Volume 104, Issue 4, October 1998
- acoustical news—usa
- acoustical news—international
- reviews of acoustical patents
- selected research articles 
- general linear acoustics 
- nonlinear acoustics 
- aeroacoustics, atmospheric sound 
- underwater sound 
- ultrasonics, quantum acoustics, and physical effects of sound 
- structural acoustics and vibration 
- noise: its effects and control 
- physiological acoustics 
- psychological acoustics 
- speech production 
- speech perception 
- bioacoustics 
- letters to the editor
Index of content:
- REVIEWS OF ACOUSTICAL PATENTS
104(1998); http://dx.doi.org/10.1121/1.423714View 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.
- SELECTED RESEARCH ARTICLES 
104(1998); http://dx.doi.org/10.1121/1.423761View Description Hide Description
Electrostrictive driver materials with large strain capability hold great promise for the advancement of sonar projector technology. However, the nonlinear induced strain of these materials can create acoustic distortion in transducers through higher-order harmonics. Electrostrictors also possess complicated prestress and temperature dependencies, and an elastic modulus that depends strongly on electric field. This investigation examined these issues with a nonlinear, frequency domain model for a flextensional transducer powered by an electrostrictive stacked actuator. A simple, linear lumped-parameter model of a flextensional shell and its surrounding acoustic medium were combined with a nonlinear model of the electrostrictive driver. This model accounted for the material’s nonlinear dependencies and behavior. Predictions of the device’s acoustic/electric response during operation compared favorably with experiments performed on a single element flextensional transducer. The model’s results showed that proper adjustment of the power supply’s parameters minimizes the level of distortion without completely sacrificing the transducer’s improved source level.
Vortex sound in bass-reflex ports of loudspeakers. Part I. Observation of response to harmonic excitation and remedial measures104(1998); http://dx.doi.org/10.1121/1.423760View Description Hide Description
At high sound pressure levels a bass-reflex port produces blowing sounds, especially in the case of small loudspeaker boxes with narrow bass-reflex ports. The blowing sounds are caused by vortex shedding of the acoustic flow at the end of the port at high flow velocities. It has been found that acoustic standing waves in the longitudinal direction of the port are excited in a pulsatile manner by the periodically generated vortices. This is demonstrated by time history measurements of the blowing sounds of a loudspeaker system with a bass-reflex port driven by a harmonic signal. Broadband turbulencesound appears to be weaker than these deterministic sounds. It has been found that, near the 1-kHz port resonance frequency, the power level of the blowing sounds can be reduced by 8 dB by using a port cross section that diverges gradually toward both port ends with a slope angle at the port ends of about 6°, and rounding the edges at both port ends.
Vortex sound in bass-reflex ports of loudspeakers. Part II. A method to estimate the point of separation104(1998); http://dx.doi.org/10.1121/1.423762View Description Hide Description
In part I of this paper, the vortex shedding that may occur in a bass-reflex port of a loudspeaker system was discussed. At the Helmholtz frequency of the bass-reflex port, air is pumped in and out at rather high velocities,vortex shedding occurs at the end of the port, and blowing sounds are generated. It was explained that the key in the design of a port with a minimum of blowing sounds is the point of flow separation from the wall at which vortices are formed. This paper presents a method for estimating the point of separation for an unsteady flow like the flow through a bass-reflex port. Assuming that the flow can be described by a potential flow up to the point where flow separation occurs, it was found that the point of separation can be estimated on the basis of measurement of the sound pressure inside the loudspeaker box and measurement of the sound pressure at a distance of 1 m from the port exit. Application of the proposed technique to a cylindrical port with rounded edges at both port ends revealed that the point of separation is determined by the particle displacement rather than by the particle velocity. It was also found that a good indicator of the onset of severe vortex shedding is the Strouhal number based on the radius of curvature of the port edges.
