摘要:

A new computational capability is described for calculating the sound‐pressure field radiated or scattered by a harmonically excited, submerged, arbitrary, three‐dimensional elastic structure. This approach, callednashua, couples anastranfinite element model of the structure with a boundary element model of the surrounding fluid. The surface fluid pressures and normal velocities are first calculated by coupling the finite element model of the structure with a discretized form of the Helmholtz surface integral equation for the exterior fluid. After generation of the fluid matrices, most of the required matrix operations are performed using the general matrix manipulation package available innastran. Farfield radiated pressures are then calculated from the surface solution using the Helmholtz exterior integral equation. The overall capability is very general, highly automated, and requires no independent specification of the fluid mesh. An efficient, new, out‐of‐core block equation solver was written so that very large problems could be solved. The use ofnastranas the structural analyzer permits a variety of graphical displays of results, including computer animation of the dynamic response. The overall approach is illustrated and validated using known analytic solutions for submerged spherical shells subjected to both incident pressure and uniform and nonuniform applied mechanical loads.

ISSN:0001-4966    

DOI:10.1121/1.399320

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

An investigation is made of the modal contribution of a finite elastic plate, simply supported at both ends, to the power spectrum. Theoretical calculations were made for a finite steel plate submerged in a fluid and excited by two types of forcing functions: (1) a line force and (2) a statistically uniform force. Numerical results calculated for transfer functions and power spectra were presented. A comparison of power spectra calculated for different loading conditions is presented.

ISSN:0001-4966    

DOI:10.1121/1.399321

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The scattering of sound by sound from Gaussian beams that intersect at small angles is investigated theoretically with a quasilinear solution of the Khokhlov–Zabolotskaya nonlinear parabolic wave equation. The analytical solution, which is valid throughout the entire paraxial field, is a generalization of a result obtained previously for parametric receiving arrays [Hamiltonetal., J. Acoust. Soc. Am.82, 311–318 (1987)]. Significant levels of scattered sum and difference frequency sound are shown to exist outside the nonlinear interaction region. An asymptotic formula reveals that sound is scattered in the approximate directions ofk1±k2, wherekjis the wave vector associated with the axis ofjth primary beam. Computed propagation curves and beam patterns demonstrate the dependence of the scattered radiation on interaction angle, source separation, ratio of the primary frequencies, and source radii. Comparisons are made with the farfield results presented by Berntsenetal. [J. Acoust. Soc. Am.86, 1968–1983 (1989)], which are valid for arbitrary interaction angles, source separations, and amplitude distributions.

ISSN:0001-4966    

DOI:10.1121/1.399322

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Two different approaches have been used in the literature to discuss the nonlinearity of the scattered wave from a pulsating bubble in a liquid: The radial displacement approach and the volume displacement approach. These two treatments have been compared and it has been pointed out that there is no theoretical basis for the conclusion reached from the volume displacement approach that the second‐harmonic amplitude of the sound emission is zero at a specific frequency.

ISSN:0001-4966    

DOI:10.1121/1.399323

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The SAFARI (seismo acoustic fast‐field algorithm for range‐independent environments) wave propagation model is applied for the first time to low‐frequency atmospheric sound propagation and tested by means of synthetic and real‐world data. Problems experienced with the use of SAFARI are discussed in detail. Good agreement of the model output with theoretical predictions is achieved for two half‐spaces (air/ground). Available meteorological data up to 5 000‐m height are applied to the model in the next step in order to compute sound transmission loss versus range and receiver height. Results are given in transmission loss contour plots covering a field of 1 000 m in height and 30 000 m in range. The influence of typical sound velocity profiles including strong gradients close to the ground is investigated. It turns out that knowledge of meteorological data is most relevant for heights of up to about 200 m for sound propagation modeling within a field of interest as above. SAFARI is finally compared with a fast‐field program developed at the U.S. Army Construction Engineering Laboratory. This test yields very good agreement between both model predictions on a test case in which a real‐world sound profile is used. It has to be emphasized that all results are for very low frequencies where finite impedance ground effects are minimal. One would expect to see differences at higher frequencies. These encouraging results give rise to recommending the use of the models in a number of interesting applications such as various types of acoustic detection system and civil noise control.

