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Nov 1989

Volume 86, Issue S1, pp. S1-S125

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back to top Session VV. Physical Acoustics IX: Propagation and Porous Material
Contributed Papers
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Study of acoustic transmission loss in sediments at low frequencies using parametric acoustic arrays (A)

V. N. Bindal, T. K. Saksena, S. K. Jain, Recta Gupta, and D. N. Santoshi

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S119-S119 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Investigations have been carried out on the acoustic transmission loss in clay, sand, and gravel in the frequency range 6–10 kHz. Parametric acoustic arrays have been used for this study and have been generated using amplitude modulation of a primary transducer of frequency about 300 kHz in which the carrier was suppressed. Source level and beam width of the parametric signal have been ∼ 155 dB re: ∣μPa/m2 and 3.6°, respectively. Experiments have been conducted in the laboratory taking the sediment in a rectangular container of dimensions 0.7×0.49×0.6 m3. The acoustic transmission loss using parametric arrays has been compared with the measured loss with a nonparametric source. The results show a dependence of transmission loss on the particle size and are discussed in the light of previous studies.
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Experimental and theoretical wave propagation in solid and hollow cylinders in fluid (A)

Thomas J. Plona, Sergio Kostek, and Shu‐Kong Chang

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S119-S119 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A comparison between experiment and theory is made involving axi‐symmetric wave propagation along solid and hollow cylinders immersed in a fluid (i.e., water). The theoretical methods to generate waveforms include both integral representation and finite‐difference modeling. The dispersive modal characteristics are calculated from the waveforms. The experiments are made on solid and hollow steel cylinders of 0.75 in. and 0.625 in. The source and receiver transducers are made from PZT rings 1.0 in. and 0.5 in. high which freely slide along the cylinders. Data are collected as a function of source‐receiver spacing and over a frequency range of 40–240 kHz. A Prony's method is used to obtain the modal characteristics. Discussions will focus on the good agreement between theory and experiment, the differences between the mode structures in the two cases and the asymptotic behaviors at short and long wavelengths. Some interesting comparisons between monopole and dipole modes will also be made.
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Transient response of a fluid‐filled borehole due to the passage of transient elastic waves (A)

C. Chang and S. K. Chang

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S119-S119 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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The interaction of transient elastic waves with a fluid‐filled borehole is investigated both experimentally and theoretically. The elastic waves incident from various angles are partially scattered by the borehole, while an acoustic wave field is introduced in the borehole fluid. Experiments were conducted in a solid block with a circular borehole inside. The transient elastic waves were generated by using either a simulated acoustic emission source or an ultrasonic transducer. The acoustic pressure in the borehole was measured by a commercial PZT hydrophone and by a home‐made PVDF hydrophone. The effect of the borehole on the elastic waves is identified by comparing the measured acoustic responses with the undistorted incident pulse, calculated by the generalized ray theory without the borehole. The theory of plane wave scattering by a borehole is used to compare with the experimental data. The transient results from both the experiments and the borehole theory agree with each other very well, and they both exhibit resonances due to the multiple reflections inside the borehole fluid.
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Apparatus to determine the complex mass density of a viscous fluid contained in a rigid porous solid from acoustic pressure measurements (A)

Robert A. Mirick, Steven R. Baker, and Oscar B. Wilson

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S119-S119 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Two experimental techniques to determine the frequency‐dependent complex mass density of a viscous fluid contained in a rigid porous solid are being investigated. The fluid‐filled solid is contained within a small (≪λ) cylindrical cavity. In one technique the moving mass of the fluid is sensed by the effect it has on the measured input electrical impedance of a moving coil transducer. In the second technique the moving mass is extracted from the measured pressure required for the fluid to oscillate with a known amplitude through the porous solid. Preliminary results have been reported for the impedance method [Grant et al., J. Acoust. Soc. Am. Suppl. 1 84, S176 (1988)]. Preliminary results for the pressure technique and a comparison of the two methods will be presented. [Work sponsored by NRL‐USRD and the Naval Postgraduate School.]
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Measurement of acoustical and microstructural properties of real and model porous materials (A)

Michael R. Stinson, Yvan Champoux, and Gilles A. Daigle

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S119-S119 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Detailed measurements of several parameters describing porous materials are currently being made. The acoustical behavior of the porous materials is determined using an impedance tube technique, modified to provide both characteristic impedance and propagation constant, as a function of frequency. To define the microstructural properties, flow resistivity and porosity are initially being measured. Several real materials are being considered, including samples of reticulated open‐pore foam, felt materials, and natural soils. A series of model porous materials, with well‐defined microstructure, is also being developed. The first in this series consists, simply, of circular pores of uniform cross section, formed in a rigid host material. The measurements will be interpreted in terms of existing rigid‐frame models of the acoustical properties of porous materials.
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The slow compressional wave in a porous medium by the finite element method (A)

