D. H. Trivett

The acoustic scattering by a submerged, spherical shell. I: The bifurcation of the dispersion curve for the spherical antisymmetric Lamb wave

The acoustic scattering by thin‐walled, evacuated, elastic spherical shells immersed in water is studied. The analytic structure of the scattering amplitude in the complex‐k plane is directly analyzed using Cauchy’s residue theorem, and dispersion curves are presented for the lowest elastic modes of the fluid‐loaded shell. It is found that fluid loading has a profound effect on the vacuum dynamical characteristics of the shell; the spherical equivalent of the first antisymmetric, flat‐plate Lamb wave for the fluid‐loaded shell bifurcates into two distinct modes near the frequency that the vacuum dispersion curve transitions from a subsonic to a supersonic phase velocity. By way of contrast, the spherical equivalent of the first symmetric Lamb wave is essentially unaffected. The salient features of the free‐field scattering process are also analyzed in terms of the resonance excitation of these modes.

A dynamic Young’s modulus measurement system for highly compliant polymers

A new system to determine experimentally the complex Youngs modulus of highly compliant elastomers at elevated hydrostatic pressures and as a function of temperature is presented. A sample cut in the shape of a bar is adhered to a piezoelectric ceramic shaker and mounted vertically inside a pressure vessel equipped with glass windows. Two independent measurement methods are then used: a resonant technique, to obtain low-frequency data, and a wave propagation technique, to obtain higher-frequency data. Both techniques are implemented utilizing laser Doppler vibrometers. One vibrometer detects sample resonances through a window located at the bottom of the pressure vessel, and a set of two separate vibrometers monitors the speed of longitudinal waves propagating in the sample, through windows located on the sides of the vessel. The apparatus is contained inside an environmental chamber for temperature control. Using this approach, Youngs modulus data can be obtained at frequencies typically ranging from 100 Hz to 5 kHz, under hydrostatic pressures ranging from 0 to 2.07 MPa (300 psi), and at temperatures between -2 degrees C and 50 degrees C. Experimental results obtained on two commercial materials, Rubatex R451N and Goodrich Thorodin AQ21, are presented. The effects of lateral inertia, resulting in dispersive wave propagation, are discussed and their impacts on Youngs modulus measurements are examined.

High‐frequency scattering from rigid prolate spheroids

The scattering of an acoustic plane wave by a rigid, immovable prolate spheroid is investigated over a broad frequency range (0<kL/2≤300). An unexpectedly large, off‐axis radiation lobe dominates the bistatic beam pattern for axial incidence and aspect ratios larger than unity, over the entire frequency range considered. This lobe is found to be due to an amplitude‐modulated creeping wave. A new asymptotic expansion of the spheroidal radial functions is introduced that gives an improved estimate of the functions for moderate values of the azimuthal index at high frequency.

A reanalysis of the acoustic scattering from elastic spheroids

The elastic response of fluid‐loaded, elastic spheroids to an end‐on incident acoustic wave is analyzed for a variety of aspect ratios and material properties. The focus of this analysis is the nature of the elastic excitations of the target, and, to a lesser extent, the nature of the inelastic background. It is concluded that the low kL/2, resonant response of the target is closely related to a bar wave, propagating at nearly one and one‐half times the shear speed at low frequencies and that the rigid body solution with recoil is the appropriate background choice for all materials. Bistatic scattering calculations are also studied, and it is concluded that the lobe structure of the elastically reradiated signal cannot be used to characterize a resonance without a priori knowledge of the target. The results contradict recent, but more phenomenologically oriented, studies.

ARAP - Deep Ocean Vector Sensor Research Array

A vertical line array of three axis acoustic vector sensors has been developed to investigate the structure of the acoustic noise fields at reliable acoustic path depths. The array has an aperture of 378 meters and the pass band is 10 Hz to 500 Hz. The array design is modular, with each vector sensor package having its own compass and two-axis tilt sensors and power supply. Telemetry within the array is sent over a passive electronic network. Data from the 224 acoustic channels, 32 compasses and 64 tilt sensors is stored on a bank of hard drives linked to the network controller by a single board computer with a LINUX operating system. Power and storage capacity are sufficient for 7 days of continuous operation. The system uses an acoustic modem to communicate commands from the surface and system status information to the surface. Design details of the vector sensor are presented. Flow noise performance results are presented. Hydromechanical analysis results are presented predicting the geometry of the array in anticipated flow fields

