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- ultrasonics, quantum acoustics, and physical effects of sound 
- transduction 
- structural acoustics and vibration 
- noise: its effects and control 
- architectural acoustics 
- acoustical measurements and instrumentation 
- acoustic signal processing 
- physiological acoustics 
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Index of content:
Volume 107, Issue 6, June 2000
107(2000); http://dx.doi.org/10.1121/1.429324View Description Hide Description
A thermoacoustically driven thermoacousticrefrigerator powered by solar thermal energy has been successfully built and tested. A 0.457 m diameter Fresnel lens focuses sunlight onto the hot end of a 0.0254 m diameter reticulated vitreous carbon prime mover stack, heating it to 475 °C, thereby eliminating the need for the most troublesome component in a heat driven prime mover, the hot heat exchanger. The high intensity sound waves produced by the prime mover drive a thermoacousticrefrigerator to produce 2.5 watts of cooling power at a cold temperature of 5 °C and a temperature span of 18 °C.
- REVIEWS OF ACOUSTICAL PATENTS
107(2000); http://dx.doi.org/10.1121/1.429359View Description Hide Description
The purpose of these acoustical patent reviews is to provide enough information for a Journal reader to decide whether to seek more information from the patent itself. Any opinions expressed here are those of the reviewers as individuals and are not legal opinions. Printed copies of United States Patents may be ordered at $3.00 each from the Commissioner of Patents and Trademarks, Washington, DC 20231. Patents are available via the Internet at http://www.uspto.gov.
- GENERAL LINEAR ACOUSTICS 
Experimental validations of the HELS method for reconstructing acoustic radiation from a complex vibrating structure107(2000); http://dx.doi.org/10.1121/1.429325View Description Hide Description
This paper presents experimental validations of the Helmholtz Equation Least Squares (HELS) method [Wang and Wu, J. Acoust. Soc. Am. 102, 2020–2032 (1997); Wu and Wang, U.S. Patent Number 5712805 (1998); Wu, J. Acoust. Soc. Am. 107, 2511–2522 (2000)] on reconstruction of the radiated acoustic pressures from a complex vibrating structure. The structure under consideration has geometry and dimensions similar to those of a real passenger vehicle front end. To simulate noise radiation from a vehicle, a high fidelity loudspeaker installed inside the structure at the location of the engine is employed to generate both random and harmonic acoustic excitations. The radiated acoustic pressures are measured over a finite planar surface above the structure by a microphone. The measured data are taken as input to the HELS formulation to reconstruct the acoustic pressures on the top surface of the structure as well as in the field. The reconstructedacoustic pressures are then compared with measured ones at the same locations. Also shown are comparisons of the reconstructed and measuredacoustic pressure spectra at various locations on the surface. Results show that satisfactory reconstruction can be obtained on the top surface of the structure subject to both random and harmonic excitations. Moreover, the more measurements and the closer their distances to the source surface, the more accurate the reconstruction. The efficiency of the HELS method may decrease with increasing of the excitation frequency. This high frequency difficulty is inherent in all expansion theories.
Wave propagation in micro-heterogeneous porous media: A model based on an integro-differential wave equation107(2000); http://dx.doi.org/10.1121/1.429326View Description Hide Description
A model hyperbolic partial differential equation with singular convolution operators and infinitely smooth solutions is studied. It is shown that short pulses, including finite-bandwidth pulses, propagate with a delay with respect to the wavefront. For a two-parameter family of such equationsGreen’s functions are obtained in a simple self-similar form. As an application, it is demonstrated that the Gurevich–Lopatnikov dispersion law for a thin-layered porous medium can be approximated by a hyperbolic equation with singular memory.
107(2000); http://dx.doi.org/10.1121/1.429327View Description Hide Description
A new method termed Directive Line SourceModel (DLSM) is presented for predicting the diffracted field produced by a sound wave incident on a rigid or pressure release half plane. In the new method the edge of the half plane is modeled as an infinite set of directive point sources continuously distributed along the edge. Because DLSM is fast, simple and intuitive, it is a promising tool for the study of diffraction. It can be applied for several types of incident radiation: omnidirectional cylindrical and spherical waves, plane waves, as well as waves from directional sources. Wedges may also be treated. Finally, DLSM can handle diffraction by an arbitrarily shaped edge profile, for example, a half plane having an edge that is jagged instead of straight. Results for plane, cylindrical and spherical incident waves, as well as for arrays of line and point sources, are presented and their agreement with known analytical solutions is demonstrated. Predictions based on DLSM compare favorably with experimental data.
107(2000); http://dx.doi.org/10.1121/1.429328View Description Hide Description
Experimental results of time-reversal focusing in a high-order multiple scattering medium are presented and compared to theoretical predictions based on a statistical model. The medium consists of a random collection of parallel steel rods. An ultrasonic source (3.2 MHz) transmits a pulse that undergoes multiple scattering and is recorded on an array. The time-reversed waves are sent by the array back to the source through the scattering medium. The quality of temporal focusing is very well predicted by a simple statistical model. However, for thicker samples, persistent temporal side-lobes appear. We interpret these side-lobes as a consequence of the growing number of crossing paths in the sample due to high-order multiple scattering. As to spatial focusing, the resolution is practically independent from the array’s aperture. With a 16-element array, the resolution was found to be 30 times finer than in a homogeneous medium. Resolutions of the order of the wavelength (0.5 mm) were attained. These results are discussed in relation with the statistical properties of time-reversal mirrors in a random medium.
Analytical regularization based analysis of a spherical reflector symmetrically illuminated by an acoustic beam107(2000); http://dx.doi.org/10.1121/1.429329View Description Hide Description
A mathematically accurate and numerically efficient method of analysis of a spherical reflector, fed by a scalar beam produced by a complex source-point feed, is presented. Two cases, soft and hard reflector surface, are considered. In each case the solution of the full-wave integral equation is reduced to dual series equations and then further to a regularized infinite-matrix equation. The latter procedure is based on the analytical inversion of the static part of the problem. Sample numerical results for 50-λ reflectors demonstrate features that escape a high-frequency asymptotic analysis.
107(2000); http://dx.doi.org/10.1121/1.429330View Description Hide Description
The backscattering of sound from two regularly arranged bubbles is studied theoretically and experimentally. In well-controlled laboratory experiments a bistatic acoustic system is used to interrogate the scatterers, which are placed on a very fine thread at the same distance from the combined beam axis of the set of transmitting and receiving transducers. The radius of each bubble is 585 μm. The frequency range is 80–140 kHz, and is varied so that the variable spans the range 0.2–21, where is the acoustic wave number. Scattering calculations are carried out using an exact, closed-form solution derived from the multiple scattering series. Several experiments are performed, and the results are in close agreement with the calculations. It is verified that multiple scattering induces an oscillatory behavior about the exact coherent scattering level, with decreasing amplitude for increasing For interbubble distance the backscattered radiation is maximized, while for the radiation is reduced considerably. These and other effects are discussed.
107(2000); http://dx.doi.org/10.1121/1.429331View Description Hide Description
For wave propagation at low frequencies in a porous medium, the Gassmann–Domenico relations are well-established for homogeneous partial saturation by a liquid. They provide the correct relations for seismic velocities in terms of constituent bulk and shear moduli, solid and fluid densities, porosity and saturation. It has not been possible, however, to invert these relations easily to determine porosity and saturation when the seismic velocities are known. Also, the state (or distribution) of saturation, i.e., whether or not liquid and gas are homogeneously mixed in the pore space, is another important variable for reservoir evaluation. A reliable ability to determine the state of saturation from velocity data continues to be problematic. It is shown how transforming compressional and shear wave velocity data to the (ρ/λ,μ/λ)-plane (where λ and μ are the Lamé parameters and is the total density) results in a set of quasi-orthogonal coordinates for porosity and liquid saturation that greatly aids in the interpretation of seismic data for the physical parameters of most interest. A second transformation of the same data then permits isolation of the liquid saturation value, and also provides some direct information about the state of saturation. By thus replotting the data in the (λ/μ, ρ/μ)-plane, inferences can be made concerning the degree of patchy (inhomogeneous) versus homogeneous saturation that is present in the region of the medium sampled by the data. Our examples include igneous and sedimentary rocks, as well as man-made porous materials. These results have potential applications in various areas of interest, including petroleum exploration and reservoir characterization, geothermal resource evaluation, environmental restoration monitoring, and geotechnical site characterization.
107(2000); http://dx.doi.org/10.1121/1.429433View Description Hide Description
Compressional waves in heterogeneous permeable media experience attenuation from both scattering and induced pore scale flow of the viscous saturating fluid. For a real, finely sampled sedimentarysequence consisting of 255 layers and covering 30 meters of depth, elastic and poroelastic computer models are applied to investigate the relative importance of scattering and fluid-flow attenuation. The computer models incorporate the known porosity, permeability, and elastic properties of the sand/shale sequence in a binary medium, plane layered structure. The modeled elasticscatteringattenuation is well described by stochastic medium theory if two-length scale statistics are applied to reflect the relative thickness of the shale layers when compared to the sand layers. Under the poroelastic Biot/squirt flow model, fluid-flow attenuation from the moderate permeability sands may be separated in the frequency domain from the attenuation due to the low permeability shale layers. Based on these models, the overall attenuation is well approximated by the sum of the scatteringattenuation from stochastic medium theory and the volume weighted average of the attenuations of the sequence member rocks. These results suggest that a high permeability network of sediments or fractures in a lower permeability host rock may have a distinct separable attenuation signature, even if the overall volume of high permeability material is low. Depending on the viscosity of the saturating fluid, the magnitude of the flow-based attenuation can dominate or be dominated by the scatteringattenuation at typical sonic logging frequencies (∼10 kHz).
- NONLINEAR ACOUSTICS 
107(2000); http://dx.doi.org/10.1121/1.429332View Description Hide Description
A modelequation that describes the propagation of sound beams in a fluid is developed using the oblate spheroidal coordinate system. This spheroidal beam equation (SBE) is a parabolic equation and has a specific application to a theoretical prediction on focused, high-frequency beams from a circular aperture. The aperture angle does not have to be small. The theoretical background is basically along the same analytical lines as the composite method (CM) reported previously [B. Ystad and J. Berntsen, Acustica 82, 698–706 (1996)]. Numerical examples are displayed for the amplitudes of sound pressure along and across the beam axis when sinusoidal waves are radiated from the source with uniform amplitude distribution. The primitive approach to linear field analysis is readily extended to the case where harmonic generation in finite-amplitude sound beams becomes significant due to the inherent nonlinearity of the medium. The theory provides the propagation and beam pattern profiles that differ from the CM solution for each harmonic component.
107(2000); http://dx.doi.org/10.1121/1.429333View Description Hide Description
Evolution equations for propagation of both unipolar and bipolar acoustic pulses are derived by using hysteretic stress-strain relationships. Hysteretic stress-strain loops that incorporate quadratic nonlinearity are derived by applying the model of Preisach–Mayergoyz space for the characterization of structural elements in a micro-inhomogeneous material. Exact solutions of the nonlinear evolution equations predict broadening in time and reduction in amplitude of a unipolar finite-amplitude acoustic pulse. In contrast with some earlier theoretical predictions, the transformation of the pulse shape predicted here satisfies the law of “momentum” conservation (the “equality of areas” law in nonlinear acoustics of elastic materials). A bipolar pulse of nonzero momentum first transforms during its propagation into a unipolar pulse of the same duration. This process occurs in accordance with the “momentum” conservation law and without formation of shock fronts in the particle velocity profile.
107(2000); http://dx.doi.org/10.1121/1.429334View Description Hide Description
An earlier paper [J. Acoust. Soc. Am. 98, 3412–3417 (1995)] reported on the comparison of rise times and overpressures of sonic booms calculated with a scattering center model of turbulence to measurements of sonic boom propagation through a well-characterized turbulent layer under moderately turbulent conditions. This detailed simulation used spherically symmetric scatterers to calculate the percentage of occurrence histograms of received overpressures and rise times. In this paper the calculation is extended to include distorted ellipsoidal turbules as scatterers and more accurately incorporates the meteorological data into a determination of the number of scatterers per unit volume. The scattering center calculation overpredicts the shifts in rise times for weak turbulence, and still underpredicts the shift under more turbulent conditions. This indicates that a single-scatter center-based model cannot completely describe sonic boom propagation through atmospheric turbulence.
107(2000); http://dx.doi.org/10.1121/1.429335View Description Hide Description
In this paper new observations of a laser-generated cavitation bubble interacting with an inertial boundary are presented. Employing schlieren photography techniques and a thin film transducer placed on the surface of the boundary, the pressure stresses induced in the solid boundary and the surrounding fluid by collapsing bubbles, created very close to the solid surface, are experimentally measured. Liquid jet development, shock wave emission, and “splash” phenomena are identified. For different creation sites close to the boundary, the relevance of each of these phenomena with respect to potentially damaging pressure stresses in the boundary is speculated on.
