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Jun 1993

Volume 93, Issue 6, pp. 3027-3544

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Analysis of scattering from structures containing a variety of length scales using a source‐model technique

Eitan Erez and Yehuda Leviatan

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3027-3031 (1993); (5 pages) | Cited 1 time

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See Also: Erratum

Show Abstract
Fictitious source models have been applied extensively in recent years to a variety of electromagnetic and acoustic wave scattering problems. This paper is introducing an extension of the source‐model technique that facilitates the solution to problems subsuming scatterers that contain a variety of length scales. This extension is in tune with the source‐model technique philosophy of using simple sources the fields of which are analytically derivable. It amounts to letting the coordinates of some of the source centers assume complex values. Positioned in complex space, the simple sources radiate beam‐type fields, which are more localized and are better approximations of the scattering from the smooth expanses of the structure. The coordinates of the other source centers retain their conventional real values. These latter sources are used, of course, to approximate the fields in the vicinity of the more rapidly varying expanses of the structure. The new approach is applied to analyze acoustic scattering from a structure comprising two adjacent pressure‐release spheres of different size. It is found to render the solution computationally more effective at the expense of only a slight increase in its complexity.
Show PACS
43.20.Bi Mathematical theory of wave propagation
43.20.Fn Scattering of acoustic waves
43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Acoustic scattering from a rough sphere

Per‐Åke Jansson

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3032-3042 (1993); (11 pages) | Cited 3 times

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The scattering of a plane acoustic wave from a random rough sphere is studied using the null field approach. The starting point is an integral representation derived from the Helmholtz equation. The incident field, the scattered field, and the free‐space Green’s function are all expanded in terms of suitably chosen basis functions. In this way a relation between the incident and the scattered fields is obtained, which is expressed by the transition matrix (T matrix). The scattered field is expanded in a power series of a small parameter, namely the product of the wave number and the root‐mean‐square height of the irregularities. Analytical expressions for the leading terms of the series have been calculated. In particular, ensemble averages of the far‐field amplitude and the scattering cross section have been determined. As the analytical results are somewhat complicated, some numerical results are presented. Numerical computations of higher‐order terms indicate that the convergence of the series is satisfactory as long as the root‐mean‐square height is small compared to a correlation length describing the average distance between the peaks of the surface.
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43.20.Fn Scattering of acoustic waves

Large membrane array scattering

G. A. Kriegsmann and C. L. Scandrett

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3043-3048 (1993); (6 pages)

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The scattering of a time harmonic acoustic wave by an array of N identical baffled membranes is considered in the limit as N→∞. The method of matched asymptotic expansions is used to construct an inner and outer expansion; the inner expansion contains the physics of the local periodic structure and the outer expansion has the proper behavior in the far field. The matching process is used to determine a set of unknown coefficients in the analysis and the result is a far‐field pattern for the structure that blends the physics of both expansions. In particular, the far‐field pattern is highly localized about the Bragg or mode angles of the inner representation. The maximum value of this function is directly proportional to the reflection coefficient determined numerically from the inner problem. Several numerical examples are presented illustrating these features. Moreover, it is observed that the asymptotic theory agrees quite well with the exact answer (obtained by using a finite difference scheme) when N=3.
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43.20.Fn Scattering of acoustic waves

An evaluation of the Kirchhoff approximation in predicting the axial impulse response of hard and soft disks

Guy V. Norton, Jorge C. Novarini, and Richard S. Keiffer

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3049-3056 (1993); (8 pages) | Cited 4 times