- GENERAL LINEAR ACOUSTICS 
104(1998); http://dx.doi.org/10.1121/1.423614View Description Hide Description
An acoustically soft or hard sphere which is covered by a penetrable concentric spherical shell disturbs the propagation of an incident wave field emanating from a point source. The source is located in the exterior of the coated sphere. The medium, occupying the shell, is considered to be lossy while the dimensions of the coated sphere are much smaller than the wavelength of the incident field. For the case of a soft sphere covered with a penetrable lossy shell, the exact low frequency coefficients of the zeroth and the first order for the near field as well as the first and second order coefficients for the normalized scattering amplitude are obtained. In the case of the coated hard sphere, the zeroth and the first order coefficients of the near field, as well as the leading nonvanishing coefficient of the normalized scattering amplitude, which is of the second order, are obtained. For both cases of the soft and the hard sphere, the scattering and the absorption cross sections are calculated. The effect of the coating is expressed in terms of specific constants. A detailed discussion of the results and their physical meaning is included. For a sphere with a soft core and a point source located more than five radii away from the scatterer, or for a sphere with a hard core and a point source located more than two radii away from the scatterer, the results obtained are almost the same as if the scatterer was excited by a plane wave.
Generalized Bremmer series with rational approximation for the scattering of waves in inhomogeneous media104(1998); http://dx.doi.org/10.1121/1.423615View Description Hide Description
The Bremmer series solution of the wave equation in generally inhomogeneous media, requires the introduction of pseudodifferential operators. In this paper, sparse matrix representations of these pseudodifferential operators are derived. The authors focus on designing sparse matrices, keeping the accuracy high at the cost of ignoring any critical scattering-angle phenomena. Such matrix representations follow from rational approximations of the vertical slowness and the transverse Laplace operator symbols, and of the vertical derivative, as they appear in the parabolic equation method. Sparse matrix representations lead to a fast algorithm. An optimization procedure is followed to minimize the errors, in the high-frequency limit, for a given discretization rate. The Bremmer series solver consists of three steps: directional decomposition into up- and downgoing waves, one-way propagation, and interaction of the counterpropagating constituents. Each of these steps is represented by a sparse matrix equation. The resulting algorithm provides an improvement of the parabolic equation method, in particular for transient wave phenomena, and extends the latter method, systematically, for backscattered waves.
104(1998); http://dx.doi.org/10.1121/1.423616View Description Hide Description
The application of a hybrid asymptotic/finite element method to the problem of scattering from two-dimensional, submerged objects is considered. The hybrid method is based on patching a short-wavelength asymptotic expansion of the scattered field to a finite elementinterpolation of the near field. In patching, the diffracted field shape functions with unknown amplitude are forced to agree smoothly with the solution in the near field along a curve at a prescribed distance from the diffraction points. A new hybrid finite element on this artificial boundary represents the effect of the outer domain on the solution within this new boundary. This allows the replacement of the original boundary value problem with an asymptotically equivalent boundary value problem, the domain of which is small and efficiently discretized. The method is applied to diffraction by a blunted wedge, which in this context represents a degenerate prism. The hybrid scattering solution shall be compared to an analytic field representation found using an exact Dirichlet-to-Neumann map.
104(1998); http://dx.doi.org/10.1121/1.423763View Description Hide Description
When only two loudspeakers are used for the reproduction of sound for a single listener, time domain simulations show that it is advantageous that the two loudspeakers are very close together. The sound field reproduced by two loudspeakers that span 10 degrees as seen by the listener is simpler, and locally more similar to the sound fieldgenerated by a real sound source, than that reproduced by two loudspeakers that span 60 degrees. The basic physics of the problem is first explained by assuming that the sound propagates under free-field conditions. It is then demonstrated that when the influence of the listener on the incident sound waves is taken into account by modeling the listener’s head as a rigid sphere, the results are qualitatively the same as in the free-field case. Consequently, two closely spaced loudspeakers are capable of accurately reproducing a desired sound field, not only at the ears of the listener but also in the vicinity of the listener’s head. This result, although counter-intuitive, is very encouraging. In particular, it suggests that many low-fidelity audio systems, such as those currently supplied with most multi-media computers, can be greatly improved.
104(1998); http://dx.doi.org/10.1121/1.423764View Description Hide Description
In two-transducer, fluid-coupled ultrasonic reflection measurements phase matched to guided modes of elastic solid structures, the effects of misalignment (i.e., differences) between the receiver angle and the incident beam angle on the receiver voltage have been studied. The received voltage is typically due to contributions from the specular reflection and any of the several possible guided wave modes excited by the incident wave field. It is found that misalignment leads to changes in the relative amplitudes of the various contributions. Further, the more highly collimated the beam (or the contributor), the more pronounced are the effects. It is shown that the signal maximum is not a reliable indicator of receiver alignment. These conclusions are based on measurements and on calculations that have been performed at incident angles selected both close to, or far from, phase-matching angles to guided wave modes on plates, curved surfaces, and cylindrical shells. Receiver voltage coordinate scans have been performed with receiver angles misaligned from the incident beam axis by 1 to 4 degrees. The receiver voltage versus scan parameter in planar and curved structures is calculated by employing complex transducer points to synthesize two-dimensional, sheet-beam transducer fields with Gaussian apertures, spectral analysis to formulate the beam-structure interaction problem, and asymptotic methods to evaluate the resulting spectral integrals. The model predictions are generally in good agreement with the experiments.
Stress effect on boundary conditions and elastic wave propagation through an interface between anisotropic media104(1998); http://dx.doi.org/10.1121/1.423765View Description Hide Description
Elastic wave propagation through a plane interface between two generally anisotropic stressed solids is considered. The case when one solid is replaced with fluid is considered separately due to its importance for the development of immersion techniques for ultrasonic stress determination. The incident wave plane can coincide with or deviate from planes of material symmetry. Static stresses are assumed to be locally homogeneous and to satisfy static boundary conditions. The analysis is applicable for both applied and residual stresses. For numerical implementation a method and an analysis of the reflection-refraction problem on the boundary of separation between two anisotropicsolids [Rokhlin et al., J. Acoust. Soc. Am. 79, 906 (1986)] is modified to take stresses into account. Simulations are performed to implement the theory described and to clarify the stress effect on elastic wave interaction with a solid interface. Changes in propagation directions, polarizations, and energy transmission and reflection coefficients due to the presence of stresses are discussed. The stress-induced birefringence is explored quantitatively. Conditions for destructive interference of shear waves are presented.
104(1998); http://dx.doi.org/10.1121/1.423766View Description Hide Description
In the pore space of packed grain material,transport properties are characterized by macroscopic parameters. Some of them, tortuosity, characteristic dimensions, viscous permeability, and trapping constant, are measured for a random packing of glass beads and compared to evaluations performed in previous studies. These parameters are used to predict the surface impedance at normal incidence of a layer of glass beads. The predictions are compared to measurements performed at normal incidence in a Kundt tube.
104(1998); http://dx.doi.org/10.1121/1.423715View Description Hide Description
The effect of spatial dispersion on the phase-speed degeneracies of acoustic waves in noncentrosymmetric crystals is explored in the framework of continuum elasticity. It is shown that the degeneracies, which in the nondispersive limit occur in directions neither lying in a symmetry plane nor parallel to a rotation inversion axis, are lifted on account of spatial dispersion. The degeneracies lying in symmetry planes change their in-plane orientation depending on the frequency, unless remaining fixed to a symmetry axis. Given that the degeneracy with regard for linear spatial dispersion occurs in some direction m, the value of degenerate phase speed coincides with the value of one of the speeds assigned to m in the nondispersive limit. Relations for the acoustic-wave parameters with dispersive corrections are derived and discussed for various particular cases.
104(1998); http://dx.doi.org/10.1121/1.423716View Description Hide Description
The propagation and scattering of torsional waves in circular rods of infinite length with a free surface that have an abrupt diameter change is discussed. A mode expansion technique involving propagating and cut-off modes is applied to obtain numerical results for a range of parameter values. The calculated results are compared with simplified models valid in the high- and low-frequency regions, respectively. The calculations show that higher radial modes have strong influence at higher frequencies. In particular, near the cut-off frequencies both amplitude and phase responses are strongly influenced. It is further found that the approximation accuracy results depend on the number of modes used in the modal expansion. It is demonstrated that in order to obtain an accuracy (as judged from convergence) that is sufficient for typical parameters chosen, all propagating modes plus a few cut-off modes should be included in the calculations.
