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Nov 1988

Volume 84, Issue S1, pp. S2-S224

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back to top Session D. Engineering Acoustics I: Theory, Practice, and Materials in Engineering Acoustics
Contributed Papers
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Signal pressure received by a hydrophone placed on a plate backed with a compliant baffle (A)

Sung H. Ko and Howard H. Schloemer

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S4-S4 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A theoretical model was developed to evaluate the signal pressure received by a hydrophone placed in front of a plate backed with a compliant baffle layer. The compliant baffle layer between the plate and the semi‐infinite fluid medium is designed for reducing pressure fluctuations from Papers nonsignal directions. Because of its acoustic softness, the signal received by the hydrophone without a plate would be degraded. Therefore, it is desirable to improve the signal reception by covering the baffle layer with a hard plate. The baffle layer considered here is the compliant‐tube array, modeled by Junger [J. Acoust. Soc. Am. 78, 1010 (1985)], to represent a homogeneous (dispersive) fluid layer. Effects of various parameters such as the angle of incidence, the aspect ratio of the compliant tube, the distance between tubes, and the damping of the tube on the received signal pressure are presented. Calculations made for the nondispersive fluid layer are compared with those made for the dispersive fluid layer.
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Optimization of a low‐frequency transmitting array (A)

Dominique Lalisse and Didier Boucher

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S5-S5 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A low‐frequency plane array is studied in water in a wide frequency band around the resonance frequency of the transducers. The array under study is made of eight length‐expander vibrators (two columns of four transducers) with a circular radiating face in a rigid box of limited dimensions. The radiating impedance matrices are calculated by an integral equation method [C. Audoly, J. Acoust. Soc. Am. Suppl. 1 83, S20 (1988)] and projectors are modeled with a classical electromechanical equivalent circuit. Due to the effects of acoustic interactions, no terms in the matrices are found to be negligible. Mechanical and electrical constraints on the transducers are identified and computed. The array is studied under three conditions: identical voltage driving, identical headmass velocity distribution, and acoustic power optimization. The results confirm that acoustic interactions have important and drastic effects around the resonance. The study of acoustic power optimization makes it possible to discuss the opportunity of using velocity control and electromechanical feedback devices in low‐frequency sonar projector arrays.
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Comparison of theoretical and experimental form functions for a viscoelastic coated infinite shell (A)

Gary W. Caille, Jacek Jarzynski, and Peter H. Rogers

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S5-S5 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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The experimentally observed backscattered farfield form functions (normal incidence) for a simulated infinite cylindrical shell coated with (a) closed‐cell nitrile rubber and (b) corprene are compared with the theoretically derived form functions. The shell is 304 stainless steel with a radius ratio of 0.97 and a length greater than 8 ft so as to avoid end effects. Air is the inner fluid. The coatings have specific acoustic impedances less than water and the nitrile coating is highly attenuating. The experiment was conducted for an approximate ka range of 1.5–15. The viscoelastic constants for the coatings were determined from the experimentally measured Young's and plane‐wave moduli. The experimental procedure for measuring the form function was validated by the excellent agreement of the observed form function with the theoretical form function for the shell only. Significant observations are that (1) these coatings increase the form functions relative to the shell at low ka values; (2) the coatings reduce or eliminate the resonances observed in the shell‐only baseline; and (3) the nitrile‐coated shell is an excellent simulation of an ideal pressure release cylinder. [Work supported by Office of Naval Research, Code 11250A.]
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Radiation from an array of simple sources (A)

Adnan Akay

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S5-S5 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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General closed‐form expressions for the farfield intensity and power radiated by finite arrays are derived for a class of linear arrays made up of simple point sources. The closed‐form solutions are made possible by a special trigonometric relationship that simplifies the intensity expression. Earlier work on out‐of‐phase sources [J. Acoust. Soc. Am. Suppl. 1 79, S30 (1986)] has been extended to include in‐phase sources with axes either transverse or coincident with the axis of the array. [Work sponsored by NSF.]
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The increase of transducer directivity using diffractive attachments (A)

Wieslaw R. Woszczyk

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S5-S5 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Increased directivity is sought for pressure microphones at frequencies above 1 kHz in order to accomplish useful directional selectivity and better signal‐to‐noise ratio in the pickup of direct transient information contained within reverberant sound fields. The goal is to improve the “reach” of pressure microphones for direct transient sound under reverberant conditions in rooms while maintaining only single‐point sampling of the sound field. Smooth off‐axis frequency responses and undistorted time‐domain responses of microphones are desired to transcribe with fidelity the complex sound field at the point of pickup. Diffractive and absorptive attachments are installed on microphones to modify their frequency and directional responses without distorting time‐domain responses. Detailed directional frequency responses and transient responses are measured at every 5 and 15 deg of angular sound incidence using impulse techniques to verify the effect of the attachments. These responses are compared to those of high‐quality microphones used for the recording of music and speech. [Work supported by SSHRC.]
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Effects of attenuation, dispersion, and high sound‐pressure levels on acoustic wave distortion in horns (A)

Frederic G. Pla and Gerhard Reethof

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S5-S5 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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High‐power sound sources have received a lot of attention in the past few years due to renewed interest in industrial applications of high‐intensity sounds such as the acoustic agglomeration of aerosols or combustion enhancement. Most high‐power sound sources require a horn to match the source impedance to the medium where the sound is radiated. Such horns introduce distortion in the initial waveform, which can be detrimental to the agglomeration or combustion enhancement process. Boundary‐layer attenuation smooths the wave shape while dispersion breaks up the symmetry of the waveform. Horn‐induced dispersion is usually the dominant dispersion mechanism, resulting in strong peaks in the waveform. Finally, due to the very high acoustic levels at the horn throat, finite‐amplitude effects are responsible for a significant amount of distortion at high frequencies. Simple examples of waveform distortion due to these various mechanisms are shown. The effects of sound‐pressure level, horn design, and frequency on distortion are illustrated for an exponential horn and several initial wave shapes. Experimental results are presented that compare very well with theory.
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Oblique reflection of a finite‐amplitude dilatational wave in an elastic half‐space (A)

Kun‐Tien Shu and Jerry H. Ginsberg

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S5-S5 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A ray theory for two‐dimensional, finite‐amplitude acoustic waves forming a mode within a hard‐walled rectangular waveguide was described previously [K. T. Shu and J. H. Ginsberg, J. Acoust. Soc. Am. Suppl. 1 83, S1 (1988)]. The present paper extends those developments to the treatment of oblique reflection and mode conversion of a finite‐amplitude dilatational wave (P wave) at the stress‐free boundary of an elastic half‐space. Due to nonlinear self‐action, cumulative growth of second harmonics occur in the incident and reflected P waves in proportion to the square of the amplitude of the first‐order incident and reflected P waves, respectively, but such growth is not encountered in the reflected vertically polarized shear wave (SV wave), nor in the many waves arising from nonlinear interaction between dilatational and shear waves. Uniformly valid expressions for strain are obtained by using renormalization techniques along the rays. The analysis indicates that the mode conversion between the nonlinear P and SV waves can be described by linear reflection theory. As a consequence of the reflection process, the nonlinear effect in the reflected P wave corresponds to a simple planar wave that originated from a weaker source at a longer range, even though the phase of that wave is governed by the actual propagation distance from the source. [Work supported by NSF and ONR.]
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Absolute measurement of particle velocity of time‐harmonic acoustic waves (A)

Joseph Vignola, Jacek Jarzynski, and Yves H. Berthelot

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S6-S6 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Experiments are being conducted to measure by optical means the amplitude and the phase of a steady‐state, time‐harmonic compressional wave being produced inside a standing wave tube filled either with air or with water. The technique used is an extension of laser Doppler anemometry in acoustics developed by Taylor [J. Acoust. Soc. Am. 59, 691–694 (1976)]. It consists of illuminating with laser beams a small probe volume in water in which slowly drifting suspended microparticles are moving in phase with the acoustic field. The light scattered from the particles is Doppler shifted and contains information about the amplitude and the frequency of the particle motion. Preliminary results indicate that signal analysis performed on a spectrum analyzer is probably not the optimum processing technique, mainly because signals are being analyzed and displayed even at times when there is no particle in the probe volume to scatter the laser light. Instead, more meaningful and repeatable results can be obtained by using a digital data acquisition system triggered to capture the signal only when light is being scattered from a particle. The data are then transferred to a computer for further processing. [Work supported by ONR.]
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Application of variational principles to the evaluation of axisymmetric surface pressure and displacement along a harmonically vibrating elastic shell (A)

