Volume 105, Issue 6, June 1999
- acoustical news—usa
- acoustical news—international
- book reviews
- general linear acoustics 
- nonlinear acoustics 
- aeroacoustics, atmospheric sound 
- underwater sound 
- ultrasonics, quantum acoustics, and physical effects of sound 
- transduction 
- structural acoustics and vibration 
- noise: its effects and control 
- architectural acoustics 
- acoustic signal processing 
- physiological acoustics 
- psychological acoustics 
- speech perception 
- speech processing and communication systems 
- music and musical instruments 
- bioacoustics 
- letters to the editor
Index of content:
- GENERAL LINEAR ACOUSTICS 
105(1999); http://dx.doi.org/10.1121/1.424627View Description Hide Description
Composite barriers, consisting of thin plates separated by light matrix structures, are widely used for fuselage construction in the aircraft industry, and in partitions in the building trade. The acoustical properties of such materials can vary considerably by altering the interior geometry, and perforations can be added to one or both sides. With perforations the interior cavities can act as Helmholtz resonators, causing a substantial modification to the overall transmission and reflection properties of such barriers. Leppington [Proc. R. Soc. London, Ser. A 427, 385–399 (1990)] devised an effective boundary condition for a perforated sandwich plate structure, valid in the limit of low frequency (acoustical waves long compared with the typical dimensions of the hole/cavity construction), and obtained transmission and reflection coefficients for infinite planar structures. This article investigates the radiation properties of perforated sandwich plates by examining a simple infinite one-dimensional model (employing Leppington’s effective boundary condition) which is loaded by a line force or moment. The radiated far field, and unattenuated subsonic plate wave coefficients, are found explicitly, and are plotted over a range of frequencies for two physical configurations, namely an aluminum plate in water and in air. It is revealed that, unlike the usual thin plate equation, the model discussed herein has two bi-directional unattenuated plate waves, and for the structure in air the two waves are of similar magnitude over most frequencies. Surprisingly, these amplitudes are shown to become very large at a frequency below that of the structure’s Helmholtz resonance frequency. Further, the field radiating into air is also significantly modified by the cavity/perforations well away from the Helmholtz frequency.
105(1999); http://dx.doi.org/10.1121/1.424631View Description Hide Description
For thin bead layers saturated by air, a pole of the reflection coefficient related to a trapped mode inside the layer and a surface wave in air is predicted, and detected with the Tamura method [J. Acoust. Soc. Am. 88, 2259 (1990)]. For semi-infinite layers, the Brewster angle is observed with the same method at oblique incidence at 15 kHz. A description of the related Zenneck wave is performed, at high and low frequencies.
Experimental verification of the opposite effect of fluid loading on the velocity of dilatational waves in thin plates and rods105(1999); http://dx.doi.org/10.1121/1.424632View Description Hide Description
In a recent paper [J. Acoust. Soc. Am. 102, 3478 (1997)], it was demonstrated by analytical means that radiation loading increases the velocity of dilatational waves in immersed thin plates, but decreases it in thin rods. The main goal of this paper is to verify experimentally the predicted opposite effect, which is particularly interesting because in almost every respect, the lowest-order dilatational modes of wave propagation in thin plates and rods are very similar. Experimental verification of the predicted small radiation-induced velocity change is rather difficult, partly because of the accompanying strong attenuationeffect caused by radiation losses, and partly because of the presence of an additional velocity change caused by viscous drag even in low-viscosity fluids like water. In spite of these inherent difficulties, the presented experimental results provide unequivocal verification of the earlier theoretical predictions.
Active impedance control within a cylindrical waveguide for generation of low-frequency, underwater plane traveling waves105(1999); http://dx.doi.org/10.1121/1.424633View Description Hide Description
A cylindrical, water-filled acoustic waveguide with an active termination was used to generate constant-frequency, plane traveling waves. The waveguide was constructed of acrylic tubing with an underwater sound projector flanged to each end. One projector acted as the primary source and generated continuous harmonic waves. The active control system measured the transfer function between two hydrophones located inside the waveguide and used a pattern search algorithm to adjust the secondary source amplitude and phase in order to drive the measured transfer function to that for a plane traveling wave. The active control system was able to reduce the reflection coefficient to below 0.05 within the frequency range 12.5–400 Hz and adjust the acoustic pressure/particle velocity ratio to match that of a plane progressive wave.