- AEROACOUSTICS, ATMOSPHERIC SOUND 
107(2000); http://dx.doi.org/10.1121/1.429336View Description Hide Description
Six sonic booms, generated by F-4 aircraft under steady flight at a range of altitudes (610–6100 m) and Mach numbers (1.07–1.26), were measured just above the air/sea interface, and at five depths in the water column. The measurements were made with a vertical hydrophone array suspended from a small spar buoy at the sea surface, and telemetered to a nearby research vessel. The sonic boom pressure amplitude decays exponentially with depth, and the signal fades into the ambient noise field by 30–50 m, depending on the strength of the boom at the sea surface. Low-frequency components of the boom waveform penetrate significantly deeper than high frequencies. Frequencies greater than 20 Hz are difficult to observe at depths greater than about 10 m. Underwater sonic boom pressure measurements exhibit excellent agreement with predictions from analytical theory, despite the assumption of a flat air/sea interface. Significant scattering of the sonic boom signal by the rough ocean surface is not detected. Real ocean conditions appear to exert a negligible effect on the penetration of sonic booms into the ocean unless steady vehicle speeds exceed Mach 3, when the boom incidence angle is sufficient to cause scattering on realistic open ocean surfaces.
107(2000); http://dx.doi.org/10.1121/1.429337View Description Hide Description
Pressure–time series from breathing-mode oscillation of large (centimeter scale or larger) underwater bubbles reveal much higher decay rates than can be explained using viscous, thermal, or radiative mechanisms which apply to microbubbles. It is shown that if one assumes energy transfer to shape oscillations (surface capillary waves) of large amplitude in subharmonic resonance with the breathing mode [M. S. Longuet-Higgins, J. Acoust. Soc. Am. 91, 1414 (1992)], then the shape oscillations can drive fluid motions outside the bubble capable of exciting turbulent instabilities. Application of an appropriate eddyviscosity from mixing-length theory to the viscous decay mechanism appears to offer a credible explanation for the observed large decay rates. An analysis is given to show that energy is transferred from the breathing mode to surface capillaries fast enough to make the proposed decay mechanism viable.
- UNDERWATER SOUND 
107(2000); http://dx.doi.org/10.1121/1.429338View Description Hide Description
This paper develops a new approach to matched-mode processing (MMP) for ocean acousticsource localization. MMP consists of decomposing far-field acoustic data measured at an array of sensors to obtain the excitations of the propagating modes, then matching these with modeled replica excitations computed for a grid of possible source locations. However, modal decomposition can be ill-posed and unstable if the sensor array does not provide an adequate spatial sampling of the acoustic field (i.e., the problem is underdetermined). For such cases, standard decomposition methods yield minimum-norm solutions that are biased towards zero. Although these methods provide a mathematical solution (i.e., a stable solution that fits the data), they may not represent the most physically meaningful solution. The new approach of regularized matched-mode processing (RMMP) carries out an independent modal decomposition prior to comparison with the replica excitations for each grid point, using the replica itself as the a priori estimate in a regularized inversion. For grid points at or near the source location, this should provide a more physically meaningful decomposition; at other points, the procedure provides a stable inversion. In this paper, RMMP is compared to standard MMP and matched-field processing for a series of realistic synthetic test cases, including a variety of noise levels and sensor array configurations, as well as the effects of environmental mismatch.
107(2000); http://dx.doi.org/10.1121/1.429339View Description Hide Description
A time-reversing array (TRA) can retrofocus acoustic energy, in both time and space, to the original sound-source location without any environmental information. This unique capability may be degraded in time-dependent or noisy acoustic environments, or when propagation losses are prevalent. In this paper, monochromatic propagation simulations (based on the parabolic equation code, RAM) are used to predict TRA retrofocusing performance in shallow-water sound channels having characteristics similar to those measured during the recent SWARM (shallow-water acoustics in a random medium) experiment. Results for the influence of source–array range, source depth, acoustic frequency, bottom absorption,internal wave strength, and round-trip time delay are presented. For a fixed channel geometry, higher frequencies, deeper sources, and lower bottom absorption improve TRA performance and allow retrofocusing at longer ranges. In a dynamic shallow-water channel containing a random superposition of linear internal waves, the size of the retrofocus is slightly decreased and sidelobes are suppressed compared to the static channel results. These improvements last for approximately 1 to 2 min for source-array ranges near 10 km at a frequency of 500 Hz. For longer time delays, the internal waves cause significant TRA retrofocus amplitude decay, and the decay rate increases with increasing internal wave activity and acoustic frequency.