Volume 106, Issue 2, August 1999
- acoustics research letters online
- acoustical news—usa
- acoustical news—international
- book reviews
- general linear acoustics 
- nonlinear acoustics 
- underwater sound 
- 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 
- psychological acoustics 
- speech production 
- speech perception 
- speech processing and communication systems 
- music and musical instruments 
- bioacoustics 
- letters to the editor
Index of content:
- ACOUSTICS RESEARCH LETTERS ONLINE
Phoneme recognition by cochlear implant users as a function of signal-to-noise ratio and nonlinear amplitude mapping106(1999); http://dx.doi.org/10.1121/1.427031View Description Hide Description
The present study measured phoneme recognition as a function of signal-to-noise levels when different nonlinear loudness mapping functions were implemented in three cochlear implant users using a 4-channel continuous interleaved sampler (CIS) strategy. Results show that phoneme recognition scores in quiet vary only slightly when different amplitude mappings from highly compressive to weakly compressive are applied. As the level of background noise is increased, recognition scores decrease more rapidly for the strongly compressive mapping than for the weakly compressive mapping. Results indicate that, although a strongly compressive mapping between acoustic and electric amplitude produces slightly better performance in quiet, a less compressive mapping may be beneficial for implant listeners in noisy listening conditions.
106(1999); http://dx.doi.org/10.1121/1.427032View Description Hide Description
The piezoelectric Class III barrel-stave flextensional transducer is an underwater acoustic source that is capable of generatingsound over a wide frequency band. This broadband performance is achieved through coupling the fundamental flexural and longitudinal modes of vibration. Near the low frequency flexural resonance, the Class III transducer is small compared to a wavelength, and its radiation is omnidirectional. However, at frequencies in the vicinity of the longitudinal resonance, the radiation is directional. In this article, we show through the use of a finite element model that the Class III transducer can be designed for omnidirectional radiation over the entire band.
Acoustic scintillations and angle-of-arrival fluctuations observed outdoors with a large planar vertical microphone array106(1999); http://dx.doi.org/10.1121/1.427080View Description Hide Description
A vertical planar array of 32 microphones (eight elements in the vertical direction and four in the horizontal; overall dimensions approximately 6 m by 3 m) was used to image the scintillations and angle-of-arrival fluctuations from a source 770 m distant. Data sets of 20-min duration were collected in a variety atmospheric conditions. On a windy afternoon, the source image underwent dramatic scintillations and fluctuations in its apparent position. For still nighttime conditions, the image was much more stable, although deep fading still occurred.
- GENERAL LINEAR ACOUSTICS 
Elastic wave propagation and scattering in heterogeneous, anisotropic media: Textured polycrystalline materials106(1999); http://dx.doi.org/10.1121/1.427024View Description Hide Description
The propagation of elastic waves through heterogeneous, anisotropic media is considered. Appropriate ensemble averaging of the elastic waveequation leads to the Dyson equation which governs the mean response of the field. The Dyson equation is given here in terms of anisotropicelastic Green’s dyadics for the medium with and without heterogeneities. The solution of the Dyson equation for the mean response is given for heterogeneities that are weak. The formalism is further specified for the case of equiaxed cubic polycrystalline metals with a single aligned axis. The Green’s dyadics in this case are those for a transversely isotropic medium. Simple expressions for the attenuations of the shear horizontal, quasicompressional, and quasishear waves are given in terms of integrations on the unit circle. The derived expressions are limited to frequencies below the geometric optics limit, but give the attenuations in a direct manner. Comparisons with previous results are also discussed. It is anticipated that a similar approach is necessary for the study of wave propagation in complex anisotropic materials such as fiber-reinforced composites. In addition, the results are applicable to diffuse ultrasonic inspection of textured polycrystalline media.