105(1999); http://dx.doi.org/10.1121/1.426960View Description Hide Description
In this paper it is demonstrated that a theoretical model for wave propagation may indeed correspond to a well-posed transient problem although the group velocity for finite frequencies becomes greater than the high frequency limit of the phase velocity, negative or even infinite. Sufficient conditions for causality are derived and the particular cases of relaxing and bubbly fluids are considered so as to show some of the properties of the group velocity concept.
105(1999); http://dx.doi.org/10.1121/1.424634View Description Hide Description
Parabolic equations for gravity and acousto-gravitywaves are derived and implemented. The wave equations for these problems contain singularities at depths at which the buoyancy frequency equals the forcing frequency. One of the advantages of the parabolic equationsolution is that it is easy to avoid numerical problems associated with the singularities. Some problems involve an infinite number of propagating modes. This artifact of neglecting viscosity is handled by including stability constraints in the rational approximations used in the implementation of the parabolic equation. The parabolic equation is tested for idealized problems involving surface, internal, and interface gravity waves. Parabolic equationsolutions are also presented for range-dependent problems involving internal waves in the ocean and acousto-gravitywaves in the atmosphere.
105(1999); http://dx.doi.org/10.1121/1.424635View Description Hide Description
The study of the guided propagation and radiation from radially layered boreholes has applications in monitoring exploratory drilling. Axisymmetric propagation through pipes immersed inside fluid-filled bores in infinite elastic spaces was modeled using full 3-D elastodynamic equations. Three propagating modes are present for frequencies below 1 kHz. These are the lowest order modes and are characterized through their dispersion, attenuation, modeshapes, and particle motions. Mode I is confined mainly to the cross section of the pipe, while modes II and III are present in the fluid layers. Modes I and II are weakly dispersive, while mode III is strongly dispersive. Mode III is the equivalent of the Stoneley mode present in fluid-filled bores without pipe and is most influenced by the properties of the surrounding elastic space. Mode III has a cutoff frequency, below which it radiates, when the shear speed of the surrounding elastic space is sufficiently low.
105(1999); http://dx.doi.org/10.1121/1.424636View Description Hide Description
A closed-form frequency-domain formalism for spatially integrated diffraction corrections is proposed. Spatially integrated diffraction corrections are necessary when trying to characterize material with ultrasonic probing. In the case of piston transducers and point receivers, the Lommel diffraction formulation is used when the excitation is monochromatic, and the arccos diffraction formulation is used when the excitation is impulsive. The Lommel and arccos formulations are usually treated separately; here, they are connected. Specifically, the arccos diffraction formulation and Lommel diffraction formulation are shown to form an approximate Fourier transform pair. Since the Lommel formulation is amenable to closed-form spatial integration, Lommel functions are used to derive diffraction corrections for unfocused piston transducers operating in receiveonly (one-way) mode or transmit/receive (two-way) mode. Results obtained from the proposed closed-form frequency-domain formalism are qualitatively compared with results based on the closed-form time-domain or impulse-response formalism. It will be shown that the proposed frequency-domain formalism has theoretical and practical value. Finally, specific computational considerations are discussed as necessary.
Directivity of a uniform-strength, continuous circular-arc source phased to the spatial position of its diameter105(1999); http://dx.doi.org/10.1121/1.424637View Description Hide Description
An expression is derived for the in-plane directivity function of a uniform-strength, circular-arc source (or receiver) wherein the output of each element of the arc is phase shifted by the wavelength distance from that element to the spatial position of the diameter of the circle which is parallel to the chord joining the end points of the arc. Numerical results are presented for a selected set of arc sizes and values of the radius-to-wavelength ratio. The results resemble the directional characteristics of a true straight line source of the chord length. The special cases of a phased semicircular arc and a phased complete circular arc are discussed. The directivity function of a uniform-strength, unphased, continuous circular-arc source, a previously established result, is also presented for comparison.