Pei‐Tai Chen and Jerry H. Ginsberg

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S6-S6 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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An earlier paper [J. H. Ginsberg and P. T. Chen, J. Acoust. Soc. Am. Suppl. 1 82, S1 (1987)] employed assumed modes to describe the displacement and pressure along an elastic plate. The amplitudes of those modes were obtained by simultaneously satisfying Hamilton's principle for the structure and a variational principle for the pressure distribution along a vibrating body. Here, the method is extended to an elastic shell structure in the form of an arbitrary body of revolution. The energy expressions for the shell are based on Love's assumptions, while the stationary quantity for the surface pressure is derived from the Kirchhoff‐Helmholtz principle. Two important facets of the derivation are the treatment of singularities in the latter and the reciprocal nature of the coupling between the pressure and surface displacement. A numerical example compares the results obtained from the variational formulation with Hayek's analytical solution [J. Acoust. Soc. Am. 40, 342–348 (1966)] for the radiation of a spherical shell in an acoustic medium under a harmonic, concentrated force. [Work supported by the Office of Naval Research, Code 1132‐F.]
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On left‐ and right‐circularly polarized waves in isotropic noncentrosymmetric elastic media (A)

Akhlesh Lakhtakia, Vasundara V. Varadan, and Vijay K. Varadan

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S6-S6 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Acoustic waves in solids can discriminate between a chiral scatterer and its mirror image. Thus it is possible to construct an acoustically chiral composite medium by embedding chiral microstructures in a host medium. The microstructure size should be large enough compared to the shear wavelength in the matrix medium so that an incident wave can sense its handedness; at the same time, the microstructure size should be small enough that, at least in some frequency range, the composite structure should appear to be effectively chiral. Isotropic composite media with chiral microstructure can be modeled as noncentrosymmetric (hemitropic) micropolar elastic solids, which have been the subject of some recent investigations. The simplest possible constitutive equations have been obtained, and the dispersion equations have been derived and studied. Approximate solutions of the inhomogeneous field equations have also been derived using dyadic algebra. [Work supported by the Research Center for the Engineering of Electronic and Acoustic Materials.]
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Detection of quantization distortion in digital audio systems (A)

Kanako Maeda, Tsuyoshi Usagawa, and Masanao Ebata

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S6-S6 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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In recent digital audio systems, 16‐bit uniform quantization is employed in most cases, because a dynamic range of 96 dB is considered satisfactory for most cases. Ideally speaking, a dynamic range of more than 120 dB is required in music reproduction. However, harmonic distortion due to quantization can be detected when fewer bits are used for the representation of the digital signal that is reproduced at a higher sound level. In this paper, the thresholds of distortion are measured using a stationary tone as a function of sound level, the bit width of the signal, and the tone spectrum. Whether harmonic distortion is detectable when the maximum level of 16‐bit digital signal is set at 120 dB SPL is examined. The effect of a dither is also examined. The results show that the dither is not always effective in reducing distortion. Furthermore, using a complex tone with exponential decay, thresholds for various conditions are measured and the results from the viewpoint of masking effects are discussed.
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Time‐domain scattering from tungsten‐carbide targets (A)

H. Üerall, J. W. Dickey, M. F. Werby, and Michael D. Collins

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S6-S6 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Scattering calculations are almost exclusively performed in the frequency domain. Although form functions are computed regularly, they are rarely utilized to obtain time‐domain results. This is surprising because the payoff for the small amount of extra effort required for Fourier synthesis includes results that are easy to interpret in terms of causality. This approach will be used to study resonances of tungsten‐carbide spheroids for aspect ratios ranging from 1–6 and for kL/2 up to 26. Tone bursts, cw pings, and Gaussian sources will be utilized to isolate resonances and to determine their nature. An analysis of arrival time as a function of aspect ratio gives credibility to the interpretation that resonances induced along the axis of symmetry are due to Rayleigh waves as first proposed by Flax et al. [J. Acoust. Soc. Am. 71, 1077–1082 (1982)]. [Work supported by the Naval Ocean Research and Development Activity.]
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Numerical calculations of echo patterns in ultrasonic detection (A)

Li Donglin, Cai Chongcheng, and Jiang Nanxiang

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S6-S6 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A calculation method has been developed to calculate the echo patterns of an ultrasonic pulse in the time domain. Another method for forming the acoustic radiation impedance function of the ultrasonic transducer used will also be presented. Typical numerical results are compared with experimental data, and they are in good agreement with each other.
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