Yu‐chiung Teng

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S120-S120 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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The finite element results for a fluid/porous‐solid/fluid system are presented by using the double‐nodel‐layer technique. The pressure‐type fluid elements are used to model the fluid fields: The porous solid is modeled by Biot's medium [J. Acoust. Soc. Am. 28, 168 (1956)]. The Biot's theory predicts that for propagation of acoustic waves in fluid‐saturated elastic porous media, a second compressional wave exists. Geertsma and Smit [Geophysics 41, 169 (1961)], using Biot's theory, predicted the generation of this second type of compressional wave at an open interface between a liquid and a porous solid. Plona [Appl. Phys. Lett. 36, 259 (1980)] observed the second type of compressional wave in his liquid/porous‐solid/liquid experiment at ultrasonic frequencies. Given suitable physical parameters in the finite element calculations, excellent agreement with the Plona's measurement has been obtained. The finite element results for a sealed interface, showing a diminishing of the second type compressional wave, also agree with the experimental work performed by Rasolofosaon [Appl. Phys. Lett. 52, 780 (1988)].
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Dependence of the acoustic to seismic coupling ratio on the angle of incidence and geophone depth (A)

W. Pat Arnott, James M. Sabatier, and John O. Messer

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S120-S120 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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An atmospheric sound wave can couple with the poroelastic ground, resulting in ground (or seismic) motion. This is the phenomenon of acoustic to seismic coupling. Microphones are used to measure the sound pressure level at the surface and geophones are used to measure the resulting seismic motion at or below the surface. The seismic:acoustic transfer function (SATF) characterizes a particular site. SATF measurements are reported as a function of the angle of incidence of the sound wave for frequencies 15 Hz to 1 kHz. The angle of incidence varied from 50° to 80°, and normal incidence. Surface vertical and radial‐horizontal geophones were used. In addition six vertical geophones at depth intervals of 10 cm, starting at 10 cm, were used. A seismic p‐wave survey indicated a first layer depth of 44 cm having a wave velocity of 159 m/s overlying a layer of velocity 379 m/s. This gives a critical angle of incidence of 65°. It was anticipated and confirmed that the SATF would increase in magnitude for certain frequency bands as the angle of incidence approached and passed over the critical angle. Comparisons of experimental results and multilayered elastic and poroelastic media theory will be addressed as time permits. [Work supported by ONR.]
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Experimental studies of low‐frequency propagation over flat terrain (A)

D. Keith Wilson and Dennis W. Thomson

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S120-S120 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Experimental studies of atmospheric acoustic propagation have included monitoring the level of a 27.7‐Hz source at a distance of 770 m. Two of the extended duration experiments consisted of recording SPL at 1‐min intervals for 75 h over dry ground (Oct. 1988), and for 144 h over frozen, snow‐covered ground (March 1989). Comprehensive surface layer micrometeorological measurements, including monitoring the turbulent momentum, heat, and moisture fluxes adjacent to the path, as well as continuous Doppler sodar measurements of the boundary layer wind profiles, were made. These measurements are used to reconstruct the time‐dependent “along path” sound speed profiles. Interpretation of the acoustic data includes comparisons with theoretical predictions from a fast field program (FFP).
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Low‐frequency, long‐range sound propagation modeling over a locally reacting boundary with the parabolic approximation (A)

J. S. Robertson, M. J. Jacobson, and W. L. Siegmann

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S120-S120 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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There is substantial interest in the analytical and numerical modeling of low‐frequency, long‐range atmospheric acoustic propagation. Ray‐based models, because of their frequency limitations, do not always give an adequate prediction of quantities such as sound pressure or intensity levels. However, the parabolic approximation method, widely used in ocean acoustics, and often more accurate than ray models for frequencies of interest, can be applied to acoustic propagation in the atmosphere. Modifications of an existing implicit finite‐difference implementation for computing solutions to the parabolic approximation are discussed. A locally reacting boundary is used together with a one‐parameter (the flow resistivity) ground impedance model. Intensity calculations are performed for a number of flow resistivity values in both quiescent and windy atmospheric sound channels. Variations in the value of this parameter are shown to have substantial effects on the spatial variation of the acoustic signal. [Work supported by NASA.]
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An investigation of the relationship between upward refraction above a complex impedance plane and the spherical wave evaluation for a homogeneous atmosphere (A)

Richard Rasper, Gordon E. Baird, and Wenliang Wu

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S120-S120 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A residue series solution based on Fock's work in electromagnetic propagation has been used by several investigators to examine sound propagation into a shadow zone. In this paper it is demonstrated that this solution merges smoothly into the Sommerfeld solution for sound propagation above a flat surface as the sound velocity gradient goes to zero. A principle thrust of this investigation is the behavior of the sound propagation above a complex impedance plane as the gradient becomes finite. Initial work indicates that the surface wave pole may contribute to the residue series solution under certain conditions.
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Scattering of sound by atmospheric turbulence (A)

Walton E. McBride, Henry E. Bass, Richard Respet, and Kenneth E. Gilbert

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S120-S120 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A computer simulation of the effect of small scale turbulence on atmospheric sound propagation over a complex impedance boundary is developed. The atmosphere is broken up into spherically symmetric eddies characterized by a Gaussian profile. Single scatter is assumed and a closed form of the first Born approximation for scattering is obtained, giving each eddie's contribution to the total fluctuation of the sound pressure at a receiver downrange. The numerical simulation was accomplished with the concept of a “realization,” or snapshot of the turbulent medium. Each eddie's scatter contribution was added up for a particular configuration of eddies, giving that realization's total sound pressure fluctuation. The eddies were then given a random change in their coordinates. The total sound pressure was calculated for this realization, and the process repeated. A complex impedance boundary was added and the predictions of the standard deviations of the amplitude fluctuations, amplitude probability distributions, and structure functions were then tested against experimental data. Good agreement was found whenever the average intensity of the fluctuations was well above the background noise level. [Work supported by U. S. Army Construction Engineering Research Laboratory.]
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