ISSN:0001-4966    

DOI:10.1121/1.399324

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In recent years, short‐range measurements of excess attenuation from a point source have been used together with semiempirical formulas for frequency dependence of impedance to enable the acoustical characterization of ground surfaces, including snow, in terms of a single parameter. This has then been advocated as a basis for predicting ground and propagation effects at longer ranges. An alternative method is described for determining acoustical properties of ground surfaces including sands, soils, and snow from iterative least‐squares fitting of the level difference spectrum obtained between a pair of vertically separated microphones within 2 m of a broadband point source. The method is based on a three‐parameter model for the surface normal impedance as a function of frequency, together with well‐established formulations for propagation from a point source above either local or extended reaction surfaces. The three parameters are porosity, effective flow resistivity, and tortuosity. Independent (nonacoustic) measurements of porosity compare tolerably well with the acoustically determined values for soils that are homogeneous to several centimeters depth. For such soils, fitting comparisons reveal the superiority of the three‐parameter impedance model to the single‐parameter semiempirical model. Where there are obvious surface crusts, a double‐layer model based upon a two‐parameter approximation for the characteristic impedance of each layer is found to give better agreement with short‐range propagation measurements than the three‐parameter homogeneous approximation.

ISSN:0001-4966    

DOI:10.1121/1.399325

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A method is introduced to calculate the influence of wind and temperature gradients in stratified media on sound propagating above an absorbing ground surface. It is based on the ‘‘two‐way wave equation’’ for the Fourier transforms of the sound pressurePand its derivativeV. The vector containingPandVis stepwise extrapolated through the medium in the direction perpendicular to the ground surface, fulfilling the boundary conditions at the ground surface, at a top level, and at the source height. The propagation equations forPandVappear as simple plane‐wave equations, and computer (CPU) time within each layer is very low. Therefore, many thin layers (in the order of centimeters if desired) can be applied, and any complicated gradient can be used. Calculations for a homogeneous atmosphere, with a computer program based on this model, show an excellent agreement with previous models. When a wind profile is present, results are mainly compared with measurements by Parkin and Scholes [P. H. Parkin and W. E. Scholes, J. Sound Vib.1, 1–13 (1964);2, 353–374 (1964)]. They show very good agreement in the no‐wind and downwind cases. In the upwind situation, agreement is very good below 500 Hz. Above this value the model does not predict sound to penetrate into the shadow region, while Parkin and Scholes found (low) sound levels.

ISSN:0001-4966    

DOI:10.1121/1.399326

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The parabolic equation method is an effective approach when the acoustic wave field is incident at low grazing angles onto a rough surface. The method consists of an integral equation and an integral, the first of which yields the surface field derivative. The main part of this paper is concerned with an approximation to this equation, valid when wavenumber times surface height is up to order unity. The approximation has several advantages. First, it allows a decomposition of the equation into deterministic and stochastic components. The stochastic part depends only locally upon the surface in certain regimes, and this can give rise to a great reduction in computational expense. Some basic statistical moments of the stochastic component are also considered. These are nonstationary, but for the incident field a simple stationary transformation is found, which can therefore be compared with Monte Carlo simulations using far fewer realizations. These results are demonstrated computationally. The final part of the paper describes the numerical implementation of the full parabolic equation method. Both of the integrals contain singularities, and these are treated semianalytically.

ISSN:0001-4966    

DOI:10.1121/1.399327

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Observed spatial and temporal characteristics of ultralow‐frequency (ULF) ocean‐bottom seismo‐acoustic ambient noise are required in order to construct realistic quantitative predictive models of the phenomena involved. Few such data exist or have been studied, especially for frequencies below about 0.1 Hz. Analysis of noise data is presented in the band 0.002 to 0.4 Hz from a 2‐week period, 11/28–12/12/67, recorded from long‐period, three‐component seismometers and a hydrophone of the Columbia‐Point Arena ocean‐bottom seismic station (OBSS, 38° 09.2’N–124° 54.4’W, 3903‐m depth). Two intense NE Pacific storms with hurricane force winds occurred during the emphasized time period. Time variations of spectra and of amplitude and phase coherencies of the four‐component OBSS data are related to the storm histories and to local weather/wave conditions and are used to identify motion (seismic wave) types and directions of propagation.

ISSN:0001-4966    

DOI:10.1121/1.399328

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Fast‐field programs (FFPs) have emerged as an important tool for predicting transmission loss in an ocean waveguide. Such models have been primarily used for time‐harmonic sources; however, pulses or other broadband sources may be treated by Fourier synthesis. A new technique is developed that provides adirectsolution by marching the solution forward in time. As an example of the method, a pulse incident on an interface between two homogeneous half‐spaces is considered. Snapshots of the pulse in time illustrate graphically the effects on the reflected and transmitted waves. Second, an interesting hyperbolic cosine profile is considered that leads to repeated focusing as the pulse propagates out in range.

ISSN:0001-4966    

DOI:10.1121/1.399329

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP