Volume 103, Issue 5, May 1998
Index of content:
- UNDERWATER SOUND 
103(1998); http://dx.doi.org/10.1121/1.422752View Description Hide Description
It is demonstrated that seismicinterfacewaves on the surface of a natural beach can be used to identify the position of a buried object. For this experiment, the waves were created with a sediment-coupling transducer and received on a three-element horizontal line array of triaxial geophones. The source and its coupling to the medium provided a high degree of signal repeatability, which was useful in improving signal-to-noise ratio. Reception of all three directions of particle velocity made it possible to augment conventional beamforming techniques with polarization filters to enhance interface-wave components. Reverberation in the beach was found to be large, though, and coherent background subtraction was required to isolate the component of the sound field reflected by the target. Propagation loss measurements provided comparisons of reflected signal power with predictions made previously, and the two were found to agree closely.
103(1998); http://dx.doi.org/10.1121/1.422753View Description Hide Description
A method is presented for efficiently computing or estimating low-order elastic wave scattering, from a pointlike inhomogeneity in a bounded medium, into interface as well as bulk modes. The particular system considered is a half-space of fluid overlying a half-space of a linear elastic solid, with the obstacle placed in the solid very near the interface. Boundary constraints are enforced as an improvement to the bulk scattering vertex, so that the stratified-medium Green’s function is used to propagate scatteredwaves at each perturbative order. Analytic approximation of the first-order scattering amplitude, based on the symmetries of the scattering vertex and the interface-wave pole structure, makes it easy to identify the qualitatively different components of the scatteredwave, and their dependence on medium properties.
A fast integral equation approach to acoustic scattering from three-dimensional objects using a natural basis set103(1998); http://dx.doi.org/10.1121/1.422754View Description Hide Description
Boundary value integral equations and the moment method solution technique provide a succinct and elegant strategy for the solution of acoustic surface scattering problems. This paper explores the application of a subdomain basis set, based on the phase of the incident radiation, which is used to solve for the unknown normal pressure gradient on the surface of the scatterer using the moment method. The advantage of this basis set lies in the fact that it can accurately represent the fast phase character of the pressure gradient on the surface of the scatterer. This allows for far fewer discretizations/basis elements in the moment method formulation and therefore significantly reduces the density of unknowns that must be numerically computed. The main contribution of this paper is to show how to efficiently employ this natural basis and to explore the physical justification for using this approach. The method is compared with other techniques including the Kirchhoff solution technique, high frequency single scattering techniques, the Nussenzveig–Fock technique, and the parabolic equation approximation. Unlike these methods no approximation to the physical system need be introduced.
103(1998); http://dx.doi.org/10.1121/1.422755View Description Hide Description
Acoustic radiation produced by end-fired explosive line charges is examined experimentally and theoretically. Three explosive compositions are studied: PETN detonating cord, RDX detonating cord, and thin ribbons of HLX. Specific output levels are formulated for each of the three explosive materials over the four 1-oct bands spanning 50–800 Hz. Bubble period relationships are derived for each material and compared to previous studies. In terms of directivity effects, long line charges are found to behave like beam-steered continuous arrays over moderately wide bands. A math model is developed which predicts acoustic levels and spectrum shapes for the shock wave component over a wide range of view angles and linear charge densities. This model is driven by two parameters which are extracted from the experimental data for the three compositions studied. The result is a practical means of designing explosive line arrays to desired source levels, bandwidths, and beam patterns.
103(1998); http://dx.doi.org/10.1121/1.422756View Description Hide Description
Nonlinear scattering due to bubble oscillations was theoretically and experimentally investigated for bubble size distribution measurements in water. If two primary acoustic waves of different frequencies are incident on a bubble, the difference frequency component of primary waves is produced in the scattered field. The incoherent scattering at the difference frequency from bubbles was theoretically studied. It was shown that the difference frequency amplitude is proportional to the density of bubbles having resonance frequencies close to the difference frequency. It allows the use of difference frequency measurements to determine the bubble density in water. For experimental investigations two focused transducers were used to increase the levels of primary waves in the interaction zone. One of the primary frequencies was kept constant 2.25 MHz while another was changed from 2.22 to 1.93 MHz, so that the observed difference frequency was varied from 30 to 320 kHz. This frequency band corresponds to the resonant bubble radii from 109 to 10 μm. For the bubble clouds produced in a laboratory tank by an electrolysis-type and a slit-type bubble maker the bubble densities were well estimated with the present nonlinear acoustic method.