- NONLINEAR ACOUSTICS 
A new theoretical approach to the analysis of nonlinear sound beams using the oblate spheroidal coordinate system105(1999); http://dx.doi.org/10.1121/1.424638View Description Hide Description
In this article, the authors propose a new modelequation which describes well progressive sound beams from a planar piston source with a circular aperture. The theory is based on the oblate spheroidal coordinate system and is easily extended to the analysis of nonlinear propagation of finite amplitude sound beams. The resultant beam equation facilitates numerical calculations in the far field. An experiment is performed in water using a 5-MHz planar ultrasonic transducer with a 9-mm radius aperture, and the results are compared with the theoretical prediction for sound pressure amplitudes of the fundamental and second harmonics. They are in excellent agreement with each other along and across the beam axis.
105(1999); http://dx.doi.org/10.1121/1.424639View Description Hide Description
An approximate quasistatic equation, analogous to the Burgers equation, is derived to account for the combined effects on tube wave propagation of (a) dispersion/attenuation in permeable formations, and (b) quadratic nonlinearity of the fluid and of the formation. Numerical results for weak nonlinearity and narrow-band pulses indicate that pulse self-demodulation does occur, but over relatively large distances because of the relatively low-frequency band relevant for tube wave propagation in characteristic borehole geometries The self-demodulated pulse shape can be very significantly distorted from that predicted by the conventional Burgers equation, depending upon the choice of relevant parameters such as the permeability, the carrier frequency, and the mudcake membrane stiffness. Numerically exact analytical formulas for the self-demodulated pulse shape, as well as for the energy in the second harmonic band, are derived for cases in which the pulse duration is long and the nonlinearity is relatively weak. These formulas are valid for any arbitrary dispersion/attenuation mechanism, and not just tube waves in permeable formations, as long as the propagation wave vector may be specified uniquely as a function of frequency.
105(1999); http://dx.doi.org/10.1121/1.424640View Description Hide Description
Static friction in rocks is modeled as a threshold phenomenon. An inverse problem in ground motion is analyzed for propagation in a medium with an arbitrary number of fractures at which static friction thresholds determine the onset of slippage. It is shown that seismic sourcedynamics inferred from surface motion is unique. This result is used to show that the threshold property of static friction is not a mechanism for hysteresis in rocks. The coda for the surface motion is shown, in the case of a single fracture and a deep source of finite duration, to be either a periodic function in time, or to asymptote in time to such a function, depending on an algebraic relation between ground and fracture parameters. The fundamental period of the coda is independent of the sourcetime series and is equal to four times the signal transit time between the single fracture and ground surface. Only odd harmonics of the fundamental frequency are present in a Fourier series of these periodic functions.
105(1999); http://dx.doi.org/10.1121/1.424641View Description Hide Description
The effects of a flat sea surface on the secondary wave field for a nonsaturation limited parametric source with a piston-type transmit transducer have been assessed both theoretically and experimentally. When the virtual array of the parametric source interacts with the sea surface, the amplitude of the secondary signal is reduced. In addition to the normal interference between the direct and surface reflection paths, there is also one extra mechanism which causes the reduction of the secondary signal level. This is the destructive summation of the secondary field generated before and after intersection with the water surface. A numerical model and the Westervelt model are used to predict the secondary field which is a function of grazing angle. Experiments in an in-door laboratory tank have been carried out to measure the primary and secondary fields of a parametric array under these circumstances. It is shown that the surface reflection affects the beam pattern of the secondary field and the induced reduction in the signal level depends on the characteristics of the parametric source and the geometry of the problem. The level of reduction caused by the effects of the surface interaction approaches a constant at large grazing angles.
