摘要:

A method of generating very strong focused negative pressure fields in water for cavitation studies from shock waves produced by an electromagnetic transducer is described. By using a reflecting concentrator with a phase-inverting central element, focal pressures of around−16 MPafrom a+5-MPasource have been measured. The combination of an electromagnetic transducer and a phase-inverting parabolic concentrator provides a reliable and reproducible source of cavitation bubbles for study. A qualitative study of the cavitation field produced by the device, using a laser schlieren technique, is presented.

ISSN:0001-4966    

DOI:10.1121/1.420310

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The paper presents formulas for the statistical moments of a plane sound wave propagating in moving random media (the turbulent atmosphere and ocean, turbulent flows of gases and fluids, etc.) with arbitrary spectra of temperature and medium velocity fluctuations. These statistical moments, which are most often of interest, are the following: the variances of log-amplitude and phase fluctuations, the correlation functions of log-amplitude and phase fluctuations, the mean sound field, the coherence function of a sound field, and the sound scattering cross section per unit volume. These statistical moments are calculated for Gaussian spectra of temperature and medium velocity fluctuations. It is shown that the contributions to the statistical moments due to sound scattering by medium velocity fluctuations may dramatically differ from those due to sound scattering by temperature fluctuations.

ISSN:0001-4966    

DOI:10.1121/1.420311

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Formulas for the statistical moments of a spherical sound wave propagating in a medium with arbitrary (anisotropic) spectra of temperature and medium velocity fluctuations are obtained. These statistical moments are: the variances of log-amplitude and phase fluctuations, the correlation functions of log-amplitude and phase fluctuations, the mean sound field, and the coherence function of the sound field. Then, the statistical moments of a spherical sound wave are calculated analytically and numerically for Gaussian spectra of temperature and medium velocity fluctuations. It is shown that the temperature and medium velocity contributions to these statistical moments can differ not only quantitatively but also qualitatively. The useful relationships between the statistical moments of spherical and plane waves propagating in a moving random medium are derived. Some of the theoretical results obtained are compared with experimental data from the literature on sound propagation through the turbulent atmosphere.

ISSN:0001-4966    

DOI:10.1121/1.420312

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A unified theory of sound propagation in saturated marine sediments is developed on the basis of a linear wave equation, which includes a new dissipation term representing internal losses arising from interparticle contacts. This loss mechanism, which shows a “memory” or hysteresis, is proposed as being responsible for the acoustic properties of sediments. To accommodate the memory, the loss term in the wave equation is formulated as a temporal convolution between the particle velocity and a material response function,h(t),which varies ast−n,where0

ISSN:0001-4966    

DOI:10.1121/1.420313

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The effects of internal waves on low-frequency (25-Hz) wide-band source localization performance are investigated at a very long range (1000 km). Sound-speed perturbations induced by internal waves obeying the Garrett–Munk spectral model are incorporated into the normal mode solution to the wave equation using the adiabatic approximation. Source range estimates are limited by very random internal waves and do not improve with increasing SNR. In an attempt to combat this limitation due to environmental uncertainty, processors can benefit from the spectrum of the acoustic source. According to the hybrid Cramer–Rao lower bound (CRLB), significant improvements on range estimates are possible when using a coherent wide-band acoustic source with bandwidth of 10 Hz on the 25-Hz source. The CRLB provides valuable analytical insight into the limits on source range estimates, but the optimal signal processing algorithm is necessary to compute the actual limits on source localization. Using a coherent wide-band source with many discrete frequencies spanning the bandwidth, significant gains in range estimates are achieved by optimally incorporating this signal spectrum into the localization algorithma priori.

ISSN:0001-4966    

DOI:10.1121/1.420314

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Wave breaking at the surface of the ocean entrains bubbles, significantly modifying the phase speed and attenuation of acoustic waves propagating through the resulting two-phase medium. An autonomous buoy system was developed that directly measures sound speed at 3.33, 5, and 10 kHz at seven depths ranging from 0.7 to 7 m through the use of a travel-time technique. Simultaneous measurements at each depth are obtained at a 2-Hz rate, allowing observation of the unsteady sound-speed field from individual bubble injection events, as well as the calculation of mean sound speeds. The travel-time technique allows a direct measurement of the sound speed, eliminating the uncertainties common with inferring sound speeds from bubble population data. The sound speed buoy was deployed in the North Atlantic during the winter of 1993–94 as part of the Acoustic Surface Reverberation Experiment (ASREX). Our aim was to characterize the highly variable near-surface sound-speed field under varying environmental conditions. Forty-three days of data were obtained spanning several storm cycles with wind speeds and significant wave heights reaching 20 m/s and 8 m, respectively. During periods of intense wave breaking, average sound speeds below 1000 m/s were observed at the 0.7-m measurement depth while instantaneous sound speeds during individual events approached values as low as 300 m/s. Furthermore, the data suggest that the dispersive effects of bubbles may extend to frequencies as low as 5 kHz near the surface during storms. Strong correlations of the mean and rms sound speed with the overlying wind and wave fields were found.

ISSN:0001-4966    

DOI:10.1121/1.420315

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A perturbative method has recently been presented for improving parabolic equations to account for intermediate backscattering induced by slow range dependence of the acoustic index of refraction. In realistic cases, it yields solutions to the full wave equation from a truly one-way algorithm, and generalizes a number of special results of this kind which have been known for many years. Numerically validating the method, though, involves certain unique difficulties, because the physical corrections it introduces to local evolution are accompanied by medium dependence of the projection operator that defines the input and output wave fields. A method for benchmarking is presented here, which permits numerical isolation of range-local effects from the endpoint projections, and also from particular details of the choice of one-way model used. To illustrate that the physical content of the new terms can be sensibly validated in this manner, an exact solution is considered, where they are shown to correct the “idealized” one way equation on which current parabolic algorithms are based. This solution is used to illustrate the relation of energy conservation to one-way evolution in the presence of slow range dependence. In particular, it demonstrates that energy conservationaloneis not a sufficient requirement to produce correct parabolic evolution, even when it pertains.

ISSN:0001-4966    

DOI:10.1121/1.420316

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An analytical model is developed for the vertical coherence of the ambient noise field generated by a plane of sources at a finite depth beneath the ocean surface. To clarify the effects of source depth on the noise field, the relatively simple case of a semi-infinite ocean with an isovelocity profile is considered. The expression derived for the coherence is exact; it depends on the source depth explicitly, and it includes the homogeneous and inhomogeneous components of the field. When the sources are shallow, that is, the source depth is much less than a wavelength and the source–image pairs act as dipoles, the coherence is an oscillatory function of frequency, consistent with an earlier theory of noise coherence in deep water. With deeper sources, the dipole description fails and the coherence function becomes approximately independent of frequency. This change of character suggests that the spatial structure of the noise field at depth in the ocean could be inverted to yield information on the acoustic properties of the bubble sources associated with breaking surface waves.

ISSN:0001-4966    

DOI:10.1121/1.420317

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A method for determining the track of a source emitting continuous broadband acoustic energy in an oceanic waveguide is presented. The method involves cross correlating the measured signals at horizontally separated receivers over a period of time and identifying the traces on the resulting correlogram in terms of the ray paths at each receiver that produce them. Range and bearing information is contained in the structure of the multipath correlation traces. Environmental parameters are used as inputs to a ray model to obtain the ray travel timest̄as functions of rangeR.Thet̄j(R)functions for the ray multipaths are used to obtain simulated correlogram time delays given a source track and receiver geometry. Constant-velocity, constant-depth source tracks are parametrized by four variables, and a nonlinear optimization algorithm is used to find the track that provides the best fit between measured and simulated correlation traces. The method is applied to measurements of a passing surface ship in the shallow waters of the English Channel. Successful localization is obtained using 4-min intervals and correlation traces produced by rays with 3–15 traversals of the water column. Receiver separation, unknown because of experimental uncertainties, is also obtained.

ISSN:0001-4966    

DOI:10.1121/1.420318

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An earlier paper [J. Acoust. Soc. Am.99, 2055–2066 (1996)] tackled the calculation of ambient noise and noise coherence in a range-independent reflecting or refracting environment with horizontal distributions of noise sources. The approach was analytical producing some closed-form solutions and never more than a single well-behaved integral to solve. In this paper the azimuth and elevation dependence of the noise over a sloping bottom is investigated. The analytical and graphical results explicitly show the effects of upslope and downslope bottom reflections on the “noise notch,” and in passing they explain the flatness of the angle distribution sought by Dashen and Munk [J. Acoust. Soc. Am.76, 540–554 (1984)]. It is also clear that, contrary to one’s intuition, three-dimensional horizontal bending effects on the noise directionality may be significant because of the large contribution from distant sources.

ISSN:0001-4966    

DOI:10.1121/1.420319

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP