Volume 84, Issue S1, November 1988
Index of content:
- PROGRAM OF THE SECOND JOINT MEETING OF THE ACOUSTICAL SOCIETIES OF AMERICA AND JAPAN
- Session B. Architectural Acoustics I: Vern O. Knudsen Lecture
- Invited Paper
84(1988); http://dx.doi.org/10.1121/1.2026257View Description Hide Description
After introducing dimensional factors of the acoustic system in a concert hall, the capability of calculating acoustical quality at any seat based upon the theory of preference is presented. The effort to describe important qualities of sound in terms of the processes of the auditory pathways and brain has been brought to bear on the problem. If enough were known about how the auditory and the central nervous systems modify the nerve impulses from the cochlea, the design of concert halls could proceed according to guidelines derived from knowledge of these processes. An attempt to analyze this is made through study of the auditory‐evoked potentials over the left and right human cerebral hemispheres. In addition, results of recording the slow vertex responses (SVR), which were obtained by adjusting the temporal and spatial physical factors, indicated that the information related to subjective preference appeared in the latency components. The longest‐latency responses were observed at the most‐preferred condition or at the most subjectively diffuse condition. The correlation between brain activity and subjective preference seems to indicate that subjective preference can be traced back to a primitive response in the “inner universe.” Thus, such a theory with temporal and spatial factors may be generalized in designing physical environments for every human activity.
- Session C. Bioresponse to Vibration I: Hand‐arm Vibration
- Invited Papers
84(1988); http://dx.doi.org/10.1121/1.2026299View Description Hide Description
Signs and symptoms of the hand‐arm vibration syndrome (HAVS) will be described followed by recent research developments. Much attention has been focused on elucidating the pathophysiological basis of vibration‐induced injury to vascular, neural, muscular, and skeletal tissues in the hand and arm. New information suggests that chronic neural damage is less reversible after long‐term cessation of vibration exposure than vascular damage. If confirmed, current hand‐arm vibration standards (based mainly on prevention of onset of finger blanching symptoms) should be re‐evaluated to also preclude onset of chronic neurologic deficits. Progress has been made in designing and validating new objective measures of peripheral somatosensory dysfunction, as well as improving and testing existing methods for assessing neural and vascular pathology. The Stockholm Workshop symptom scale (revised Taylor‐Pelmear scale) for assessing the vascular component of HAVS will be described as well as a suggested new scale for evaluating neural damage.
84(1988); http://dx.doi.org/10.1121/1.2026300View Description Hide Description
Peripheral circulatory disorders such as Raynaud's phenomenon are thought to be basal symptoms of vibration hazards. The elucidation of their pathogenesis and their diagnosis between attacks is regarded as important. A system for monitoring the body surface circulation noninvasively and continuously has been developed, which enables the quantitative dynamic assessment of skin blood flow. This apparatus is based on the thermal diffusion method and uses a probe incorporating a Peltier stack. After fundamental studies on this system, the peripheral circulatory functions of workers using vibrating tools were studied. The workers were divided into three groups: group A, without any symptoms; group B, with numbness and/or pain but without Raynaud's phenomenon; and group C, with Raynaud's phenomenon. The finger blood flow (FBF) of group C at rest was significantly lower than that of group A (p < 0.01). In a handgrip test and with exposure to local vibration (60 Hz, 50 m/s2), the FBF of group A significantly changed compared to its value at rest, but that of group C showed no significant change.
Vibrotactile perception thresholds in hands occupationally exposed to vibration: Interpretation of sensorineural loss84(1988); http://dx.doi.org/10.1121/1.2026301View Description Hide Description
The tactile performance of the hand is now known to be critically dependent on neural activity in one population of slow‐adapting (SAI) and two populations of fast‐adapting (FAI, FAII) mechanoreceptors. A technique has recently been developed to establish the sensitivity of these receptor populations at the fingertip from vibrational perception thresholds determined psychophysically with sinusoidal stimulation [A. J. Brammer et al., J. Hand Surg. 12A, 870–875 (1987)]. A comparison of data from 98 vibration‐exposed and 34 normal hands, all screened to exclude confounding factors, has revealed two patterns of abnormal threshold elevation. The first, involving similarly elevated thresholds at all frequencies (2–200 Hz), is indicative of sensorineural losses of similar magnitude in each type of receptor‐nerve system, and so is suggestive of peripheral nerve degeneration. The second pattern involves elevated thresholds at frequencies mediated only by one or sometimes two receptor types (commonly SAI and/or FAII). A mechanoreceptor‐specific mechanism is unlikely to occur within a nerve trunk, and so this frequency‐dependent pattern is suggestive of selective damage to the nerve endings.
A study on the setting position of vibration pickups for the measurement of hand‐arm transmitted vibration84(1988); http://dx.doi.org/10.1121/1.2026341View Description Hide Description
In order to reduce hand‐arm vibration disease, it is important to know the amount of vibration transmitted from a hand‐held vibration tool to the hand‐arm system. There are many problems in determining the amount of exposure in practice. For example, (1) the handle vibration does not always correspond to the vibration disease. (2) If the vibration pickup is set between the handle and palm, this system will not be usable for a long time, because the vibration pickup will become an obstacle for the worker. In this study, L eq8 (8 h of equivalent hand‐arm vibration level according to ISO 5349) is used to evaluate the transmitted vibration, and the back side of the hand is recommended for the placement of the vibration pickup to measure the amount of transmitted vibration without disturbing the worker during the long‐time use of the measuring device. The reasons for this proposal are indicated by the results of field measurements of actual hand‐held vibration tools (chain saw, breaker, etc.) and by the analytical results of the model experiment. [Work supported by Ministry of Labor of Japan.]
- Contributed Papers
84(1988); http://dx.doi.org/10.1121/1.2026342View Description Hide Description
ISO 7505 (Forest Machinery—Chain Saws—Measurement of hand‐transmitted vibration. International Oragnization for Standardization, Geneva, 1986) specifies for the measurement of frequency‐weighted handle acceleration a maximum permissible accelerometer weight of 50 g, which is comparable to that of most chain saw handles. To explore the magnitude of errors so introduced, a series of difference measurements has been conducted with a variable mass accelerometer mount. With this device the effective accelerometer weight can be cycled between 6 and 25, 50, or 100 g, in a time in which other parameters affecting handle vibration during saw operation with no load (viz: engine speed and hand grip) usually vary little. Reductions in frequency‐weighted acceleration from the minimum resolvable (± 5%) to 50% were recorded when changing accelerometer weight from 6 to 50 g, depending on handle design and material. Investigation of the vibration spectra (6–1250 Hz) and transmissibility (30–700 Hz) of one handle suggests that the data are consistent with a localized mass loading, introduced by the accelerometer, reducing the amplitude of one or more flexural modes of handle vibration.
84(1988); http://dx.doi.org/10.1121/1.2026343View Description Hide Description
Materials performance affecting the level of surface vibration ordinarily in direct contact with the hand under specified conditions will be reported. The intent of this study was to develop a method by which elasticmaterials can be evaluated for isolation properties relating to test surface vibration using nonrigid loading. The hand and arm, as a complex system, are used in place of the more traditional and simpler rigid mass load. This research and the scope of these findings report performance as a more realistic means for determining isolation, i.e., vibration reduction caused by an intervening material. The findings reported use this method to identify materials that exhibit isolation and display this positive effect, if it occurs, by graphic means over the frequency range of 1 to 800 Hz. The Z‐axis (basicentric) hand‐arm apparent mass, evaluated while grasping a 3.81‐cm‐diam handle with 4.5‐Newton total grip force accelerated at 4 m/s2, will be used as the reference load to determine material isolation effectiveness, reported as transmissibility.
- Session D. Engineering Acoustics I: Theory, Practice, and Materials in Engineering Acoustics
84(1988); http://dx.doi.org/10.1121/1.2026344View Description Hide Description
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.
84(1988); http://dx.doi.org/10.1121/1.2026389View Description Hide Description
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.
84(1988); http://dx.doi.org/10.1121/1.2026390View Description Hide Description
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.]
84(1988); http://dx.doi.org/10.1121/1.2026391View Description Hide Description
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.]
84(1988); http://dx.doi.org/10.1121/1.2026392View Description Hide Description
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.]
Effects of attenuation, dispersion, and high sound‐pressure levels on acoustic wave distortion in horns84(1988); http://dx.doi.org/10.1121/1.2026393View Description Hide Description
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.
84(1988); http://dx.doi.org/10.1121/1.2026394View Description Hide Description
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 (Pwave) 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 Pwaves in proportion to the square of the amplitude of the first‐order incident and reflected Pwaves, respectively, but such growth is not encountered in the reflected vertically polarized shear wave (SVwave), 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 SVwaves can be described by linear reflection theory. As a consequence of the reflection process, the nonlinear effect in the reflected Pwave 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.]
84(1988); http://dx.doi.org/10.1121/1.2026437View Description Hide Description
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.]
Application of variational principles to the evaluation of axisymmetric surface pressure and displacement along a harmonically vibrating elastic shell84(1988); http://dx.doi.org/10.1121/1.2026438View Description Hide Description
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 surfacepressure 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.]
84(1988); http://dx.doi.org/10.1121/1.2026439View Description Hide Description
Acoustic waves in solids can discriminate between a chiralscatterer 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.]
84(1988); http://dx.doi.org/10.1121/1.2026440View Description Hide Description
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.
84(1988); http://dx.doi.org/10.1121/1.2026441View Description Hide Description
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.]
84(1988); http://dx.doi.org/10.1121/1.2026442View Description Hide Description
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.