Volume 100, Issue 6, December 1996
Index of content:
 ACOUSTICAL NEWS—USA


 ACOUSTICAL NEWS—INTERNATIONAL



Acoustic Particle Velocity Sensors: Design, Performance, and Applications
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 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.

Pitch, periodicity, and auditory organization
View Description Hide DescriptionThe perception of pitch forms the basis of musical melody and harmony. It is also among the most precise of all our human senses, and with imagination, this precision can be used experimentally to investigate the functioning of the auditory system. This tutorial presents auditory demonstrations from the zoo of pitcheffects:pitch shifts, noise pitch, virtual pitch, dichotic pitch, and the pitches of things that are not there at all. It introduces models of auditory processing, derived from contemporary psychoacoustics and auditory physiology, and tests these models against the experimental effects. It concludes by describing the critical role played by pitch in the important human ability to disentangle overlapping sources of sound.

On the relation between the wavefront speed and the group velocity concept
View Description Hide DescriptionThe relation between the wavefront speed and the group velocity concept is studied in this work. The relationship between the more well‐known velocity concept named as the phase velocity and the speed of propagation of a front of an acoustic pulse is discussed. This is of interest since it concerns transient wave propagation and is, in general, not well known. The form and properties of a pulse can be obtained by means of a Fourier integral and estimates based on quantities derived for monochromatic waves, such as the phase velocity, can be severely misleading and confusing. The wavefront velocity is defined as the high‐frequency limit of the phase velocity. This quantity can be far less than the value of the phase velocity for finite frequencies which for example is the case for bubbly fluids. Then the group velocity concept is discussed, which was introduced in order to characterize the propagation of water waves of essentially the same wavelength. However, more confusion occurs in that it is sometimes believed that a wavefront is propagating with the group velocity (a limit process not mentioned) since it can be related to the propagation of energy. This interpretation of energy propagation is based on sinusoidal waves and involves time as well as space averages and is not applicable for pulses. However, by means of the expression for the group velocity given by Stokes it is shown that the speed of a wavefront can be found from the group velocity at a limiting high frequency. This result can be understood geometrically from the definition of the group velocity given by Lamb which is conservation of wavelength. A wavefront is a discontinuity and limiting short wavelengths will be found there.

Reciprocity theorems for two‐way and one‐way wave vectors: A comparison
View Description Hide DescriptionFor acoustic applications in which there is a ‘‘preferred direction of propagation’’ (the axial direction) it is useful to arrange the two‐way and one‐way wave equations into the same matrix‐vector formalism. In this formalism, axial variations of the wave vector are expressed in terms of lateral variations of the same wave vector. The two‐way wave vector contains the field quantities pressure and velocity (axial component only), whereas the one‐way wave vector contains waves propagating in the positive and negative axial direction. By exploiting the equivalent form of the two‐way and one‐way matrix‐vector equations, it appears to be possible to derive two‐way and one‐way reciprocity theorems that have an equivalent form but a different interpretation. The main differences appear in the boundary integrals for unbounded media, in the contrast terms, and (for the correlation‐type theorems) in the handling of evanescent waves.

Numerical determination of scattered field amplitudes for rough surfaces
View Description Hide DescriptionIn theory, when an incident plane wave strikes a perfectly reflecting periodic surface, the resulting scattered field is comprised of a discrete spectrum of plane waves. Upon applying Dirichlet boundary conditions to the surface, one can construct what is referred to as a spectral‐coordinate (SC) formalism for the scattered amplitudes. A Fredholm integral equation of the first kind is involved, and the integration is performed over a single surface period. Since the Rayleigh approximation is not utilized in the construction of this formalism, one may use this method to determine the exact scattered field above the highest surface excursion. The problem will be approached numerically by directly discretizing the mixed SC representation, then solving the system using a pseudoinverse SVD technique. It is very important to note that the scattered amplitudes are obtained without constraining the value of normalized energy. This particular approach is unique. It differs from others in which the discretizations are implemented entirely in coordinate space or entirely in spectral (i.e. Bragg) space. It is thus an additional computational tool designed for cases when a mixed representation is appropriate. Although this numerical scheme has been developed for arbitrary periodic surfaces, the results presented in this paper are restricted to sinusoidal surfaces. Particularly interesting features of this approach are the high level of accuracy attained for near‐grazing incident fields and the maintenance of stability even for badly conditioned systems of equations.