106(1999); http://dx.doi.org/10.1121/1.427025View Description Hide Description
The Anderson fluid sphere scattering model [J. Acoust. Soc. Am. 22, 426–431 (1950)] is reexamined to clarify three issues which have been the source of misunderstanding among underwater acousticians. First, the accuracy of the Morse large range approximation for the spherical Hankel functions is investigated. It is shown that the minimum range for use of the approximation is strongly mode number dependent, and should be carefully evaluated in short range and/or high frequency applications. Second, the precise characterization of the forward scatter region is studied. When the scattered field and the incident plane wave are combined, it is shown that little advantage is obtained in detection and localization applications by using forward scattering, rather than backscattering. Third, the translational response, or “rebound,” of the sphere under the action of the incident field is examined. By demonstrating that Anderson’s theory is a limiting case of Faran’s scattering model [J. Acoust. Soc. Am. 23, 405–418 (1951)] for an elastic sphere, which contains the rebound response, it is shown that the response is completely explainable within Anderson’s theory, and is consistent with a description which uses a normal mode expansion around a fixed origin.
106(1999); http://dx.doi.org/10.1121/1.428041View Description Hide Description
In the past, various two- and three-dimensional Cartesian, poroelastic finite element formulations have been proposed and demonstrated. Here an axisymmetric formulation of a poroelastic finite element is presented. The intention of this work was to develop a finite element formulation that could easily and efficiently model axisymmetric sound propagation in circular structures having arbitrary, axially dependent radii, and that are lined or filled with elastic porous sound absorbing materials such as foams. The formulation starts from the Biot equations for an elasticporous material expressed explicitly in axisymmetric form. By following a standard finite element development, a u-U formulation results. Procedures for coupling the axisymmetric elements to an adjacent acoustical domain are described, as are the boundary conditions appropriate for unfaced foams. Calculations described here show that the present formulation yields predictions as accurate as a Cartesian, three-dimensional model in much reduced time. Predictions made using the present model are also compared with measurements of sound transmission through cylindrical foam plugs, and the predicted results are shown to agree well with the measurements. Good agreement was also found in the case of sound transmission through a conical foam plug.
106(1999); http://dx.doi.org/10.1121/1.427026View Description Hide Description
A theory of compressional and shear wave propagation in consolidated porous media (rocks) is developed by extending ideas already introduced in connection with unconsolidated marine sediments. The consolidated material is treated as an elastic medium which exhibits a specific form of stress relaxation associated with grain boundaries and microcracks. The stress relaxation, which is linear in the sense that it obeys superposition, shows hysteresis, as characterized by a material response function. Two linear wave equations are derived, one for compressional and the second for shear waves, from which expressions for the wave speeds and attenuations are established. In both cases, the attenuation is found to scale with the first power of frequency, consistent with many observations of attenuation in sandstones, limestones, and shales; the wave speeds show weak logarithmic dispersion. These expressions for the wave speeds and attenuations satisfy the Kronig–Kramers dispersion relationships, as they must if the response of the medium to disturbances is to be causal. Some comments are offered on the nature of the material response, notably that it appears to be primarily associated with grain-boundary interactions occurring at a molecular level, rather than being related to the macroscopic properties of the material, such as density or porosity.
106(1999); http://dx.doi.org/10.1121/1.427135View Description Hide Description
In this paper, laser-ultrasonic techniques are employed to develop a quantitative understanding of the underlying principles of the propagation of guided circumferential waves in two-layered cylindrical components. The high-fidelity, broad-bandwidth, point source/receiver and noncontact nature of these optical techniques are critical elements to the success of this work. The experimental procedure consists of measuring a series of transient, circumferentially propagating waves in a cylindrical waveguide and then operating on these transient waveforms with signal-processing techniques to develop the dispersion relationship for that waveguide; this procedure extracts the steady-state behavior from a series of transient measurements. These dispersion curves are compared to theoretical values. There is good agreement between the experimental and theoretical results, thus demonstrating the accuracy and effectiveness of using laser-ultrasonic techniques to study the propagation of guided circumferential waves.