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To test the ability of the Kirchhoff approximation for estimating the various components in the near‐field impulse response of a circular disk, the predictions from a time domain formulation of the Helmholtz–Kirchhoff solution [Trorey, Geophys. 35, 762–864 (1970)] are benchmarked against results obtained via the Fourier synthesis of highly accurate frequency domain solutions [Kristensson and Waterman, J. Acoust. Soc. Am. 72, 1612–1625 (1982)]. In these numerical experiments, a collocated point source and receiver lie on the symmetry axis of an acoustically hard (rigid) or soft (pressure release) disk. A time‐domain analysis is carried out in order to unambiguously evaluate the Kirchhoff approximation for different components of the scattered field. It is found that, while Helmholtz–Kirchhoff predicts the correct reflected component, it fails to accurately predict the strength of the diffracted component. The magnitude of the error depends on whether the disk is soft or hard and on the source/receiver height above the disk. The error in the diffracted component exceeds, in some cases, 100%. Furthermore, it is observed that the Helmholtz–Kirchhoff approach does not include the secondary or multiply diffracted arrivals which are more pronounced for the hard disk.
Show PACS
43.20.Fn Scattering of acoustic waves
43.20.Px Transient radiation and scattering
43.30.Hw Rough interface scattering

Directional attenuation of SH waves in anisotropic poroelastic inhomogeneous media

Ari Ben‐Menahem and Richard L. Gibson, Jr.

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3057-3065 (1993); (9 pages) | Cited 2 times

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The equations of motion for anisotropic poroelastic inhomogeneous media admit separable SH wave motion for the case of azimuthal isotropy coupled with coaxial inhomogeneity and poroelasticity with a diagonal permeability tensor. In homogeneous examples of such media, the low‐frequency waves exhibit differential attenuation that is more pronounced along the symmetry axis. If this differential attenuation can be observed, it may be used to determine the permeability of the medium. Media with a vertical gradient of both elastic and permeability properties show different effects. For low‐frequency waves, the attenuation acts so as to reverse the effects of elastic parameter gradients on SH wave amplitudes.
Show PACS
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.20.Jr Velocity and attenuation of elastic and poroelastic waves
43.20.Bi Mathematical theory of wave propagation
43.40.Ph Seismology and geophysical prospecting; seismographs

A new technique for measuring Rayleigh and Lamb wave speeds

Tribikram Kundu and Bruce Maxfield

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3066-3073 (1993); (8 pages) | Cited 3 times

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A new technique is proposed in this paper to measure Rayleigh and Lamb wave speeds in solid half‐spaces and plates. In this technique two transducers are positioned above the specimen in a pitch‐catch orientation. The time of flight of the signal from the transmitter to the receiver is recorded. Then the rate of change of this time as the distance between the reflector and the transducer varies is experimentally determined. This rate remains constant when leaky Rayleigh of Lamb waves are generated but it varies when these waves are not generated. Thus surface waves are detected in an indirect manner. An expression is derived to relate the surface wave speed to the signal flight time change rate with the transducer specimen distance. Using this expression Rayleigh and Lamb wave speeds have been accurately determined in isotropic metals and anisotropic composites.
Show PACS
43.20.Jr Velocity and attenuation of elastic and poroelastic waves
43.20.Ye Measurement methods and instrumentation
43.35.Pt Surface waves in solids and liquids
43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants

Transient acoustic pressure radiated from a finite duct

P. Stepanishen and Rene A. Tougas, Jr.

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3074-3084 (1993); (11 pages)

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The time‐dependent pressure radiated from a finite, rigid, circular duct of constant cross section is determined for the specified motion (velocity or acceleration) of a piston source within the duct. The modal composition of the internal field variables is represented as an eigenfunction expansion over the duct cross section, and a time‐ and space‐dependent Green’s function is used to develop a generalized boundary condition which describes the effect of the external surroundings at the duct exit. Solution of this boundary value problem results in a duct transfer function which is then cascaded with the spatial transfer function connecting the duct exit port and a selected external field point. In this paper, inversion to the time domain is accomplished via the FFT algorithm. Numerical examples demonstrating the calculation of the external time‐dependent pressure are presented based upon the specification of a gated sinewave piston acceleration over an assumed piston spatial profile. Confirming the theoretical analysis, the numerical results show that the time‐dependent pressure for the cross modes develops its peak value off‐axis and is always zero on‐axis, while the plane wave mode always peaks precisely on‐axis.
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43.20.Mv Waveguides, wave propagation in tubes and ducts
43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods
43.20.Px Transient radiation and scattering