- AEROACOUSTICS, ATMOSPHERIC SOUND 
105(1999); http://dx.doi.org/10.1121/1.424642View Description Hide Description
For a realistic model of a stratified moving atmosphere with arbitrary vertical profiles of the adiabatic sound speed and wind velocity vector, an equation is derived for the sound scattering cross-section per unit volume, σ, as a function of apparent scattering angle The effects of these profiles on σ are studied numerically. It is shown that if the wind velocity is zero, but the adiabatic sound speed varies with height, then σ can be affected significantly for Furthermore, if the wind velocity varies with height, but the adiabatic sound speed does not, then σ can be affected significantly for as well as near These and other numerical calculations have shown that in many cases acoustic remote sensing of the structure parameters of temperature and wind velocity fluctuations in the atmosphere should be based on the derived equation for σ rather than on that used in the literature. The derived equation for σ is also compared to those obtained by Clifford and Brown [J. Acoust. Soc. Am. 55, 929–933 (1974)] and by Ye [J. Acoust. Soc. Am. 102, 754–758 (1997)] for a model of a stratified moving atmosphere as two homogeneous layers in relative motion. It is explained why the predictions by Clifford and Brown are different from those by Ye.
- UNDERWATER SOUND 
105(1999); http://dx.doi.org/10.1121/1.424643View Description Hide Description
An acoustic monitoring program of herring migration in Drogden channel, near Copenhagen, Denmark was conducted from June 1996 until the end of May 1997. Fixed 100-kHz side-looking sonars provided nearly continuous surveillance in a 1-km-wide by 12-m-deep navigation channel. Water temperature, salinity, and current profiles were simultaneously monitored at this site. The sonars were positioned to insonify regions near the seabed at ranges up to 800 m, such that the typical reverberation was due to low-grazing angle seabed backscatter. It was found that under normal, weakly stratified flow conditions, fish schools attributable to herring (Clupea harengus) were observed from the 50- to 500-m range. This could be done despite interference from the dense vessel traffic, specifically direct echoes from hulls, propeller cavitationnoise, and bubbly wakes. At close ranges (<150 m) hyperbolic trajectories attributable to individual herring were observed, with horizontal advection speeds in close agreement with measured current magnitudes. It was further observed that occasional intrusions of saline bottom waters created strong upward-refracting conditions that significantly limited the range for fish school detection. Ray-tracing analysis is used to define the insonified areas and describe the backscattered reverberation under normal and stratified flow conditions. It is shown using simulations of fish-school echoes that seabed-reflected multipaths can create an upward bias in fish-school densities calculated using echo-integration techniques.
105(1999); http://dx.doi.org/10.1121/1.424644View Description Hide Description
A time-seriesmodel for acoustic seafloor backscattering is described. The method analytically expresses the elementary time-backscattered response of every seafloor surface and every seafloor volume infinitesimal element. For chosen geometric, acoustical, and acquisition parameters, they are summed to produce in the time domain a realization of the reverberation time-pressure field received at the source. The approach is based on the Kirchhoff approximation for the seafloor interface backscattering and on the Small Perturbation theory for the seafloor volume. It only accounts for single backscattering mechanisms of the compressional wave with the seafloor. The model is implemented using calculated height fields for the water-sediment interface and distributed seafloor volume inhomogeneities. The analytical description of the model and its limitations is described in this paper.
105(1999); http://dx.doi.org/10.1121/1.424645View Description Hide Description
A time-evolution model of seafloor scatter is numerically implemented and experimentally evaluated. The model is based on analytically expressing the elementary time-backscattered response of every seafloor surface and every seafloor volume infinitesimal element. The implementation of the model is based on a statistical realization of the seabed interface and volume inhomogeneities, from which the time series are computed by coherent summation of the scatter from small elements over the insonified area and volume. The analytical expressions and the implementation are evaluated for the image solution case, for which an almost perfect agreement is found. Examples are shown of how the beam width and seabed roughness affect the time-series return from both the surface and from the volume. The results of the model are compared with data from two different bottom types recorded with a parametric sonar. Reasonable accordance is found between the model and the data.