Gary W. Caille

Test chamber for determining damage thresholds for high‐amplitude underwater sound exposure in animal models

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 simulate 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 plane 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 Hz) to examine damage on a cellular and tissue level, as well as at scaled frequencies (∼1–5 kHz) 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 air in their lungs. The lungs will drastically alter the acoustic field in the chamber and change the loading on the sources and the chamber could alter the vi...

(3,1) Drive PVDF acoustic displacement sensor

A (3,1) drive piezofilm vibration sensor is introduced. Operated above the lumped element resonance frequency of 600 Hz, the sensor delivers a voltage signal proportional to displacement over the frequency range of 2 kHz to 8 kHz. It is anticipated that the sensor response is flat above 8 kHz, but calibration has not been performed at higher frequencies. The sensor is very sensitive, detecting acoustic displacements as small as 10−5 nm. Because of its simple design the sensor is robust and easy to assemble.

Average acoustic field in shallow water I: Active sonar performance prediction

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, incl...

Laser vibrometer analysis of sensor loading effects in underwater measurements of compliant surface motion

The application of contact motion sensors, such as accelerometers, in the measurement of the vibration of compliant surfaces underwater may lead to errors in the evaluation of certain types of surface motion. An underwater scanning laser Doppler vibrometer (USLDV) was used to measure the scattered velocity field due to a mock sensor (rigid, neutrally buoyant cylindrical body) on a compliant surface (the outer surface of a thin cylindrical shell coated with a layer of soft rubber). Axially propagating waves were launched in the shell by a ring of 10 uniformly distributed shakers located near one end of the shell and driven with a short pulse. The outer surface of the coating was scanned over a short line segment in the axial direction with and without the mock sensor attached. The extracted scattered field, consisted of high wavenumber fluid‐solid interface waves accompanied by rotational motion of the mock sensor. [Work supported by ONR]

New underwater acoustic tank facility at Georgia Tech

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.]

Normal‐mode analysis for signal fluctuations in the Yellow Sea

This paper presents a normal mode approach for analyzing signal fluctuation in shallow water with a thermocline which fluctuates due to tides and internal waves. In the Yellow Sea 1996 experiment, variations of thermocline structure and internal waves were observed by CTD and thermistor chains. The cw and FM acoustic signals were measured over a 12‐h period with two 16‐hydrophone vertical arrays deployed 3.11 and 10.32 km from the source. The cw pulses were used to observe fluctuation at four specified frequencies. Amplitude fluctuations of more than 20 dB were observed. The FM signals were used to isolate individual normal modes by pulse compression and mode filtering. The influence of internal waves on the normal‐mode structure and signal fluctuation was examined. It was found that the variation of the thermocline was significant in determining acoustic signal fluctuation. The normal‐mode structure and dispersion relations in the channel depend strongly on the thermocline. Variation of the thermocline t...

Preliminary report on the summer ’96 joint China–U.S. internal wave/acoustic wave experiment in the Yellow Sea

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 primary objective of this experiment was to validate the predicated modal coupling and fluctuations induced by shallow‐water internal waves. The Yellow Sea provides an ideal environment for such research because it has a very flat and homogeneous bottom and a very strong, sharp thermocline. The environment lends itself to relatively simple models for both the acoustic field and the internal wave field. Propagation was measured over distances up to 55 km in the frequency range of 50 Hz–5 kHz. The receivers were three moored and two suspended hydrophone arrays. Internal wave activity was monitored using thermistor chains, SAR satellite imagery, and high‐frequency sonar. Propagation data were taken as a function of range, as a function of time at a fixed range, and as a function of azimuth in the presence of differing levels of internal wave activity. Sup...

Radiation from small acoustic sources located close to a submerged, compliantly coated cylindrical shell

Measurements were made of the radiation from individual, small acoustic sources located close to a submerged, compliantly coated, thin cylindrical shell in the 1 5, the field is simple, and is well approximated by a source and its negative image. For ka<4, the pressure field is characterized by significant spatial variations, owing to contributions from the elastic response of the coated cylinder. Finite‐element model results are compared with the measurements, and are used to provide insight into the interactions between the source and the coated shell. [Work supported by ONR.]

Acoustic wave–internal wave interaction experiment in the Yellow Sea

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.]

On the scattering of sound from a submerged thin walled cylinder coated with a compliant layer

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.]