Investigation of a three-phase medium with a negative parameter of nonlinearity

Nonlinear acoustic properties of a composite medium, consisting of hollow microspheres suspended in Castor Oil, at elevated hydrostatic pressure are experimentally investigated. The acoustic nonlinear parameter B∕A of the medium is found to be highly dependent upon hydrostatic pressure. B∕A varies from a small negative value near ambient pressure to a very large negative value (around −6000) in the vicinity of 7×104 Pascals, above ambient pressure. With a further increase in hydrostatic pressure the magnitude of B∕A decreases passing through zero and finally becomes positive. Finite amplitude wave propagation in this medium at low hydrostatic pressures is characterized by waveform steepening in the backward direction leading to rarefactive shockwaves and, at high hydrostatic pressures, by steepening in the forward direction leading to compressive shockwaves. Estimates of B∕A are obtained from both the measurement of thermodynamic properties and from waveform distortion during propagation.

Transient response of an elastic spheroid—Surface waves and quasicylindrical modes

The low kL/2 transient response of an elastic spheroid to an acoustic wave incident along the axis of symmetry is analyzed. The emphasis is upon a direct theoretical and experimental determination of the group velocity of the lowest elastic mode of the spheroid. With these results, a critical examination is made of the consequences of two different physical pictures that have been proposed for the low kL/2 elastic response of such targets (the surface wave and ‘‘quasicylindrical’’ mode pictures). Large discrepancies are found between predictions of the group velocity based on the surface wave picture and both the T‐matrix analysis and the experimental results presented here. In contrast, the predictions of the quasicylindrical mode picture are found to be in excellent agreement with these results. It is concluded that, although the surface wave description is useful at high kL/2, the quasicylindrical mode picture provides a more meaningful description of resonance phenomena in the low‐to‐medium kL/2 region.

High‐frequency scattering from liquid/porous sediment interfaces

A method for remote determination of the mean grain size of sand sediments is demonstrated. The method uses the backscattered signal from a bounded acoustic beam incident onto a water–sediment interface. The possibility of grain size determination stems from a localized enhancement in the backscattering for acoustic wavelengths of the same order as grain size. A phenomenological model that recaptures the overall structure of the experimental data is developed. The sediment is pictured as having a network of channels running through it, and the model developed is novel in its inclusion of channel effects as well as rough surface effects in modeling the backscattered signal.

Investigation of media with large, negative parameters of nonlinearity and their application to the enhancement of a compact, omnidirectional, parametric source

This paper reports on two media whose acoustic properties are highly pressure dependent and, thus, have large nonlinear parameters (B/A∼−3000 to −5000). One is Xanthan gelatin filled with, 10%, Expancel microspheres. The other is a polyurethane rubber, Polytek 74, with 20% Expancel microspheres. The microspheres have a high bulk modulus at ambient pressure due to the hoop stiffness of the spherical shell. As pressure is increased some of the microspheres buckle, leading to significantly decreased bulk modulus. The decreasing bulk modulus with increasing hydrostatic pressure (negative dc/dp) results in both negative B/A and low sound speed (150 m/s at the minimum), which further enhances nonlinear distortion. These media are used to enhance the efficiency of an underwater parametric source. Experiments on a spherical source with a thin layer (2 cm thick) of the Xanthan gelatin material resulted in a generated difference frequency source level 16 dB higher at 300 Hz, than would be obtained driving the transducer directly at the difference frequency. Experiments on the same source with a thin layer of the microvoided, polyurethane rubber are in progress. This material has a smaller coefficient of nonlinearity but operates over a broader range of hydrostatic pressures.

Development of a dynamic Young’s modulus measurement system for compliant polymers

A new system to experimentally determine the complex Young’s modulus of elastomers at elevated hydrostatic pressures and as a function of temperature is presented. A sample cut in the shape of a rod is glued to a piezoelectric ceramic shaker and mounted vertically inside a pressure vessel equipped with glass windows. Two independent measurement methods are then used: a resonant technique, to obtain low frequency data, and a wave propagation technique, to obtain higher frequency data. Both techniques are implemented utilizing laser Doppler vibrometers; one vibrometer detects sample resonances through a window located at the bottom of the pressure vessel, and a set of two separate vibrometers monitors the speed of longitudinal waves propagating in the sample (by measuring symmetric lateral displacements), through windows located on the sides of the vessel. The apparatus is contained inside an environmental chamber for temperature control. Using this approach, Young’s modulus data can be obtained at frequenc...