Elastic wave and excitation mechanism of surface waves in multilayered media
View Description Hide DescriptionThe elastic wave field and the excitation mechanism of the surface waves in multilayered elasticsolid media are studied in this paper. On the basis of Abo‐zena [Geophys. J. R. Astron. Soc. 58, 91–105 (1979)] and Menke [Geophys. J. R. Astron. Soc. 59, 315–323 (1979)], the elastic wave field is further investigated in the B, P, C coordinate system. The so‐called new type of propagator matrix introduced by Menke to avoid loss of the precision problem is improved. It presented an important result and some new properties. The dispersion characteristics and excitation mechanisms of the surface waves (Rayleigh and Love waves) are also investigated via numerical simulation. The excitation intensities of the surface waves strongly depend on the frequency range of the source. The source frequency should be controlled in a proper range to effectively excite the surface waves. Two quantities, β_{1} (the ratio of B to P components of displacement) and β_{2} (the ratio of B to P components of stress), are defined for the Rayleigh wave. It is found that β_{1} and β_{2} are sensitive to the material property of the medium and the layered geometry, and they are two important physical quantities for exploring the structures of the interfaces and the velocity distributions of layers under the free surface. The relative error in estimating the thickness of each medium by β_{1} and β_{2} is less than 10%. The effects of the thickness of each layer of media and other factors on the dispersion characteristics of Rayleigh and Love waves and the values of β_{1} and β_{2} are also analyzed.

A lumped parameter model for the acoustic power output from a vibrating structure
View Description Hide DescriptionPrevious applications of lumped parameter models to acoustic radiation problems assume that the characteristic dimension of the vibrating structure is small in comparison to the acoustic wavelength. In this paper, the frequency range of the lumped parameter model is extended by dividing the surface of the structure into elements and characterizing the amplitude of the radiation from each element by its volume velocity. The model is derived by truncating all but the lowest‐order (monopole) terms of a multipole expansion for the acoustic power output. The multipole expansion differs from those derived previously because it is based on elemental quantities rather than global quantities. By comparing the full multipole expansion for the power output to the lumped parameter model, the error in the lumped parameter model as a function of the acoustic and structural wavelengths (k and K) and the size of the largest surface element (L) is determined. This approach is general and provides a means of determining bounds on the accuracy of any lumped parameter model based on elemental quantities. For example, the analysis predicts that when the overall volume velocity of a vibrating structure is nonzero, the maximum possible error in the lumped parameter model is equal to C(kL)(KL), where C is a constant. Likewise, when the overall volume velocity of a vibrating structure is zero, the model predicts that the maximum possible error in the lumped parameter model is equal to C′(KL)(L/R _{12}), where C′ is another constant, and R _{12} is the largest distance between any two points on the structure. The results of the analysis show that it is desirable to formulate acoustic models in terms of elemental volume velocities, because the power output predicted by any such model converges absolutely to the correct solution as the element mesh is refined.

An electromagnetic liquid shock wave generator for the production of a pulsed water jet
View Description Hide DescriptionIn this study the feasibility of using a novel adaptation of an electromagnetic source (EMAS) for the production of a pulsed water jet is investigated. This device consists of a high‐voltage, 4 to 8‐μF, 20‐kV capacitor which is discharged through a flat pancake coil. An insulated metal disk is placed in close proximity with the coil and is mounted in a water‐filled vessel. Lorentz forces due to the eddycurrents induced in the disk accelerate the disk away from the coil. The disk acceleration produces a pressure shock wave which is focused by a convergent reflector onto a nozzle. The reflection of this shock wave from the water–air interface produces a discrete water jet. A theoretical analysis of the electrical characteristics of the transducer is presented. A finite‐element package was used to study different convergent reflector shapes. Experimental results from a prototype generator are given, including pressure measurements from a needle hydrophone and shadowgraph photographs of the water jet.

Absorption of sound by noise in one dimension
View Description Hide DescriptionExperimental results are presented for the excess attenuation of monofrequency waves in the presence of high‐intensity noise in one dimension. The theory by Rudenko and Chirkin predicts a Gaussian attenuation of a weak signal in the presence of shockless noise, thus showing that in one dimension translational invariance has been broken. The theory is modified to incorporate wall losses providing, only then, excellent agreement with experiment. The agreement is shown as the frequency, noise level, and distance from the source are varied. In addition, the spectral intensity of the high‐frequency tail of fully developed shockless noise is observed to be an f ^{−3} power law in the frequency f, in accord with theory. This power law is a consequence of the far off equilibrium nature of the system.

Analysis of nonlinear effects in a piezoelectric resonator
View Description Hide DescriptionA theoretical model that explains the impedance variation due to nonlinear elasticity of piezoelectric materials is described. A modified equivalent circuit is used in order to explain the behavior of a piezoelectricresonator near its series resonance, driven by a high‐amplitude signal. A simple relation between the motional impedance Z _{1} and the motional branch current I _{1} has been obtained, defining nonlinear impedance coefficients. This model is able to explain and predict: (1) the amplitude‐frequency effect, (2) the decreasing of the mechanical quality factor, and (3) the appearance of hysteretic phenomena. An experimental method is proposed to measure the above coefficients, and is used to verify the theoretical model on disk ceramicresonators. Experimental measurements have been carried out to compare the nonlinearity of different ceramic materials.

Low‐frequency acoustic wave generation in a resonant bubble layer
View Description Hide DescriptionThe nonlinear response of a bubble layer subject to harmonic and biharmonic excitation at frequencies smaller than the individual bubbleresonance frequency is considered. The nonlinear resonance properties of the layer and generation of difference‐frequency signal are studied analytically and numerically. It is found that, for bubble volume fractions β≊10^{−3} and pump amplitudes of the order of 10^{−1} atm, the power of the low‐frequency signal (including waves radiated in both directions) may reach 10% of the total power of the incident biharmonic wave. The efficiency is restricted by the rapid formation of shocks already at relatively low driving amplitudes, which may not occur in a more complete model accounting for the inertia of the bubble pulsations.

Sound propagation over screened ground under upwind conditions
View Description Hide DescriptionA screen on an absorbing ground is investigated experimentally and theoretically under upwind conditions. The experimental data are the result of scale modelexperiments in a 1:25 scale model. The sound propagation is measured using a triggered spark source with signal spectrum averaging in the frequency domain. The meteorological data representing the wind conditions have been determined by means of hot‐wire anemometry in positions on both sides of the screen as well as directly over the screen. The theoretical model used for comparison is a hybrid approach. The sound field without a barrier is determined by means of numerical integration of a Hankel transformsolution for a stratified atmosphere [like a fast field program (FFP) but taking the near field into account]. This solution is used on both sides of the screen and is combined with a screen diffraction calculation. Results from the calculation model are able to explain the overall tendency in the experimental results.

A numerical model for sound propagation through a turbulent atmosphere near the ground
View Description Hide DescriptionIn this paper a series of numerical simulations of the effect of turbulence on the propagation of acoustic waves in the atmosphere are presented. First the technique of representing the turbulence as a set of realizations of a random field generated by a limited number of Fourier modes is described. Through each individual realization, the acoustic waves are propagated in a wide‐angle parabolic approximation to obtain the sound‐pressure level. Ensemble averaging is then performed to compute the statistical properties of the acoustic field: mean sound‐pressure level, intensity fluctuations, and amplitude distributions. The method is applied first to a nonrefractive atmosphere, both in the presence of a rigid boundary and of an impedance ground, and then to an upward refractive atmosphere with an impedance ground. The model, which contains no adjustable parameters, is tested using the experimental data of Parkin and Scholes, Daigle, and Wiener and Keast. Good agreement between numerical simulations and experiments is obtained.

Inversion scattering in finite space domain
View Description Hide DescriptionThe general integral representation of a scattered acoustic field with arbitrary boundary conditions is introduced. Assuming weakness of the scatterer, two linearized methods for the recovery of an object’s shape and position are derived and briefly discussed. The methods are applied to one‐ and two‐dimensional problems in order to compare them with the standard use of the first Born approximation (1BA). By presenting the numerical examples, this paper discusses the influence of the power spectrum of the incident pulse on exactness of the recovered object, the influence of ‘‘weakness’’ of scatterer, and comparison of proposed methods to 1BA.

