1962)] and Weston [J. Sound Vib.18, 271–287 (1971)] have been adapted to describe the intensity‐range variation in the deep water situation, where the sound speed at the surface is equal to the sound speed at the base of the water column. The modified equations take account of the surface decoupling loss which occurs at frequencies below 100 Hz for source depths less than 25 m. The levels predicted by these equations are compared with experimental data obtained in deep water in the Indian Ocean. It is found that the mean difference in measured and predicted levels at frequencies of 25 and 50 Hz is less than 2 dB, and rms level differences are within 4 dB.

ISSN:0001-4966    

DOI:10.1121/1.393554

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

In the problem of scattering from a pressure release boundary, the Helmholtz integral equation may be expressed as a Fredholm equation of the second kind with the unknown surface velocity as the independent variable. The method of successive approximations was employed by Meecham [J. Ration. Mech. Anal.5, 323–333 (1956)] to obtain a series solution to this equation, where the first term of this series is, in fact, the unshadowed Kirchhoff approximation to the solution. Meecham attempted to formulate the region of validity of the Kirchhoff approximation by determining when the remaining terms of the series could be neglected, however, his arguments used to select ‘‘smallness’’ have been questioned. An exact solution to the integral equation for a sinusoidal boundary is employed to examine the series, term by term, for convergence; and it is found that (1) the alternating nature of the series does not bring convergence when the series diverges absolutely, and (2) the absence of any propagating side orders and reflection anomalies is not a sufficient condition for convergence. Finally, to highlight the extent of the error made when using the Kirchhoff approximation, comparisons are made between reflection coefficients computed using the exact solution and several terms of the series. It is found that in the shadowed regions, the Kirchhoff approximation is in reasonable agreement with the exact solution, even when Meecham’s series diverges.

ISSN:0001-4966    

DOI:10.1121/1.393555

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Propagation in a shallow‐water channel of constant sound speed, overlying a homogeneous viscoelastic half‐space with frequency‐independent loss factors, is examined as a function of range, frequency, water depth, and a parameter representing the initial rate of increase of bottom reflection loss with increasing grazing angle. Particular attention is focused on the ‘‘transition’’ frequency below which the transmission loss increases rapidly. It is found to separate a ray‐theoretical domain, where the directional spectrum of transmitted sound is quasicontinuous, from a modal domain where, in fact, only the first mode carries significant energy. A simple algebraic formula is derived for this transition frequency that also yields good estimates of both the ‘‘optimum frequency’’ of Jensen and Kuperman [J. Acoust. Soc. Am.73, 813 (1983)] and the low‐frequency rolloff observed in some data.

ISSN:0001-4966    

DOI:10.1121/1.393556

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP