摘要:

Results of an experimental investigation are presented that describe the characteristics of discrete sound generated by a constant area ejector that uses an underexpanded rectangular jet as the primary flow. The variation of screech tones, which are commonly observed in the spectrum of free shock‐containing jets with stagnation pressure and area ratio, show a staging behavior due to the presence of the ejector duct. Such a behavior is explained with the use of a feedback loop. In addition to screech tones, a different type of resonant oscillation within the duct is identified. These consist of acoustic resonances whose frequencies and mode shapes are related to the unsteady flow in the duct and the size of the duct. Two dominant symmetric (varicose) and antisymmetric (flapping) modes of the primary jet determine the nature of the ejector tones that are observed in the spectrum. Using a phase‐locked schlieren system, different longitudinal and transverse duct modes are photographed.

ISSN:0001-4966    

DOI:10.1121/1.414655

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

It is well known that the acoustic scattering from a swimbladder fish is dominated by the gas‐filled swimbladder. In this paper, the sound backscattered by a fish swimbladder of an arbitrary geometrical shape is studied. Particular attention is paid to the scattering at low frequencies, i.e., the acoustic wavelength is much larger than the size of swimbladder. A surface average approach is presented to study the sound scattering by the fish swimbladder. A formula is derived for the low‐frequency scattering function of an arbitrary swimbladder, incorporating the familiar resonance feature of a gas‐filled scatterer. The theory is applied to a prolate spheroid gas bubble, yielding favorable results compared to that obtained by a rigorous approach. As an application, the theory is also used to study the low‐frequency acoustic scattering of some fish, and the results are compared with some existing experimental data. It is shown that the theoretical results agree reasonably well with the data for frequencies near or below resonance frequencies, but significant discrepancy remains for higher frequencies.

ISSN:0001-4966    

DOI:10.1121/1.414656

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A coupled normal‐mode approach for the prediction of pulse propagation in a complex coastal environment is presented. In this broadband approach, the received time‐domain sound‐pressure signal is generated by Fourier synthesis using the product of the source signal spectrum and the source‐to‐receiver ocean transfer function. For a given frequency, the basic formulation involves decomposing the acoustic pressure into slowly varying complex envelopes that modulate (mode by mode) analytic, rapidly varying, adiabatic‐mode solutions. A coupled set of differential equations governing these complex modal envelopes is derived and the numerical solution techniques used are discussed. This coupled‐mode approach has been used to model the modal arrival structure in the 1992 Barents Sea Coastal Acoustic Tomography Test. Model results and comparison to the data measured by a vertical hydrophone array are discussed.

ISSN:0001-4966    

DOI:10.1121/1.414673

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

During August 1992, a combined acoustics/physical oceanography experiment was performed to study both the acoustical properties and the ocean dynamics of the Barents Sea Polar Front in the region near Bear Island. Oceanographic observations from shipboard hydrography and moored sensors allowed the construction of the internal wave frequency spectrum for the area. A rapidly sampled tomographic section from a 224‐Hz, 16‐Hz‐bandwidth acoustic source to a 16‐element vertical receiving array enabled the monitoring of travel‐time fluctuations over the internal wave frequency band. To describe the measured acoustic fluctuations, theoretical expressions have been developed for the travel‐time variances which are functions of the internal wave oceanographic field, the local acoustic propagation characteristics, and the acoustical system’s properties. Both ray and mode theory expressions are generated, as the experiment was performed in shallow water and both ray and mode arrivals were resolvable. Comparison of the theory with the data shows generally good agreement. However, due to the fact that the internal wave study is a secondary objective of this experiment, the data are not of the quality eventually hoped for. Directions for future research in this area are outlined and discussed.

ISSN:0001-4966    

DOI:10.1121/1.414657

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

In a recent paper, Lynchetal. used modal and ray based perturbation techniques to compare predicted variances of acoustic travel times due to internal waves to measured variances in the Barents Sea Polar Front experiment [Lynchetal., J. Acoust. Soc. Am.99, 803–821 (1996)]. One of the interesting results of this work is that the modal and ray travel‐time variances are substantially different for rays and modes with the same grazing angle. Specifically, the maximum modal travel‐time variance shows a resonant effect in which the variance increases with increasing frequency. Unlike the modal solution, the ray travel‐time variance has a geometrically constrained maximum, independent of frequency. In this paper, the linear first‐order solutions for the ray and modal variances due to the internal waves are reviewed, and in an Appendix the effects of the linearizing assumptions are examined. The ray and mode solutions are then shown to be consistent by considering a truncated sum of modes that constructively interfere along a geometric ray path. By defining the travel‐time perturbation due to a truncated sum of modes, the travel‐time variance of the modal sum is derived. With increasing frequency the maximum value of this variance converges to a frequency‐independent result with a similar magnitude to the ray maximum variance.

ISSN:0001-4966    

DOI:10.1121/1.414563

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Recently, an efficient, numerically robust algorithm for calculating acoustic normal modes called MODELAB was implemented [J. Acoust. Soc. Am.97, 1576–1585 (1995)]. In this paper, a new approach is introduced and shown to significantly reduce the most computationally intensive aspect of MODELAB: the requirement of fine wave number presampling during the search for eigenvalues. Analytic derivatives (up to sixth order) are efficiently calculated using a new recursion relation for the characteristic equation. The derivatives are used to form Padé approximants, which in turn, are used to locally extrapolate the characteristic equation to find successive eigenvalues. Results indicate that an algorithm using this technique can be more than a factor of five times faster than the MODELAB algorithm, and the new technique is shown to be particularly efficient at near‐degenerate eigenvalues, which previously had been the most difficult to identify.