Mode coupling by internal waves for multimegameter acoustic propagation in the ocean
View Description Hide DescriptionWe have performed broadband parabolic‐equation simulations with and without sound‐speed fluctuations induced by internal waves. The simulations have a center frequency of 75 Hz, a bandwidth of 30 Hz, and propagation range R of 1000, 2000, and 3000 km. In these cases it is found that long‐range acoustic propagation through internal waves is strongly nonadiabatic. In terms of modal travel times, low modes have a negative bias (they have a higher effective group speed than without internal waves) because they couple into higher, faster modes, while the higher modes show a positive bias, indicating preferential coupling into lower, slower modes. The lowest modes show the least travel‐time spread and bias, and these quantities increase rapidly with increasing mode number. Empirically and approximately it is found that bias grows like R ^{2} and spread grows like R ^{3/2}. This average slowing down and spreading of the higher modes causes a depth broadening of the reception finale. The modeled modal power distributions over frequency at multimegameter ranges are markedly different from the initial distributions at the source. Power is distributed roughly equally across the 30‐Hz frequency band for each mode, with 5.6‐dB scintillations consistent with an exponential probability distribution function for intensity. In spite of the dramatic spread and bias in the higher modes, it is found that a synthesis of these modes results in coherent wavefronts, whose characteristic timing fluctuations at the 3000‐km range are two orders of magnitude less than those of the corresponding modes. The modal spreads found in these simulations imply limits to the precision with which modal travel times can be measured using standard techniques. A nonstandard approach, using the geometric mean of independent realizations after transformation to the frequency domain, effectively averages the phase fluctuations and eliminates much of the spread. The strong modal coupling nonetheless suggests that nonadiabatic modal inversions will be necessary to do modal tomography at multimegameter ranges.

High resolution adaptive beamforming for three‐dimensional acoustic imaging of zooplankton
View Description Hide DescriptionThis paper investigates the use of adaptive beamforming techniques to increase the performance of a sonar imaging system developed at the Scripps Institution of Oceanography to monitor the migration of zooplankton. The original system, known as Fish Television (FTV), images a volume using an 8×8 grid of beams in elevation and azimuth. Imaging currently performed with the FTV system is limited by a 2‐deg resolution in both azimuth and elevation. In this paper, both a conventional delay‐and‐sum beamformer and a minimum variance adaptive beamformer are evaluated for use with the FTV sonar system to increase resolution and reduce sidelobe interference. Through simulation it is shown that by using adaptive beamforming, the image resolution can be improved to 1/4 deg in azimuth and elevation for targets with signal‐to‐noise ratios of at least 20 dB. Results using acoustic data from the Gulf of Eilat are compared to the original images of the FTV sonar system to illustrate the improvement in resolution, accuracy, and dynamic range achieved by adaptive beamforming.

A normal mode model for acousto‐elastic ocean environments
View Description Hide DescriptionA normal mode method for propagation modeling in acousto‐elasticocean waveguides is described. The compressional (p‐) and shear (s‐) wave propagation speeds in the multilayer environment may be constant or have a gradient (1/c ^{2} linear) in each layer. Mode eigenvalues are found by analytically computing the downward‐ and upward‐looking plane wave reflection coefficients R _{1} and R _{2} at a reference depth in the fluid and searching the complex k plane for points where the product R _{1} R _{2}=1. The complex k‐plane search is greatly simplified by following the path along which R _{1} R _{2}=1. Modes are found as points on the path where the phase of R _{1} R _{2} is a multiple of 2π. The direction of the path is found by computing the derivatives d(R _{1} R _{2})/dk analytically. Leaky modes are found, allowing the mode solution to be accurate at short ranges. Seismic interface modes such as the Scholte and Stonely modes are also found. Multiple ducts in the sound speed profile are handled by employing multiple reference depths. Use of Airy function solutions to the wave equation in each layer when computing R _{1} and R _{2} results in computation times that increase only linearly with frequency.