Plane compression front steepening in nonlinear media forms both a shock and a reflected wave

C. L. Morfey and V. W. Sparrow

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3085-3088 (1993); (4 pages) | Cited 1 time

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An exact solution is given for the reflected wave formed in a nonlinear medium when a plane compression front steepens into a shock. The solution predicts both a shock and a reflected wave. In the small‐amplitude limit the reflected wave strength varies as (ΔP)3, where ΔP is the strength of the initial wave front.
Show PACS
43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves
43.28.Mw Shock and blast waves, sonic boom
47.40.Nm Shock wave interactions and shock effects

On the existence of stationary nonlinear Rayleigh waves

M. F. Hamilton, Yu. A. Il’insky, and E. A. Zabolotskaya

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3089-3095 (1993); (7 pages) | Cited 6 times

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The existence of stationary nonlinear Rayleigh waves is investigated theoretically on the basis of new model equations for the propagation of finite amplitude Rayleigh waves in isotropic solids [E. A. Zabolotskaya, J. Acoust. Soc. Am. 91, 2569–2575 (1992)]. The spectral components of the proposed stationary waveforms are governed by coupled quadratic algebraic equations that are similar in form to those used by Parker and Talbot [J. Elast. 15, 389–426 (1985)]. However, whereas the theoretical investigation of Parker and Talbot predicted the existence of stationary nonlinear Rayleigh waves, the present investigation does not, unless artificial constraints are imposed on the frequency spectrum. Differences between nonlinearity in Rayleigh wave propagation in isotropic solids and nonlinearity in sound wave propagation in fluids is briefly discussed.
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43.25.Fe Effect of nonlinearity on acoustic surface waves
43.25.Dc Nonlinear acoustics of solids

Deformation and location of an acoustically levitated liquid drop

Yuren Tian, R. Glynn Holt, and Robert E. Apfel

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3096-3104 (1993); (9 pages) | Cited 14 times

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A theoretical method to determine the location and static deformation of an acoustically levitated liquid drop in air is presented. The interaction between drop and sound field, involving nonspherical acoustic scattering and drop volume variation, is the crux of this analysis, which is valid for drops with aspect ratio as large as 2. Numerical calculations are presented of drop shape and location as functions of sound pressure, surface tension, and drop volume in both gravity (1g) and gravity‐free (0g) environments. The numerical results agree well with our experimental measurements and those of other researchers.
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43.25.Uv Acoustic levitation

Application of the Gaussian beam approach to sound propagation in the atmosphere: Theory and experiments

Yannick Gabillet, Hartmut Schroeder, Gilles A. Daigle, and André L’Espérance

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3105-3116 (1993); (12 pages) | Cited 6 times

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The Gaussian beam approach solves the wave equation in the neighborhood of the conventional rays using the parabolic approximation. The solution associates with each ray a beam having a Gaussian amplitude profile normal to the ray. The approximate overall solution for a given source is then constructed by a superposition of Gaussian beams along nearby rays. The solution removes ray‐tracing artifacts such as perfect shadows and infinite energy at caustics without the computational difficulties of numerical solutions to the wave equation. In this paper, the Gaussian beam approach is applied to atmospheric sound propagation in the presence of refraction above a ground surface. A brief overview of the method is presented. Calculations obtained from Gaussian beam tracing are compared to those obtained from the fast field program (FFP) and to experimental measurements. The experiments were made above a concave surface indoors that simulates propagation under downward refraction (inversion or downwind) in the cases of a hard and finite impedance surface. These experiments include measurements in the presence of a barrier. Measurements were also made in a wind tunnel in the presence of wind and temperature gradients. The results suggest that beam tracing can be applied to complex atmospheric sound propagation problems with advantages over conventional ray tracing and full‐wave solutions.
Show PACS
43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors
43.20.Bi Mathematical theory of wave propagation
43.50.Vt Topographical and meteorological factors in noise propagation

Active and passive acoustic behavior of bubble clouds at the ocean’s surface

A. Prosperetti, N. Q. Lu, and H. S. Kim

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3117-3127 (1993); (11 pages) | Cited 5 times

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The emission and scattering of sound from bubble clouds is studied theoretically. It is shown that clouds having a size and air content similar to what might be expected as a consequence of the breaking of ocean waves can oscillate at frequencies as low as 100 Hz and below. Thus cloud oscillations may furnish an explanation of the substantial amount of low‐frequency wind‐dependent oceanic ambient noise observed experimentally. Detailed results for the backscattering from bubble clouds—particularly at low grazing angles—are also presented and shown to be largely compatible with oceanic data. Although the cloud model used here is idealized (a uniform hemispherical cloud under a plane water free‐surface), it is shown that the results are relatively robust in terms of bubble size, distribution, and total air content. A similar insensitivity to cloud shape is found in a companion paper [Sarkar and Prosperetti, J. Acoust. Soc. Am. 93, XXX (1993)].
Show PACS
43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.30.Ft Volume scattering
43.30.Lz Underwater applications of nonlinear acoustics; explosions

Backscattering of underwater noise by bubble clouds

K. Sarkar and A. Prosperetti

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3128-3138 (1993); (11 pages) | Cited 2 times

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This paper is a continuation of an earlier one [Prosperetti et al., J. Acoust. Soc. Am. 93, XXX (1993)] in which the low‐frequency backscattering of sound by hemispherical bubble clouds at the ocean’s surface was studied. Here, clouds of various geometrical shapes (spheroids, spherical segments, cones, cylinders, ellipsoids) are considered and results in substantial agreement with the earlier ones and with the experiments of Chapman and Harris [J. Acoust. Soc. Am. 34, 1592–1597 (1962)] are found. The implication is that the backscattering levels are not strongly dependent on the shape of the clouds, which strengthens the earlier conclusion that bubble clouds produced by breaking waves can very well be responsible for the unexpectedly high backscattering levels observed experimentally. The accuracy of the Born approximation used by others for similar problems is also examined in the light of the exact results. Significant differences are found for gas concentrations by volume of the order of 0.01% or higher. Finally, shallow nonaxisymmetric plumes are briefly considered.
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43.30.Ft Volume scattering
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Comparison of the Rayleigh and T‐matrix theories of scattering of sound from an elastic shell

Luc Kazandjian

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3139-3148 (1993); (10 pages) | Cited 2 times

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The Rayleigh and T‐matrix mathematical formalisms of scattering of sound from an elastic shell are established and compared. It is shown that the two formalisms have a unique solution and are theoretically equivalent when used to calculate the pressure field sufficiently far away from the obstacle. From a numerical point of view, the closeness of the results obtained with the two methods does not depend on the validity of the Rayleigh hypothesis. These results enable one to analyze the validity of the wave superposition and retracted boundary integral formalisms when the auxiliary retracted surface is located inside the inscribed sphere. Some comments are made about the numerical results expected from these methods.
Show PACS
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.20.Fn Scattering of acoustic waves
43.20.Tb Interaction of vibrating structures with surrounding medium

Coherence of acoustic scattering from a dynamic rough surface

David R. Dowling and Darrell R. Jackson

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3149-3157 (1993); (9 pages) | Cited 8 times

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Numerical studies of the angular and temporal coherence of rough‐surface acoustic scattering are presented at kh=12, 20, and 32, where k is the acoustic wave number and h is the root‐mean‐square surface height fluctuation. The computations are based on point‐source illumination of a one‐dimensional pressure‐release Pierson–Moskowitz dynamic rough surface, and the Kirchhoff approximation. In the region near specular, for a source‐location grazing angle of 20°, the angular variation of the scattered field coherence is found to predominantly depend on the product of k and a fixed length scale. The computed results show only a mild dependence on kh. For the same geometry, the temporal variation of the scattered field coherence is found to depend predominantly on (g Δt/U)kh, where g is the acceleration of gravity, Δt is the time shift, and U is the wind speed. A general method for scaling the scattered field coherence in terms of k, Δt, U, and the angular separation of the field points is suggested based on the findings. The effects of wind direction and surface‐wave frequency cutoff on acoustic scattering are also noted.
Show PACS
43.30.Hw Rough interface scattering
43.20.Fn Scattering of acoustic waves

On the validity of the wedge assemblage method for pressure‐release sinusoids

Richard S. Keiffer

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3158-3168 (1993); (11 pages) | Cited 2 times

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In the past, the wedge assemblage (WA) method for calculating the acoustic scattering from rough, long‐crested, or corrugated surfaces has been compared with both experimental data and exact theory with good results. Nevertheless, significant questions about what physics is included in the method and its realm of validity remain unanswered. In this paper, the WA method is applied to scattering from pressure‐release sinusoidal surfaces in order to further explore these topics. Comparisons with accurate benchmark calculations are carried out over a broad range of kh and kΛ (k is the acoustic wave number; h and Λ are the amplitude and wavelength of the sinusoid, respectively) indicate that the primary limitation of the WA method stems from its current failure to include multiple scattering effects. It is also shown that quite good agreement with the benchmark can be achieved by a ‘‘diffraction‐only’’ WA model even when kh≪1 and ‘‘reflection‐like’’ scattering patterns are observed.
Show PACS
43.30.Hw Rough interface scattering
43.20.Fn Scattering of acoustic waves

The underwater sound generated by heavy rainfall

Jeffrey A. Nystuen, Charles C. McGlothin, and Michael S. Cook

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3169-3177 (1993); (9 pages) | Cited 4 times

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The underwater acoustic signature of heavy rainfall is very different from that of light rainfall. During heavy rainfall sound levels are observed to rise with increasing rainfall rate at all frequencies monitored (4–21 kHz) and the 15‐kHz spectral peak observed during light rainfall is absent. The sound levels are most highly correlated (r≊0.8) with heavy rainfall rate for frequencies less than 10 kHz. Lower correlations between sound levels and heavy rainfall rate were observed for frequencies above 10 kHz under several different conditions. When wind speed exceeds 10 m/s, wave breaking mixes bubbles downward and creates a layer of bubbles. This bubble layer attenuates subsequent surface‐generated sound (from the raindrop splashes) for frequencies above 10 kHz. Extremely heavy rainfall (total rainfall above 150 mm/h) also generates a subsurface bubble layer. This rainfall‐generated bubble layer is evidence of rainfall‐induced turbulent mixing of the ocean surface layer and has implications for air/sea exchange processes (momentum, heat, and gas exchange). Finally, previous studies have shown that light rain generates acoustic energy above 10 kHz and that this sound is poorly correlated with total rainfall rate. A simple empirical acoustic rainfall rate algorithm for heavy rain is offered. This algorithm may be site specific. Furthermore, it overestimates rainfall rate early in the rain events examined here (convective rain events), and then underestimates rainfall rate later in the same events. This observation is shown to be consistent with likely changes in the drop size distribution during the lifetime of the rain event. The sound produced by large drops within heavy rain (drop diameter greater than 2.2 mm) is shown to dominate the underwater sound field. The empirical acoustic rainfall rate algorithm is therefore more correctly a measure of the rainfall rate from large raindrops. Fortunately, the rainfall rate from large raindrops is highly correlated with the total rainfall rate, making acoustic monitoring of underwater sound an effective measure of rainfall rate in oceanic regions.
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43.30.Lz Underwater applications of nonlinear acoustics; explosions
43.30.Nb Noise in water; generation mechanisms and characteristics of the field

Measurement of the sound produced by a tipping trough with fresh and salt water

William M. Carey, James W. Fitzgerald, Edward C. Monahan, and Qin Wang

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3178-3192 (1993); (15 pages) | Cited 11 times

