Volume 101, Issue 2, February 1997
 ACOUSTICAL NEWS—USA



Directory of Graduate Education in Acoustics
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 ACOUSTICAL NEWS—INTERNATIONAL
 REVIEWS OF ACOUSTICAL PATENTS


Reviews Of Acoustical Patents
View Description Hide DescriptionThe 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.

Temporal integration of loudness, loudness discrimination, and the form of the loudness function
View Description Hide DescriptionTemporal integration for loudness of 5kHz tones was measured as a function of level between 2 and 60 dB SL. Absolute thresholds and levels required to produce equal loudness were measured for 2, 10, 50, and 250ms tones using adaptive, twointerval, twoalternative forcedchoice procedures. The procedure for loudness balances was new and employed ten interleaved tracks to obtain concurrent measurements for ten tone pairs. Each track converged at the level required to make the variable stimulus just louder than the fixed stimulus. Thus, the data yield estimates of the justnoticeable difference (jnd) for loudness level and temporal integration for loudness. Results for four listeners show that the amount of temporal integration, defined as the level difference between equally loud short and long tones, varies markedly with level and is largest at moderate levels. The effect of level increases as the duration of the short stimulus decreases and is largest for comparisons between the 2 and 250ms tones. The loudnesslevel jnds are also largest at moderate levels and, contrary to traditional jnds for the level of two equalduration tones, they do not appear to depend on duration. The latter finding indicates that loudness discrimination between stimuli that differ along multiple dimensions is not the same as level discrimination between stimuli that differ only in level. An equalloudnessratio model, which assumes that the ratio of loudnesses for a long and short tone at equal SPL is the same at all SPLs, can explain the level dependence of temporal integration and the loudness jnds. It indicates that the loudness function [log(loudness) versus SPL] is flatter at moderate levels than at low and high levels in agreement with earlier findings for 1kHz tones [M. Florentine et al., J. Acoust. Soc. Am. 99, 1633–1644 (1996)].

Resonant scattering of acoustic waves by ellipsoid air bubbles in liquids
View Description Hide DescriptionThis paper reports the first analytic study of resonantscattering of acoustic waves by ellipsoid air bubbles in liquids. Based on the Kirchhoff integral theorem, the function of sound scattering is derived in a simple closed form and is shown to bear analogy with the wellknown expression for sound scattering from spherical air bubbles. The resonance frequency and the quality factor Q are also derived. For the special cases such as spherical and prolate spheroid bubbles, the results reduce to the previously known results.

Determination of grainsize distribution from ultrasonic attenuation: Transformation and inversion
View Description Hide DescriptionAlthough researchers often describe a particular medium with only a single grain size or an average grain size, distributions of grain sizes can have significant effects on the physical characteristics of the material.Ultrasonic attenuation due to energy scattered at the grain boundaries depends on the ultrasonic wavelength, the grain size, and certain material properties such as elastic constants. This paper examines the relationship between the wavelength dependency of ultrasonic attenuation and grainsize distributions. A transformation is developed which solves a general class of Fredholm equations of the first kind. The equation relating attenuation to grainsize distributions belongs to this class of equations. The transformation allows attenuation to be calculated by discrete convolution, and provides a method for estimating the grainsize distribution directly from attenuation measurements.

Partial coherence of transient ultrasonic fields in anisotropic random media: Application to coherent echo detection
View Description Hide DescriptionIn this paper the spatial coherence properties of the specklenoisebackscattered from random media insonified by a broadband pulse are studied. The approach presented is an adaptation of the optical notion of coherence introduced by Zernike. How the statistical parameters (particularly the secondorder moment) of the medium determine those of the specklenoise is studied. This is applied to fully or partially incoherent media, and media with anisotropic scatterer distribution such as unidirectional and crossply composites; also investigated is the possibility of detecting a coherent reflector among specklenoise through measurements of coherence length. Experimental as well as theoretical results are presented. Measurements were carried out with a linear array of 128 programmable transmitters/receivers.

Singular points of intensity streamlines in twodimensional sound fields
View Description Hide DescriptionContinuing earlier work on this subject, a more rigorous discussion is given of the singular points of streamlines and the critical points of the stream function. The results for vortex and saddle points obtained earlier in piecemeal fashion and by way of examples are obtained systematically and by generally utilizing the applicable theory of differential equations and calculus. New results are also obtained. For example, a saddle point can occur when the phase of pressure and velocity differ by π/2, and in certain parts of the sound field, specifically inside a closed streamline, the number of vortex points and saddle points are related. Finally, the streamlines and singular point are considered for a discrete source: the line source.

Acoustic radiation force on a spherical particle in a viscous heatconducting fluid. I. General formula
View Description Hide DescriptionThe acoustic radiation force exerted by an axisymmetric sound field on a spherical particle is calculated assuming that the surrounding fluid is viscous and heat conducting. The incident sound fieldpressure amplitude is supposed to be small enough such that nonlinear effects like generation of subharmonics do not occur. No restrictions are imposed on the particle size, which means that the particle can be of an arbitrary radius with respect to the sound, viscous, and thermal wavelengths in the surrounding fluid. The obtained formula for the radiation force is general in that it is applicable to first, any axisymmetric sound field, such as a plane, traveling or standing wave and a spherical wave, and, second, any of the following types of dispersed particles: a gas bubble, a liquid drop, a rigid or elastic sphere, a spherical shell, etc. The force is expressed in terms of the linear scattering coefficients to be determined by the particle type. Thus, to obtain the force on a specific particle the problem of linear scattering for that particle must be solved. Problems of this sort are known not to be mathematically difficult, but can be laborious enough if a particle at issue has a complicated internal structure. The radiation forces on particles of most interest are examined in papers that follow [J. Acoust. Soc. Am. 101, 722–740 (1997)].

Acoustic radiation force on a spherical particle in a viscous heatconducting fluid. II. Force on a rigid sphere
View Description Hide DescriptionThe general expression derived in Part I of this study [J. Acoust. Soc. Am. 101, 713–721 (1997)] for the acoustic radiation force on a spherical particle immersed in a viscous heatconducting fluid is applied here to the case of a rigid sphere. Three types of incident sound field are considered: a plane traveling wave, a plane standing wave, and a diverging spherical wave. To demonstrate dissipative effects on the force clearly, for each of these fields two limiting cases of interest are investigated. In the first of these, the sphere radius is assumed to be large compared with the viscous and thermal wavelengths in the surrounding fluid and the thermal wavelength inside the sphere. In the second case, the sphere radius is assumed to be small compared with the above wavelengths. In addition, in both cases, it is assumed that the sound wavelength is much larger than the above parameters. Since viscouseffects have been examined in earlier papers by the present author, in this paper, the main attention is given to thermal effects. It is shown that these can drastically change the radiation force, resulting in, for example, the movement of light spheres both from the sound source and to it in a plane traveling wave and levitation in a diverging spherical wave.

Acoustic radiation force on a spherical particle in a viscous heatconducting fluid. III. Force on a liquid drop
View Description Hide DescriptionThe general expression derived in Part I of this work [J. Acoust. Soc. Am. 101, 713–721 (1997)] for the acoustic radiation force exerted by a sound field on a spherical particle in a viscous heatconducting fluid is applied here to a liquid drop. To demonstrate dissipative effects clearly, two limiting cases are studied. In both cases, the sound wavelength is taken to be much larger than other characteristic parameters of the system, such as the drop radius and the viscous and thermal wavelengths both outside and inside the drop. Within this assumption, two reverse situations are considered, in the first of which the dissipative effects are assumed to be weak, and in the second, to be strong. For each of these two cases, three types of incident wave are dealt with: a plane traveling wave, a plane standing wave, and a diverging spherical wave. Emphasis is given to the thermal effects on the radiation force. These are found to be greatest in the traveling and spherical waves, while in the standing wave, for both limiting cases, the thermal effects only give rise to small corrections to the leading terms due to compressibility and viscosity.

Computing effective reflection coefficients in layered media
View Description Hide DescriptionComputations are presented which show that the effective reflection and transmission coefficients for a rough interface embedded in a layered medium can differ significantly from the mean reflection and transmission coefficients computed for the same rough interface when it separates two homogeneous halfspaces. These differences are large when the correlation length of the roughness is long compared to the skip distance of rays associated with normal modes in the layered medium. Otherwise, these differences may be generally neglected. However, increasing the rms roughness decreases the ratio of correlation length to skip distance at which the effect of the layering is important. The case of a Pekeris waveguide with a rough fluid–fluid interface and the case of a rough Dirichlet surface bounding an upwardly refracting medium are considered.

Robust wideband matchedfield processing with a short vertical array
View Description Hide DescriptionThis paper addresses the problem of matchedfield passive localization of broadband underwater acousticsources with a short vertical array. In previous work, incoherent averaging of narrowband Bartlett ambiguity surfaces has been proposed to improve the robustness of matchedfield processing (MFP) to environmental uncertainties. While computationally efficient, the effectiveness of this approach is dependent on the type of environmental mismatch present. In this paper, to provide robust source localization performance for short arrays, an alternative wideband MFP method is proposed which utilizes the fourthorder statistics of the data. The method is “coherent” in that it exploits crossfrequency dependencies in the random acoustic channel response. The proposed coherent wideband minimum variance method with environmental perturbation constraints (MVEPC) consists of minimizing the squared output power of a beamformer subject to constraints defined across the signal band. The constraints are designed to provide robustness over an ensemble of random environmental realizations. Monte Carlo simulation results using a canonical shallowwater scenario indicate that the coherent wideband MVEPC method yields improved probability of correct localization performance versus incoherent averaging of narrowband Bartlett ambiguity functions with a short array above a threshold signaltonoise ratio.

Rational squareroot approximations for parabolic equation algorithms
View Description Hide DescriptionIn this article, stable Padé approximations to the function are derived by choosing a branch cut in the negative halfplane. The Padé coefficients are complex and may be derived analytically to arbitrary order from the corresponding real coefficients associated with the principal branch defined by denotes the imaginary part of ]. The characteristics of the corresponding squareroot approximation are illustrated for various segments of the complex plane. In particular, for waveguide problems it is shown that an increasingly accurate representation may be obtained of both the evanescent part of the mode spectrum for the acoustic case and the complex mode spectrum for the elastic case. An elastic parabolic equation algorithm is used to illustrate the application of the new Padé approximations to a realistic ocean environment, including elasticity in the ocean bottom.

Scattering from an elastic shell and a rough fluid–elastic interface: Theory
View Description Hide DescriptionA nullfield Tmatrix formalism is developed for scattering a pressure wave from a stationary elastic shell immersed in a homogeneous and isotropic fluid halfspace and in proximity to a rough fluid–elastic interface. Helmholtz–Kirchhoff integral representations of the various scattered pressure and displacement fields are constructed. The surface fields are required to satisfy the elastic tensor boundary conditions and the scattered fields are required to satisfy the extended boundary condition. Spherical basis functions are used to construct a freefield Tmatrix for the elastic shell and rectangular vector basis functions are used to construct a representation of the freefield T matrix for the rough fluid–elastic interface. The freefield Tmatrices are introduced into the Helmholtz–Kirchhoff and the nullfield equations for the shellinterface system and a general system of equations for the spectral amplitudes of the various fields is obtained. The general system of equations is specialized to scattering from periodic surface roughness and an exact solution for the scattered pressure field in the fluid is obtained. Then the general system of equations is specialized to scattering from smallamplitude arbitrary roughness profiles and a perturbative solution is obtained. It is shown that the formalism contains multiple scattering effects on the rough surface and between the rough surface and the shell.

Acoustic propagation through an internal wave field in a shallow water waveguide
View Description Hide DescriptionThis paper addresses the problem of predicting and interpreting acoustic wave field properties in a stochastic oceanwaveguide, for which the soundspeed variability within the water column is treated explicitly as a random process. It is assumed that the soundspeed distribution is composed of three components: a deterministic, timeindependent profile and two stochastic components induced by internal wave activity. One random contribution represents a spatially diffuse Garrett–Munk field whose spectrum is constrained by the shallow water waveguide, while the second corresponds to spatially localized soliton packets. A highangle elastic parabolic equation method is applied to compute single frequency realizations of the pressure field using this threecomponent representation of the soundspeed distribution. Ensembleaveraged transmission loss and scintillation index measures for the full pressure field and its modal components are estimated for different source depths and for both flat and sloping bottoms. Probability distributions of the mode amplitudes for different ranges are also presented. These statistical measures are incorporated into the analysis of rangedependent mode coupling between the internal wave and acoustic fields, and evidence is presented which supports a recent prediction that the scintillation index grows exponentially with range due to the competition between mode coupling and mode stripping found in shallow water waveguides. Fullfield estimates of the scintillation index are also presented for a shallow water region on the continental slope off the New Jersey coast.

A note on nonlinear radiation from a gas bubble in liquids
View Description Hide DescriptionThis paper discusses nonlinear radiation from a gas bubble in liquids. The discrepant results from two previous approaches, the volume displacement approach and the radial displacement approach, were compared and discussed. By rederiving the governing equation used in the two approaches, it is shown that under the usual approximations the sound emission resulting from the volume displacement approach is correct, but the previous procedure which leads to the right answer is not rigorous; the lack of rigor has drawn considerable criticisms in the past.

Optimal recovery of the elasticity tensor of general anisotropic materials from ultrasonic velocity data
View Description Hide DescriptionAn ultrasonicwave approach is presented for the optimal identification of the 21 independent elasticity constants of the most general linear homogeneous anisotropicelastic solid from wave speed measurements of obliquely incident ultrasonic bulk waves. Since the symmetry of acquisition planes is not taken into account, this inversion process is generalized to materials that do not possess three mutually orthogonal planes of symmetry. Minimization of an overdetermined system of nonlinear algebraic equations is solved by a constrained optimization method. Various classes of symmetry are considered. Several critical factors (initial guesses, scatter in experimental data) which can influence the accuracy of the elastic property reconstruction algorithm have been investigated by means of numerical examples that simulate in the best way a typical experiment. The sensitivity of the reconstruction algorithm to each identified elasticity constant is detected a posteriori by means of the estimation of a confidence interval associated with each identified constant. The generalized reconstruction method is applied to the experimental data for materials with various symmetry classes.

Modal analysis of Lamb wave generation in elastic plates by liquid wedge transducers
View Description Hide DescriptionA modal analysis is presented to describe the excitation of Lamb waves in an elastic plate using a liquid wedge transducer. Analytical expression for the displacement of a given mode is derived for the excitation by a uniform bounded beam. In contrast to previous studies, the contribution of the reflected wave is included in the input exciting forces using a perturbation theory. The conversion efficiency, defined as the ratio of the guided mode power to the incident power, is related to a single parameter which depends on the rate of attenuation due to leakage from the guided wave into the liquid wedge. Numerical results relevant to the fundamental Lamb modes are obtained as a function of frequency for various incident beam widths and plate thickness. Using optical interferometric detection, direct measurements of the Lamb modes displacements have been carried out in aluminium plates to verify the theoretical analysis.