104(1998); http://dx.doi.org/10.1121/1.423767View Description Hide Description
The flexural mode is the lowest-order borehole mode with dipole excitation. The low-frequency asymptote of the flexural wave speed is used to estimate the formation shear speed in well logging. The borehole flexural mode measurement system consists of a mandrel that holds the transmitter and an array of receivers. This mandrel is, generally, designed to minimize the interference between any mandrel and the borehole flexural modes. A slotted sleeve housing that makes the sleeve arrival significantly slower than the formation arrival enables the processing of the recorded waveforms based on a model of fluid-filled boreholes without any mandrel. The objective of this work is to investigate the basic physics of mandrel effects on borehole flexural mode for the condition that the mandrel is in the form of a rod or pipe that is not necessarily slower than the formation arrivals. To this end, we describe an experimental and theoretical study of flexural modes in a borehole with a concentric mandrel and water annulus. The mandrels are of simple geometry such as a rod or a pipe, not intended to model any particular sonic tool in its entirety. However, the presence of a rod or a pipe can approximately represent the waveguide nature of the mandrel and its possible influence on borehole flexural modes. A fluid-filled (without any tool) borehole and a mandrel immersed in an infinite fluid (without any borehole) are two independent systems. Each supports a flexural mode that can be excited by a dipole source. These two uncoupled modes have distinct velocity dispersions. In a coupled mandrel-in-borehole system, two flexural modes are generated with a dipole source. These two coupled modes may resemble or significantly differ from the two uncoupled modes depending on the proximity of dispersions of these two uncoupled modes.
104(1998); http://dx.doi.org/10.1121/1.423717View Description Hide Description
Bellows are generally used for vibration isolation in ducts. However, conical expansions and contractions lead to acoustic transmission loss as well. In this paper plane wave propagation through a bellow has been investigated. The wave equation for a stationary medium has been solved exactly with the help of a practical assumption. But, for the case of mean flow, with its convective as well as dissipative effects, an equivalent simple expansion-contraction chamber model has been developed based on some parametric studies. Effects of different parameters like thickness of the bellow wall, radius, length, and Mach number of mean flow have been studied. A reasonable agreement has been found between the theoretical and experimental results.
104(1998); http://dx.doi.org/10.1121/1.423718View Description Hide Description
This article considers modeling the effect of complex subsystems on the dynamics of the main structure to which they are attached. It is proposed that the substructure (say a piece of equipment) be replaced by an equivalent set of forces which react back on the main structure. These forces are given as time convolutions of the displacements at the equipment attachment points. The convolution integral, which represents a time domain DtN (Dirichlet-to-Neumann) map, is approximated in the high modal density limit with determined error bounds. This approximation leads to a family of equipment representations. The simplest requires few measured equipment properties, though more information can lead to greater accuracy. Our approximate DtNs are demonstrated numerically in finite element simulations.
104(1998); http://dx.doi.org/10.1121/1.423719View Description Hide Description
This paper extends the Helmholtz equation least-squares (HELS) method previously developed by Wang and Wu [J. Acoust. Soc. Am. 102, 2020–2032 (1997)] to reconstruction of acoustic pressure fields inside the cavity of a vibrating object. The acoustic pressures are reconstructed through an expansion of the acoustic modes generated by the Gram–Schmidt orthonormalization with respect to the particular solutions to the Helmholtz equation. Such an expansion is uniformly convergent because the selected acoustic modes consist of a uniformly convergent series of Legendre functions. The coefficients associated with these acoustic modes are determined by requiring the assumed-form solution to satisfy the pressureboundary condition at the measurement points. The errors incurred in this process are minimized by the least-squares method. Numerical examples of partially vibrating spheres and cylinders with various half-length to radius aspect ratios subject to different frequency excitations are demonstrated. The reconstructed acoustic pressures are compared with the analytic solutions and numerical ones obtained by using the standard boundary element method(BEM) codes.
- NONLINEAR ACOUSTICS 
104(1998); http://dx.doi.org/10.1121/1.423720View Description Hide Description
A time-domain numerical model is presented for simulating the finite-amplitude focused acoustic pulse propagation in a dissipative and nonlinear medium with a symmetrical source geometry. In this method, the main effects responsible in finite-amplitude wave propagation, i.e., diffraction, nonlinearity, and absorption, are taken into account. These effects are treated independently using the method of fractional steps with a second-order operator-splitting algorithm. In this method, the acoustic beam propagates, plane-by-plane, from the surface of a highly focused radiator up to its focus. The results of calculations in an ideal (linear and nondissipative) medium show the validity of the model for simulating the effect of diffraction in highly focused pulse propagation. For real media, very good agreement was obtained in the shape of the theoretical and experimental pressure-time waveforms. A discrepancy in the amplitudes was observed with a maximum of around 20%, which can be explained by existing sources of error in our measurements and on the assumptions inherent in our theoretical model. The model has certain advantages over other time-domain methods previously reported in that it: (1) allows for arbitrary absorption and dispersion, and (2) makes use of full diffraction formulation. The latter point is particularly important for studying intense sources with high focusing gains.