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See Also: Erratum

Show Abstract
Recent experiments confirm the production of sound by breaking waves at lower frequencies (30 to 500 Hz) with a dipole characteristic. The noise produced has a broadband characteristic associated with the impact and subsequent sounds that have discrete spectral characteristics. Breaking waves are known to produce bubble plumes and bubble clouds; the dynamic evolution of which provides a mechanism for sound production. Since the initial plume and cloud have appreaciable void fractions, compressible resonant oscillations of these structures as a whole or in parts are possible. These bubble plumes would act as compact acoustic monopole sources of sound and due to the pressure release surface would have an effective dipole characteristic. Sufficient energy exists in the initial breaking vorticity and turbulence to excite these regions and to explain measured source levels. These effects have been simulated with a tipping trough experiment that demonstrates the production of low‐frequency sound from salt and fresh water tipping trough events. These experimental results are shown to be consistent with the theory of sound radiation from the collective oscillations of bubble plumes.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field

Acoustic properties of fine‐grained sediments from Emerald Basin: Toward an inversion for physical properties using the Biot–Stoll model

Robert C. Courtney and Larry Mayer

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3193-3200 (1993); (8 pages) | Cited 2 times

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Acoustic data from two long cores, comprising marine clays and silts taken from Emerald Basin off Nova Scotia, are presented. High‐resolution measurements of compressional wave velocity, attenuation, and power law exponent are made using ultrasonic frequencies between 100 to 1000 kHz. The observed values of the frequency dependence of attenuation suggest that a nonconstant Q mechanism is needed to explain these data, and Biot–Stoll theory is used to model the experimental results. An inversion scheme is used to constrain physical parameters in the Biot–Stoll dispersion relation. The inversion shows that there is a restricted range of permeability and grain size. By assigning reasonable values for grain size in the inversion, the Biot–Stoll model predicts unique values for the permeability and frame bulk modulus that agree well with estimates made by other means.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics

Approximate evaluation of the spectral density integral for a large planar array of rectangular sensors excited by turbulent flowa)

William Thompson, Jr. and Robert E. Montgomery

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3201-3207 (1993); (7 pages)

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An approximate numerical procedure has been developed for rapidly evaluating the spectral density integral that predicts the output of a planar array of many sensors excited by turbulent boundary layer pressure fluctuations. This procedure is particularly useful in cases where the transfer function factor of the integrand is not a simple function of the wave numbers in the flow and transverse directions. The procedure exploits the facts that the entire integrand is a separable function of these two wave numbers and, when the number of sensors is large, the array function factor of the integrand is a rapidly varying function of wave number, characterized by many similar shaped lobes. In addition, a model for multilayered media is employed to provide the transfer function for boundary conditions that closely correspond to reality. Results generated by this procedure were compared to those from an exact evaluation of the integral which is possible if the transfer function is taken to be constant; there was agreement to within 0.2 dB or better over a broad frequency interval. Some results for a realistic transfer function are presented, such as the case of an elastomeric layer backed by an elastic plate with the sensors embedded at an arbitrary position within the layer.
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43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
43.30.Lz Underwater applications of nonlinear acoustics; explosions

Ultrasonic propagation, scattering, and defocusing in suspensions

J. Adach, R. C. Chivers, and L. W. Anson

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3208-3219 (1993); (12 pages)

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Fine resolution narrow‐band pressure amplitude distribution measurements have been performed using a hydrophone in the field of a weakly focused bowl transducer radiating into castor oil and 1% and 2% by volume suspensions of 320‐μm polystyrene beads in castor oil over the frequency range 1.0 to 2.5 MHz at 20 °C. The axial distributions permitted determination of the excess attenuation due to scattering in the suspensions as a function of frequency. The frequency range included a resonance. The excess attenuation measured was compared with the theoretical predictions of Waterman and Truell [J. Math. Phys. 2, 512–537 (1961)] and good agreement was obtained. The values of the excess attenuation obtained were used to predict axial pressure amplitude distributions in the suspension. Although the excess attenuation was determined from far‐field measurements, the agreement between the predicted axial distributions and those measured experimentally extended well into the near field. Lateral distributions were measured at seven frequencies at the position of the geometrical center of curvature of the source and at the position of the true focus in the 1% suspension. Comparison with similar scans in castor oil alone revealed no detectable defocusing due to the presence of the scatterers.
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43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions

Ultrasonic investigations of some polymeric materials

D. P. Singh and Anand Pal Singh

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3220-3223 (1993); (4 pages)

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Ultrasonic investigations of some polymeric materials such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, and polyethylene glycol at 303 K have been done using ultrasonic velocity and density data taken from the literature. Various acoustical parameters such as molar sound velocity, molar adiabatic compressibility, acoustic impedance, van der Waal’s constant, molar sound volume, free volume, internal pressure, and cohesive energy have been determined. A large number of thermodynamical parameters such as molar volume, available volume, geometrical volume, intermolecular free length, relative association, and surface tension have been evaluated. Jacobson’s free length theory and Schaaff’s collision factor theory have been used to predict the values of ultrasonic velocities in the systems under study. The obtained results have been compared with the experimental results available in literature. The variation of acoustical and thermodynamical parameters of these polymeric materials, with change in their molecular weight, provide a deep insight into the intermolecular interactions going on in these systems.
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43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions
62.80.+f Ultrasonic relaxation

Slow wave propagation in air‐filled permeable solids

Peter B. Nagy

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3224-3234 (1993); (11 pages) | Cited 7 times

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The propagation of slow compressional waves in air‐saturated permeable solids was studied by experimental means between 10 and 500 kHz. The velocity and attenuation coefficient were measured as functions of frequency from the insertion delay and loss of airborne ultrasonic waves transmitted through thin slabs of 1–5 mm in thickness. Porous ceramics of 2–70 Darcy and natural rocks of 200–700 mDarcy permeability were tested. In the low‐frequency (diffuse) regime, the experimental results are consistent with theoretical predictions; the phase velocity and attenuation coefficient are essentially determined by the permeability of the specimen and both increase proportionally to the square root of frequency. In the high‐frequency (propagating) regime, the experimental results are consistent with the theoretical predictions for the phase velocity but not for the attenuation coefficient. The phase velocity asymptotically approaches a maximum value determined by the tortuosity of the specimen while the attenuation coefficient becomes linearly proportional to frequency instead of the expected square‐root relationship. It is suggested that the observed discrepancy is due to the irregular pore geometry that significantly reduces the high‐frequency dynamic permeability of the specimens.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants

Analysis of axisymmetric waves propagating along a hollow cylindrical ultrasonic transmission line

Nobuhiro Kanbe, Yoshiro Tomikawa, Kazunari Adachi, and Takehiro Takano

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3235-3241 (1993); (7 pages)

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A new ultrasonic device for feeding powder with the use of a hollow acrylic cylinder, on which an axisymmetric progressive elastic wave is excited, has previously been proposed. Its characteristics have also been reported. To find vibrational behaviors of the hollow cylinder for feeding powder, the elliptic particle movement on the inside surface was analyzed. The authors have found many forms of elliptic motion. Some of them can move objects inside the cylinder toward the vibration source, as expected, but, on the contrary, the others move in the wave propagation direction.
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43.35.Pt Surface waves in solids and liquids
43.35.Zc Use of ultrasonics in nondestructive testing, industrial processes, and industrial products

Ultrasonic tomography using scanned contact transducers

D. P. Jansen, D. A. Hutchins, and R. P. Young

J. Acoust. Soc. Am. Volume 93, Issue 6, pp. 3242-3249 (1993); (8 pages) | Cited 1 time

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Ultrasonic tomographic images have been obtained from objects of rectangular cross section by scanning spring‐loaded transducers across the sample faces. Hemispherical brass caps, placed on each transducer, facilitated sliding along the surface. The complete system was under the control of a microcomputer, resulting in an entirely automatic data collection system. Selected images obtained from this system are presented for metal and rock samples, as well as samples with artificial anomalies.
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43.35.Sx Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography
43.35.Wa Biological effects of ultrasound, ultrasonic tomography
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