ISSN:0001-4966    

DOI:10.1121/1.414658

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The author has previously reported [J. Acoust. Soc. Am.88, 324–334 (1990)] on a model for acoustic backscatter from the volume beneath a smooth sediment seabed. In the present paper, the model is generalized to an in‐plane scattering model such that the source and receiver need not be colocated but must lie in the same vertical plane. In the model, scattering is caused by fluctuations in sediment porosity. The model allows for penetration of the incident wave into the bottom at subcritical grazing, and retransmission of scattered spherical waves through the (planar) interface. The frequency and grazing angle dependence of the acoustic scatter are determined primarily by the correlation function of the porosity fluctuations, whereas the magnitude of the scatter is controlled by the porosity variance. Results are obtained from the model over the frequency band 5–200 kHz, for a variety of incident and scattered grazing angles, using two sample porosity correlation functions. The first is an exponentially decaying correlation function and the second is a point scatterer correlation function. The results obtained illustrate the significance of the correlation function in determining the scattering strength of the bottom volume.

ISSN:0001-4966    

DOI:10.1121/1.414659

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Accurate numerical solutions generated with a new computational model are presented for several range‐dependent underwater acoustic problems involving penetrable ocean bottoms. The multifilament source model [Boagetal., ‘‘Analysis of acoustic scattering from fluid cylinders using a multifilament source model,’’ J. Acoust. Soc. Am.83, 1–8 (1988)], originally developed to treat acoustic scattering by arbitrarily shaped cylinders embedded in an infinite medium, has been extended to solve short‐range ocean acoustic problems. Source model solutions for two different range‐dependent problems agree with solutions that were obtained with Evans’ COUPLE model [R. B. Evans, ‘‘A coupled mode solution for acoustic propagation in a waveguide with stepwise depth variations of a penetrable bottom,’’ J. Acoust. Soc. Am.74, 188–195 (1983)]. These comparisons demonstrate the source model’s ability to describe propagation in a shallow water environment with rapidly varying bathymetry and a lossy, penetrable bottom.

ISSN:0001-4966    

DOI:10.1121/1.414660

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Multiyear Arctic Sea ice is a heterogeneous material whose acoustic properties are functions of time and space. Over the past few years a number of investigators have studied the reflectivity of sound at the ice–water interface for sound incident from the water. The studies have considered reflections of low‐ and high‐frequency sound for fixed times of the year. From data collected in a recent year‐long field experiment conducted in the Arctic, the reflection coefficient at the ice–water interface for sound incident from the water column has been determined as a function of time. The purpose of this paper is to study the temporal evolution of the reflection coefficient with a view to explain the observed fluctuations in the magnitude of the reflection coefficient. A thermodynamic model of ice growth was used to obtain the temperature and salinity structure of the ice for each month of the year and an elastic model was used to calculate the corresponding compressional and shear wave speeds. The reflection coefficient was calculated and the results were compared for various changes in the input parameters to the models. Correlation exists between the model and field measurements of reflection coefficient during the months of September–April. The model does not predict the behavior observed during June–August. It is believed that the reason for the discrepancy is associated with a change in water column properties during this time.

ISSN:0001-4966    

DOI:10.1121/1.414661

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

According to spectral analyses of many crosswell acoustic tomograms and cores collected from ten seabed locations [Yamamoto, J. Acoust. Soc. Am.98, 2235–2248 (1995)], the 3‐D power spectra of velocity and density fluctuations in the seabed sediments are strongly anisotropic and dipping in general. Anisotropy, dip, and spectral properties of the fluctuations vary greatly depending on the sediment type and the geographical location. Based on the Born approximation and the Wood sediment model, an analytical solution has been obtained for the acoustic wave field scattered from the velocity and density fluctuations within sediment volume having arbitrary 3‐D power spectra. Relative density fluctuations are proportional to relative velocity fluctuations in the sediment. The proportionality constant varies from over ten for soft sediments to one for dense sediments. Thus density fluctuation is the dominant mechanism for scattering from a sediment volume. The grazing angle dependence of acoustic backscattering is strongly affected by the fluctuation anisotropy. Dip in the 3‐D power spectra of fluctuations causes azimuthal dependence of acoustic backscattering. The model agrees excellently with the backscattering data reported by Jackson and Briggs [J. Acoust. Soc. Am.92, 962–977 (1992)] and Mourad and Jackson [J. Acoust. Soc. Am.94, 344–358 (1993)]. Model‐data comparisons confirmed the three theoretical findings: The dominant volume scattering of density fluctuations in soft sediments, the anisotropy‐grazing angle relationship and the dip‐azimuthal angle relationship of acoustic backscattering. In addition, the model predicts that the moderate frequency dependence of acoustic volume scattering is affected by a power low spectra of velocity and density fluctuations and attenuation within the sediments.

ISSN:0001-4966    

DOI:10.1121/1.414662

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP