Journal of Computational Acoustics

The Biot model is widely used to model poroelastic media. Several authors have studied its applicability to cancellous bone. In this article the feasibility of determining the Biot parameters of cancellous bone by acoustic interrogation using frequencies in the 5–15 kHz range is studied. It is found that the porosity of the specimen can be determined with a high degree of accuracy. The degree to which other parameters can be determined accurately depends upon porosity.

In many long-range propagation experiments the source and receiver are placed close to the depth of the waveguide (SOFAR) axis to minimize the interaction of the acoustic field with the ocean's surface and bottom. The time-of-arrival patterns of these experiments consist of resolvable, geometrical-like arrivals followed by an axial crescendo of unresolved energy. It is impossible to explain this late-arriving energy using the geometrical acoustics because of the presence of cusp caustics repeatedly along the waveguide axis. The interference of the wave fields corresponding to the rays located in the vicinity of the caustics near the waveguide axis produces a special "axial wave" that propagates along this axis. The purpose of the paper is to obtain the integral representation of the axial wave for an arbitrary deep-water waveguide in a range-independent medium in long-range acoustic propagation in the ocean when the source and receiver are located close to the depth of the sound-channel axis. The integral representation for the axial wave is derived with the use of solutions of the Helmholtz equation concentrated near the sound-channel axis and which decrease exponentially outside a strip containing the axis. These solutions have the form of the exponentials multiplied by parabolic cylinder functions whose arguments are sections of series in powers of ω^-1/2, where ω is a cyclic frequency. Numerical results are obtained for the Munk canonical sound-speed profile.

Matched-field processing (MFP) localizes sources more accurately than plane-wave beamforming by employing full-wave acoustic propagation models for the cluttered ocean environment. The minimum variance distortionless response MFP (MVDR–MFP) algorithm incorporates the MVDR technique into the MFP algorithm to enhance beamforming performance. Such an adaptive MFP algorithm involves intensive computational and memory requirements due to its complex acoustic model and environmental adaptation. The real-time implementation of adaptive MFP algorithms for large surveillance areas presents a serious computational challenge where high-performance embedded computing and parallel processing may be required to meet real-time constraints. In this paper, three parallel algorithms based on domain decomposition techniques are presented for the MVDR–MFP algorithm on distributed array systems. The parallel performance factors in terms of execution times, communication times, parallel efficiencies, and memory capacities are examined on three potential distributed systems including two types of digital signal processor arrays and a cluster of personal computers. The performance results demonstrate that these parallel algorithms provide a feasible solution for real-time, scalable, and cost-effective adaptive beamforming on embedded, distributed array systems.

This paper is concerned with the use of the reassigned wavelet transform for mode identification in shallow water acoustic propagation. Mode identification is important for inverse procedures in underwater acoustics. An efficient way to recognize the modal structure of the acoustic field when a single hydrophone is available is to refer to the time frequency analysis of the recorded signal using wavelet transform. However, the standard wavelet transform in some cases may result in an obscure representation of the dispersion curves. Thus, a reassigned process is proposed which brings important improvements in the time frequency representation of the signal. This is achieved by moving the calculation point of the scalogram in the center of gravity of the energy concentration, associated with each one of the propagating modes. This argument is supported by two illustrative examples corresponding to propagation of low frequency tomographic signals, in shallow water.

Broadband and narrowband time-domain asymptotics are proposed for pulse propagation in range-independent ocean environments. The broadband approximation results by applying the stationary-phase method to the Fourier transform of the Green's function, expressed in terms of normal modes. The narrowband approximation is obtained by incorporating the shape function of the emitted signal — assumed Gaussian — into the phase term and applying the steepest-descent method. The roots of the frequency-derivative of the phase are located in the complex plane by using a second-order expansion of the eigenvalues. The performance of the two approximations is studied numerically. While the broadband approximation improves with increasing bandwidth, the narrowband approximation improves when the bandwidth decreases. Both approximations improve with increasing range, and they can be used for delivering time-domain results more efficiently than with standard Fourier synthesis.

A modal method is developed for solving, analyzing, and controlling vibration and sound radiation of coupled fluid-structure systems. The method recasts the coupled equation of a coupled fluid-structure system in the classical matrix structural dynamic equation by modeling the acoustic load vector as direct linear function of the acceleration, velocity, and displacement vector. With the Rayleigh damping assumption of the coupled fluid-structure system the resulting equation can be uncoupled via a transformation to modal coordinates and analyzed by solving independent equations of single degree of freedom system. The modal radiation efficiencies, effect of modal interaction on sound radiation, mode shapes, and modal control of the coupled fluid-structure system are presented and discussed. Numerical example of the vibration and sound radiation of a fluid-loaded stiffened plate is presented solely as a vehicle to demonstrate the method. The comparisons in terms of computed sound power of the present method with the standard coupling method and available published results show a very good agreement. The mode shapes and the self- and mutual-radiation efficiencies of modes of the fluid-loaded stiffened plate are given and discussed. The study of the effect of modal interaction on sound power shows that the power radiated by a single mode is to increase total radiated power and the interaction of modes may lead to an increase or a decrease or no change in the total radiated power. Numerical results also show that the modal control achieves good reductions in the mean square velocity and the sound power of the fluid-loaded stiffened plate.

A new method for predicting the scattered acoustic field due to a plane wave incident upon an infinitely long cylinder lying near an penetrable plane interface is presented. The method generalizes the method of images which is restricted to rigid and soft plane interfaces. Validity domains, physical interpretations, simulations and numerical results are described for sedimentary medium-fluid plane interfaces. And, they are well compared with high frequency asymptotic results based on the Geometrical Theory of Diffraction(G.T.D.).

This paper is concerned with the numerical modeling of elastic wave propagation in layered media. It considers two isotropic homogeneous elastic solids in perfect contact. The interface is parallel to the free surface. Two finite difference methods are developed. The usefulness of the methods are investigated for long time simulations and the accuracy of the results are compared with the response from an approximate model.