- NONLINEAR ACOUSTICS 
106(1999); http://dx.doi.org/10.1121/1.427027View Description Hide Description
This analysis consists of the development of the fluid flow about a spherical particle placed at the velocity node of a standing wave. High-frequency acoustic fields are being used to levitate particles in Earth gravity, and to stabilize particles in low-gravity situations. While a standing wave in an infinite medium may be purely oscillatory with no net flow components, the interaction with particles or solid walls leads to nonlinear effects that create a net steady component of the flow. In the present development, the perturbation method is employed to derive the flow field for the situation when a spherical particle is positioned at the velocity node. As found in an earlier analysis [Riley, Q. J. Mech. Appl. Math 19, 461 (1966)] applicable to a solid sphere at the velocity antinode, there is a thin shear-wave region adjacent to the spherical boundary. However, this thin Stokes layer does not cover the entire sphere in the same manner as in the previous case. In the polar regions flow reversal takes place but the Stokes layer opens to the surrounding field. On an equatorial belt region there are closed streamlines.
- UNDERWATER SOUND 
106(1999); http://dx.doi.org/10.1121/1.427028View Description Hide Description
A perturbative solution to the problem of plane-wave scattering from a soft, randomly rough, cylindrical surface is obtained by use of the Rayleigh hypothesis and the assumption that coupled radiation modes can be present in the scattered wave. The wave amplitudes of the cylindrical-wave expansion of the scattered field are expressed as an asymptotic series in the root-mean-square of surface irregularities. The end result of the analysis is a concise recursive formula for the coefficients of that series. The solution is energy consistent up to fourth-order in the smallness parameter. Specific information about the validity and applicability of the solution is given by use of the mean boundary condition error, which is defined in this paper with a view to assessing the conformity of near-field results with the boundary condition. The numerical results show that the fourth-order approximation is more accurate than the second-order one, but it may, under certain conditions, exhibit higher mean boundary condition error.
Multiaspect identification of submerged elastic targets via wave-based matching pursuits and hidden Markov models106(1999); http://dx.doi.org/10.1121/1.427029View Description Hide Description
This paper investigates classification of submerged elastic targets using a sequence of backscatteredacoustic signals corresponding to measurements at multiple target-sensor orientations. Wavefront and resonant features are extracted from each of the multiaspect signals using the method of matching pursuits, with a wave-based dictionary. A discrete hidden Markovmodel (HMM) is designed for each of the target classes under consideration, with identification of an unknown target effected by considering which model has the maximum likelihood of producing the observed sequence of feature vectors. HMMs are stochastic models which are well suited to describing piecewise-stationary processes, and are appropriate for multiaspect classification due to the strong aspect dependence of the scattered fields for most realistic targets. After establishing the physical and geometric correspondence between multiaspect sensing and the HMM parameters, performance is assessed through consideration of measured acoustic data from five similar submerged elastic targets. Results are presented with and without additive noise.
106(1999); http://dx.doi.org/10.1121/1.427030View Description Hide Description
An experiment was performed just off the research pier at the Scripps Institute of Oceanography to determine the acoustic effects of small bubbles in very shallow water (∼6 m depth). The distance offshore was ∼300 m. The propagation lengths were 2–10 m, and the frequency range was from 39 to 244 kHz. During the experiment, rip currents passed through the field of measurement instruments. These rip currents were laden with bubbles created in the surf between the instruments and the shore. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, the bubble distributions are calculated using a new iterative approach [Caruthers et al., in press, J. Acoust. Soc. Am.] that is based on the well-known resonant bubble approximation. Calculated bubble distributions varied from an essentially uniform lack of bubbles during quiescent periods to highly inhomogeneous and dense bubbly regions within rip events. Such observed distributions were consistent with measurements made by other investigators during the experiment.
106(1999); http://dx.doi.org/10.1121/1.427082View Description Hide Description
The propagation of waves in a conducting piezoelectricsolid is studied for the case when the entire medium rotates with a uniform angular velocity. For comparison, both the conventional electrically quasistatic theory and the fully dynamic Maxwell equations are taken into consideration. In completion, a generalized thermoelastic theory of piezoelectric bodies is incorporated. The governing dispersion relations are obtained to determine the effects of moderate rotation, thermal, and constant electrical conductivity on the finite phase velocity of the waves. Analysis is carried out for plane waves in an infinite medium but also for surface waves of a half-space. Finally, the radial vibrations of a hollow cylinder are addressed. The evaluations are specified for hexagonal crystals of (6 mm) class and a simple arrangement of the direction of wave propagation and the crystal and rotational axes.
Yellow Shark Spring 1995: Inversion results from sparse broadband acoustic measurements over a highly range-dependent soft clay layer106(1999); http://dx.doi.org/10.1121/1.427083View Description Hide Description
In May 1995, SACLANTCEN performed broadband (200–800 Hz) acoustic measurements in the Giglio basin off the coast of Italy as part of the Yellow Shark inversion experiments. In this paper, inversion of sparse, broadband transmission loss (TL) measurements is investigated to determine bottom properties in strongly range-dependent (RD) situations commonly encountered in shallow water. The data are from the Elba–Formiche transect where water depth varies from 65 m at the acoustic projector location, to approximately 120 m at the 4-element vertical arrays deployed at ranges of 8, 16, 24, 32, and 40 km along the transect. The experimental site has a soft clay-layer bottom which varies in thickness from 3 to 10 m with a sound speed less than the water column. A modal analysis including coupling effect is given to explain the frequency bands for which high TL was observed as a function of range and depth. The TL measurements were inverted by matching the RD fields with model results. Using parabolic equation modeling, bottom geoacoustic parameters were varied in a marching search to fit the TL measured at the five ranges. The experimental results demonstrate that RD bottom properties such as sound speed and thickness of the slow clay layer can be obtained from broadband TL measurements sparsely distributed in range and depth.
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
Measurements and predictions of the phase velocity and attenuation coefficient in suspensions of elastic microspheres106(1999); http://dx.doi.org/10.1121/1.427139View Description Hide Description
The phase velocities and attenuation coefficients for suspensions of narrowly sized polymer microspheres are reported over a broadband spectrum from 3 to 30 MHz. The six suspensions used in this work contain microspheres with respective average diameters near 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, and 100 μm. The results of these measurements are compared with theoretical expressions for the phase velocity and attenuation coefficient derived from the scattering properties of an elastic sphere in water using the weak scattering limit of the Waterman and Truell dispersion relation [J. Math. Phys. 2, 512–537 (1961)]. This single-scattering limit of the theory is found to be sufficient for predicting the ultrasonic transport properties of these suspensions to a considerable degree of accuracy.
106(1999); http://dx.doi.org/10.1121/1.427607View Description Hide Description
A noncontacting resonant-ultrasound-spectroscopy (RUS) method for measuringelastic constants and internal friction of conducting materials is described, and applied to monocrystalline copper. This method is called electromagnetic acoustic resonance (EMAR). Contactless acoustic coupling is achieved by energy transduction between the electromagnetic field and the ultrasonic vibrations. A solenoidal coil and static magnetic field induce Lorentz forces on specimen surfaces without using a coupling agent. By changing the field direction, a particular set of vibration modes can be selectively excited and detected, an advantage in identifying the vibration modes of the observed resonance peaks. Contactless coupling allows the measure of intrinsic internal friction free from energy loss associated with contact coupling. The elastic constants and internal frictionmeasured by EMAR are compared with those by the usual RUS method for a rectangular-parallelepiped copper monocrystal. Both methods yielded the same elastic constants despite fewer resonant peaks in the EMAR case. The two methods gave essentially the same shear-mode internal friction, but the RUS method gave higher volume-mode internal friction.
Theoretical and experimental studies of surface waves on solid–fluid interfaces when the value of the fluid sound velocity is located between the shear and the longitudinal ones in the solid106(1999); http://dx.doi.org/10.1121/1.427084View Description Hide Description
The existence of two surface waves propagating on a plane solid–fluid interface is demonstrated when the value of the fluid sound velocity is located between the shear and the longitudinal ones in the solid. First, the Scholte–Stoneley dispersion equation is studied analytically and numerically to find the roots corresponding to the Stoneley and the Rayleigh waves. The anatomy of each one is then described with the formalism of the evanescent plane waves: both waves are unleaky. Finally, the results are confirmed experimentally by measuring the times of flight on a Plexiglas–water interface and on a PVC–water interface.