In order to determine the effects of low frequency underwater sound on small animal models, it is desirable to expose them to well characterized fields which closely simulates the (locally) plane wave open ocean stimulus. It is also desirable to produce pure pressure and acceleration stimuli to isolate the effects of the individual components of the acoustic p]ane wave. It is necessary to produce sufficiently strong signals to enable damage thresholds to be determined. The range of frequencies over which such stimuli must be produced is quite broad since it must include the actual band of interest (100-500 W) to examine damage on the cellular and tissue level. as well as at scaled frequencies (-l-5 W) to examine the effects of organ structure. The problem is subtler than one might imagine because of the high compliance of the test animals due to the high compliance of the test animals due to air in their lungs. Careful consideration is required to ascertain what is required of the chamber to assure valid measurements and to determine the true effective sound pressure level.
The seabed dominates the shallow-water acoustics problem, but bottom properties and bottom scattering mechanism are usually poorly known. Detailed interference patterns in shallow water are thus not always physically meaningful for engineering applications. A simple analytical expression averaged over frequency or space can sometimes give better insight into some physical problems. In this presentation, an average angular power spectrum method for calculating shallow-water sound propagation, reverberation, noise, and spatial coherence is briefly introduced. The method is based on normal-mode and ray-mode analogies and originally appeared in Chinese papers which are not available in English [Zhou, Acta Acoust. Sin. 5, 86–99 (1980); Acta Ocean. Sin. 1, 212–217 (1979)]. Taking active sonar performance prediction as an example, analytical expressions for range/depth dependences of the sound propagation, average reverberation intensity, and echo-reverberation ratio will be given for several typical cases, including Pekeris shallow water, wedged continental shelf, and negative gradient waveguide. [Work supported by ONR.]
A large underwater acoustic tank facility located in the Woodruff School of Mechanical Engineering at Georgia Tech has recently been completed. The facility includes a rectangular concrete water tank 25 feet deep, 25 feet wide, and 34 feet long containing around 160,000 gallons of water. There are three computer-controlled positioners: an x-y-z-θ positioner and a z-θ positioner mounted on carriages and a bottom mounted rotator. The facility has a large rectangular nearfield array which can be used either as a receiver or a transmitter. A single vertical nearfield line array can be translated by the x-y positioner to synthesize a cylindrical nearfield receiving array. The rectangular nearfield transmitting array and the synthesized cylindrical receiving array were designed to be used with the bottom mounted rotator to measure the true farfield bistatic target strength of any target up to one meter in length as a function of the target aspect angle. Such measurements can be done from 2 kHz to over 10 kHz. The tank is being used for transducer development, materials, and flow noise studies in addition to structural acoustics. Several available multichannel data acquisition systems will be described. [Work supported, in part, by a DURIP grant from ONR.]
Simultaneous observations of internal wave activity and acoustic wave propagation in 70 m water in the Yellow Sea were made in the late summer of 1996. The objective of the experiment was to validate the predicated modal coupling and resulting fluctuations and alterations in propagation loss induced by shallow-water internal waves. Propagation over distances up to 50 km (using narrow- and broadband sources over the frequency range of 50 Hz to 6 kHz) was measured with moored and suspended arrays which spanned the water column. Internal wave activity was monitored using several thermistor chains. Details of the experiment and preliminary data will be presented. [Work supported by ONR and the Chinese Academy of Sciences.]
There is currently an abundance of both mathematical and finite-element models of the scattering of underwater sound from cylindrical bodies treated with compliant coatings. However, there is a significant lack of corresponding scattering measurements. This paper presents the results of a set of simple measurements of the monostatic backscatter from a coated, thin, ribbed cylindrical shell with flat endcaps for ka of 2–8. The coating layer had an input impedance approximately one-fifth that of water, providing a near pressure-release boundary condition. Of particular interest is a minimum in the measured target strength which results from the resonant interaction of the coating and the shell. Results from finite-element and modal expansion models of the coated shell are also presented with the aim of providing a physical understanding of the resonance. [Work supported by ONR.]
In Part I [Turek et al.] a numerical solution for a two-body acoustical multiple scattering problem based on the analytical infinite series solution of the wave equation was presented. Surface and far-field solutions involv- ing permutations of the two ‘‘degenerate’’ boundary conditions (pressure release and rigid) were compared to solutions obtained with the combined Helmholtz integral equation formulation problem (CHIEF) program. In this paper the case in which one of the spheres is fully elastic and the other pressure release is modeled and the results verified experimentally.
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