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Mar 1989

Volume 85, Issue 3, pp. 995-1399

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Wave propagation in isotropic linear viscoelastic media

Hans C. Strifors and Guillermo C. Gaunaurd

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 995-1004 (1989); (10 pages) | Cited 1 time

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Assuming isothermal conditions, a suitable form of the free energy is formulated for general, isotropic linear viscoelastic media. On applying a proper form of the principle of dissipation, a strong condition of dissipation is derived, which imposes restrictions on constitutive parameters of particular materials. The field equations of boundary value problems in the time domain and the frequency domain are formulated and, in particular, the problem of scattering of an acoustic pulse with plane or spherical wave front by a viscoelastic scatterer is discussed. The relation to the corresponding elastic scattering problem for steady, harmonic waves is established, and a general scheme of solution for viscoelastic scatterers is laid down.
Show PACS
43.20.Bi Mathematical theory of wave propagation
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.35.Mr Acoustics of viscoelastic materials

Spectral statistics in elastodynamics

R. L. Weaver

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1005-1013 (1989); (9 pages) | Cited 32 times

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The fundamentals of acoustic modal statistics are investigated with a view toward applications in understanding fluctuation phenomena in room acoustics, statistical energy analysis, and diffuse field ultrasonics. It is conjectured that the spectral fluctuations of a linearly elastic body, like those of large nuclei, will fall into the universality class described by the Gaussian orthogonal ensemble of random matrices. This conjecture is strongly confirmed by a laboratory study of the higher eigenfrequencies of small aluminum blocks. Level repulsions and spectral rigidities are observed to conform to the predictions of random matrix theory.
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43.20.Bi Mathematical theory of wave propagation
43.20.Fn Scattering of acoustic waves

Observations and modeling of the backscattering of short tone bursts from a spherical shell: Lamb wave echoes, glory, and axial reverberations

Steven G. Kargl and Philip L. Marston

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1014-1028 (1989); (15 pages) | Cited 24 times

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See Also: Erratum

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Tone bursts having durations of 3 or 4 cycles were incident on an air‐filled stainless steel shell in water. The resulting sequence of echoes included a specular reflection and echoes radiated by Lamb waves on the shell. Echo structure was studied for ka of 24 to 75, where a denotes the outer radius; b/a=0.838, where b denotes the inner radius. The amplitudes of Lamb wave echoes were modeled using an elastic generalization of the geometrical theory of diffraction (GTD) [P. L. Marston, J. Acoust. Soc. Am. 83, 25–37 (1988)]. The required Lamb wave parameters (the phase velocity cl and damping βl ) were found by the Sommerfeld–Watson method; an efficient numerical method for the computation of the required complex root νl is described. The echoes were identified by comparing arrival times with predictions; bursts reflected from a solid tungsten carbide sphere were used for a reference amplitude. Measurements with ka=24 of the largest Lamb wave echo (which was due to a flexural wave) were made at various backscattering angles γ. The echo was largest for small γ in agreement with predictions. This is a manifestation of the glory pertinent to other bistatic measurements. The ringing of longitudinal wave reverberations (which could be seen following the specular reflection) may be useful for determining the thickness of a shell.
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43.20.Fn Scattering of acoustic waves
43.20.Px Transient radiation and scattering
43.30.Jx Radiation from objects vibrating under water, acoustic and mechanical impedance
43.40.Ey Vibrations of shells

The ONION method: A reflection coefficient measurement technique for thick underwater acoustic panels

Jean C. Piquette

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1029-1040 (1989); (12 pages)

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An earlier report [J. C. Piquette, ‘‘An extrapolation procedure for transient reflection measurements made on thick acoustical panels composed of lossy, dispersive materials,’’ J. Acoust. Soc. Am. 81, 1246–1258 (1987)] described a new technique for evaluating underwater panel measurements. The method is based on least‐squares fitting to a multiple‐layer model. The present article describes further research on the technique, including a description of a successful application to measurements made under nonatmospheric hydrostatic pressure. Also described are enhancements that increase the speed of the algorithm as implemented on a digital computer, and that render the technique practical for use in reducing the massive amounts of data that can be obtained in panel calibration measurements. The method is currently being transitioned to an on‐line panel‐measurement capability in the Anechoic Tank Facility (ATF) of the Underwater Sound Reference Detachment of the Naval Research Laboratory (NRL‐USRD), located in Orlando, Florida.
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43.20.Fn Scattering of acoustic waves
43.20.Px Transient radiation and scattering
43.30.Sf Acoustical detection of marine life; passive and active
43.60.Gk Space-time signal processing, other than matched field processing

Diffraction of a plane acoustic wave by four equal circular infinite soft strips

K. A. Sharma and D. L. Jain

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1041-1047 (1989); (7 pages)

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This article deals with the solution of the problem of diffraction of a time‐harmonic obliquely incident plane acoustic wave by four equal infinite coaxial soft circular strips. A simple integral equation technique is presented to solve this two‐dimensional problem by the perturbation method when the wavelength of the incident plane wave is much larger than the radius of the circular strips. Approximate expressions for the farfield amplitude and the scattering cross section are derived.
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43.20.Fn Scattering of acoustic waves

Acoustical properties of drill strings

Douglas S. Drumheller

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1048-1064 (1989); (17 pages) | Cited 11 times

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The recovery of petrochemical and geothermal resources requires extensive drilling of wells to increasingly greater depths. Real‐time collection and telemetry of data about the drilling process while it occurs thousands of feet below the surface is an effective way of improving the efficiency of drilling operations. Unfortunately, due to hostile down‐hole environments, telemetry of this data is an extremely difficult problem. Currently, commercial systems transmit data to the surface by producing pressure pulses within the portion of the drilling mud enclosed in the hollow steel drill string. Transmission rates are between 2 and 4 data bits per second. Any system capable of raising data rates without increasing the complexity of the drilling process will have significant economic impact. One alternative system is based upon acoustical carrier waves generated within the drill string itself. If developed, this method would accommodate data rates up to 100 bits per second. Unfortunately, the drill string is a periodic structure of pipe and threaded tool joints, the transmission characteristics are very complex and exhibit a banded and dispersive structure. Over the past 40 years, attempts to field systems based upon this transmission method have resulted in little success. This article examines this acoustical transmission problem in great detail. The basic principles of acoustic wave propagation in the periodic structure of the drill string are examined through theory, laboratory experiment, and field test. The results indicate the existence of frequency bands that are virtually free of attenuation and suitable for data transmission at high bit rates.
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43.20.Hq Velocity and attenuation of acoustic waves
43.40.Cw Vibrations of strings, rods, and beams
43.40.Ph Seismology and geophysical prospecting; seismographs
43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance

An experimental study of oscillating flow through two orifices in series

Donald F. Elger and Ronald L. Adams

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1065-1073 (1989); (9 pages)

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A Stemme drop‐on‐demand ink‐jet printer nozzle consists of two closely spaced orifices in series. To provide insight into the nozzle fluid dynamics, a study of impedance and flow distribution was conducted. Air was used as a working fluid, and orifices were fabricated in equal diameter plates attached to a piston‐driven cylinder. The outer orifice and the space between the plates were open to ambient air. Hot‐wire anemometry and a standing wave resonance method were used to measure air velocity and orifice impedance, respectively. Impedance data exhibited the nonlinear behavior characteristic of a single orifice. Velocity data showed that orifice plate spacing has the largest effect on fluid velocity through the outer orifice.
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43.20.Mv Waveguides, wave propagation in tubes and ducts
47.60.Kz Flows and jets through nozzles

On the topology of the complex wave spectrum in a fluid‐coupled elastic layer

S. I. Rokhlin, D. E. Chimenti, and A. H. Nayfeh

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1074-1080 (1989); (7 pages) | Cited 10 times

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The behavior of the complex leaky Lamb wave (LLW) spectrum of a plate immersed in a fluid has been studied as a function of the ratio of fluid‐to‐solid densities. It has been shown that widely accepted assumptions concerning the similarities between the classical Lamb wave spectrum and the LLW spectrum are not valid as the fluid density approaches that of the solid. Recent experimental and theoretical results demonstrate that there is a strong anomaly in the reflectance of composite plates in water. This anomaly is related to the details of the complex LLW spectrum. The spectrum is composed of both the familiar propagating Lamb wave branches and the mostly imaginary, nonpropagating branches. From the character of the complex spectrum, it is demonstrated that topological changes in the fundamental symmetric mode are related to its nearness to the first complex branch of the spectrum. Increasing the fluid–solid density ratio leads to interaction between these modes and to mutual interchange between portions of their branches. As the density ratio increases from zero to infinity, the complex spectrum is gradually transformed from a classical Lamb wave spectrum to one appropriate for a plate with zero normal surface displacements.
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43.20.Tb Interaction of vibrating structures with surrounding medium
43.20.Bi Mathematical theory of wave propagation

Near‐boundary streaming around a small sphere due to two orthogonal standing waves

Chun P. Lee and Taylor G. Wang

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1081-1088 (1989); (8 pages) | Cited 5 times

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This article studies the acoustic streaming pattern near a small sphere due to two orthogonal standing waves, which have the same frequency but, in general, are out of phase. The results indicate a new kind of acoustic streaming arising from the circular motion in the medium caused by the two waves.
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43.25.Nm Acoustic streaming
43.25.Uv Acoustic levitation

Atmospheric acoustic noise as a function of altitude

T. A. Griffy and E. L. Hixson

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1089-1091 (1989); (3 pages)

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Previous measurements of ambient acoustic noise in the atmosphere have been limited to a few meters from the ground. Here, a hot‐air balloon was used as a platform to measure noise as a function of altitude up to 3400 m with ‘‘A’’ and ‘‘flat’’ weighting. The frequency spectrum was also measured at 3400 m. Measurements were made in the early morning over a rural area. The spectrum level was greatest at low frequency and decreased with frequency. Levels decreased with height due to absorption and a limited acoustical horizon caused by refraction.
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43.28.Bj Mechanisms affecting sound propagation in air, sound speed in the air
43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors
43.50.Cb Noise spectra, determination of sound power

The shadow zone in a stratified medium

Rufin Makarewicz

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1092-1096 (1989); (5 pages) | Cited 2 times

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Assuming that the variations of wind speed and sound speed are significantly less than the speed of sound close to the ground, the first and the second approximations of ray equations have been derived. The first approach yields the parabolic shape of a ray when the linear dependence of wind speed and sound speed upon height is assumed. In the instance of nonlinear dependence, the generalization of Ingard’s formulas, which determines the slant distance between the source and shadow boundary, is given. Both cases of linear and nonlinear shadow boundaries are defined by grazing rays when specific conditions are met.
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43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors
43.20.Dk Ray acoustics

A simple estimate of propagation loss fluctuations due to modal interference

David M. F. Chapman

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1097-1106 (1989); (10 pages) | Cited 1 time

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A simple estimate of propagation loss fluctuations due to deterministic modal interference has been devised for use with normal mode computer codes. It is based upon the relative mode amplitudes, and is easy to compute along with the incoherent mode sum that is used to estimate average propagation loss. The estimated levels of propagation loss fluctuation compare favorably with those computed from the coherent mode sum for shallow water environments.
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43.30.Bp Normal mode propagation of sound in water
43.20.Mv Waveguides, wave propagation in tubes and ducts

A nearfield asymptotic analysis for underwater acoustics

Michael D. Collins

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1107-1114 (1989); (8 pages) | Cited 1 time

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The method of stationary phase is used to show that the homogeneous half‐space field projects properly onto the normal modes near a point source. The half‐space field is useful for initializing the parabolic equation (PE) in both the time domain and the frequency domain and in both two and three spatial dimensions. It is shown to be more accurate for wide‐angle propagation than the Gaussian PE starter. A Gaussian time‐domain PE starter is derived and compared with the half‐space starter. Application of the approach to scattering problems is discussed. The half‐space field is the inner solution of a matched asymptotics solution. A simple approach for speeding up PE calculations and an efficient ray‐tracing model for source localization are motivated by the matching.
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43.30.Bp Normal mode propagation of sound in water
43.30.Dr Hybrid and asymptotic propagation theories, related experiments

Saddle point analysis of the reflected acoustic field

N. G. Plumpton and C. T. Tindle

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1115-1123 (1989); (9 pages)

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The field due to reflection of spherical sound waves at the plane interface between two homogeneous fluids is examined in terms of saddle points of an integrand. The resulting new expression for the field is valid at all ranges including the vicinity of the critical ray and the caustic due to beam displacement. A simple interpretation of the field in terms of wave fronts and eigenrays, which is valid at all ranges, is obtained. At some ranges, the wavenumber for eigenrays is complex. The new result provides an alternative to the usual specular ray and lateral wave interpretation.
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43.30.Cq Ray propagation of sound in water

Resonance scattering in suspensions

Alex E. Hay and Arjen S. Schaafsma

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1124-1138 (1989); (15 pages) | Cited 7 times

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Measured and theoretical total cross sections for aqueous suspensions of lead glass and polystyrene beads are presented. Attenuation is measured from 1 to 100 MHz in a recirculating suspension circuit using a swept‐frequency system and broadband transducer pairs. The theoretical calculations employ the phase shift formalism and the coherent removal of a rigid scatterer background. Good agreement between theory and experiment is obtained for both types of particle, and departures from the rigid sphere cross section are due to resonance scattering. The resonance features in the glass bead cross sections are dominated by broad Rayleigh wave resonances, with frequencies close to those given by the in vacuo free‐body resonance condition, and represent departures from the rigid sphere cross section of, at most, 25%. In contrast, the polystyrene bead cross sections are dominated by narrow resonance features with maximum amplitudes up to five times greater than the geometric scattering cross section. It is shown that these resonances, which we call giant resonances, occur at frequencies given by an immersed free‐body resonance condition that is derived by assuming the existence of wave trapping in the fluid at the surface of the sphere. This condition yields frequencies that are wholly real, consistent with the observed narrow resonance widths, and leads to an approximate expression for the resonance amplitudes that is in good agreement with the exact calculations. The meridional wave phase speeds are consistent in the appropriate limit with Stonely waves. The giant resonances cease to exist if the meridional wave phase speed approaches the speed of sound in the ambient fluid.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.20.Fn Scattering of acoustic waves

Low‐frequency transmission loss in the Arctic SOFAR channel for shallow sources and receivers

T. C. Yang

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1139-1147 (1989); (9 pages) | Cited 1 time

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It is demonstrated that low‐frequency (<50 Hz) transmission loss (TL) in the Arctic SOFAR channel for shallow sources and receivers (depth<four wavelengths) is insensitive to the angular dependence of the ice‐scattering loss. Existing data of shallow sources and receivers, therefore, cannot distinguish the various published ice‐scattering models based solely on the predicted angular dependence of the ice‐scattering loss. Specifically, it is shown, using the 17.75‐ and 47‐Hz TL data of DiNapoli and Mellen [Low frequency attenuation in the Arctic Ocean,’’ in Ocean Seismo‐Acoustics, edited by T. Akal and J. Berkson (Plenum, New York, 1986).] as constraints, that the TL for shallow sources and receivers is determined predominantly by the first mode attenuation and is insensitive to the higher mode attenuations associated with ice‐scattering loss at higher angles. Measurements of the first mode attenuation coefficients at 17.75 and 47 Hz are presented in this article. It is found that transmission loss based on the measured first mode attenuation coefficients agrees well with the DiNapoli data independently determined in the same ocean basin. The magnitude of the first mode attenuation coefficient has significant experimental implications. It is found that a 1‐km‐long vertical array offers little chance of distinguishing the various angular dependence models at 17.75 Hz (based on range‐averaged transmission loss data or incoherently summed modal amplitudes), whereas a 300‐m‐long vertical array may do so at 47 Hz based on long‐range (>250 km) range‐averaged propagation data. It is also found that transmission loss varies significantly with source depth; consequently, transmission loss curves determined from data for multiple source depths have an inherent (not experimentally related) uncertainty of several dB and thus cannot shed light on the angular dependence of ice‐scattering loss.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Hw Rough interface scattering

Effect of monomolecular films on the underlying ocean ambient‐noise field

Jim Rohr, Ray Glass, and Brett Castile

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1148-1157 (1989); (10 pages)

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A series of at‐sea tests has unequivocally established that the presence of a monomolecular film on the sea surface results in a pronounced reduction of ambient noise beneath the film. This reduction was observed to begin around 2 kHz, continue to at least 20 kHz, and was evident to some degree throughout sea states 1 1/2 to 6. A predictive model is developed that is generally consistent with the film’s observed behavior, and several possible mechanisms are speculated through which the film could affect the underlying ambient‐noise field.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.30.Lz Underwater applications of nonlinear acoustics; explosions

A nonlinear matched‐field processor for detection and localization of a quiet source in a noisy shallow‐water environment

G. B. Smith, C. Feuillade, D. R. Del Balzo, and C. L. Byrne

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1158-1166 (1989); (9 pages) | Cited 2 times

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Matched‐field processing is a technique that has been developed for the detection and localization of an acoustic source in an underwater environment. Conventional matched‐field processing utilizes cross correlations between the complex acoustic pressures measured from the elements of a submerged array of hydrophones and theoretically predicted complex acoustic pressures obtained from an appropriate model of the environment with an assumed source location. Modal matched‐field processing has been recently developed for the detection and localization of an acoustic source in a shallow‐water waveguide environment, where modal propagation of acoustic energy dominates. Modal matched‐field processing utilizes the cross correlations between predicted complex modal amplitudes and complex modal amplitudes calculated from the measured complex pressures. The ambient noise field in a shallow‐water environment has a correlated component that propagates to the hydrophone array through the discrete, trapped modes of the waveguide. Cross correlations due to this modal noise compete with signal cross correlations in the cross‐spectral matrix used in conventional matched‐field processing. In this article, it is shown that modal noise coming from a large number of discrete, randomly distributed, farfield sources contributes insignificantly to the off‐diagonal elements of the cross‐spectral matrix used in modal matched‐field processing. This is to be contrasted with signal cross correlations, which contribute significantly to these off‐diagonal elements. This provides a separation of signal from noise that is not available with conventional matched‐field processing. In this article, nonlinear (off‐diagonal), modal matched‐field processing algorithms are demonstrated that exploit this separation of signal and modal noise. The superiority of these off‐diagonal algorithms over conventional matched‐field processing in a low signal‐to‐noise situation is demonstrated using computer simulations of a shallow‐water Pekeris waveguide.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.60.Gk Space-time signal processing, other than matched field processing
43.20.Mv Waveguides, wave propagation in tubes and ducts

Ultrasonic interferometric characterization of highly attenuative materials

N. K. Batra and P. P. Delsanto

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1167-1172 (1989); (6 pages)

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Experimental techniques for the ultrasonic characterization of materials usually aim at the determination of either their ultrasonic velocity or their attenuation. In this article, a technique is presented that allows a simultaneous measurement of both. It provides, also, a method for a quantitative nondestructive evaluation of material irregularities. This technique is based on the propagation of ultrasonsic continuous waves through a slab of material, designed in the shape of a wedge, and on a correlation analysis between reference (incident) and received signals. Experiments performed on several specimens of various polymers confirm the efficiency and reliability of the method.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Zc Use of ultrasonics in nondestructive testing, industrial processes, and industrial products
81.70.-q Methods of materials testing and analysis

Thermoacoustic generation of narrow‐band signals with high repetition rate pulsed lasers

Yves H. Berthelot

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1173-1181 (1989); (9 pages) | Cited 2 times

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The generation of sound by absorption of laser light in water is analyzed for the case of lasers pulsating at high repetition rates. It is shown that pulsating the laser at any arbitrary repetition rate is, in general, not a very effective way to produce a narrow‐band signal. In order to produce a signal with a narrow frequency band, one should instead pulsate the laser at an optimum repetition rate that is determined by the optical frequency of the laser and, to a lesser extent, by the laser beam diameter. Expressions for the optimum repetition rate are derived from both a frequency domain analysis and a time domain analysis. It is found that, with the present laser technology, significant gain in signal‐to‐noise ratio can be achieved by pulsating the laser at its optimum repetition rate, as compared with the more conventional methods for generation of narrow‐band signals, such as the method of laser intensity modulation. As a result, it now seems possible to generate continuous thermoacoustic highly collimated sound beams with high repetition pulsed lasers in such a way that these signals are easily detectable several kilometers away from the source. Spatially periodic laser deposition configurations on the water surface are also discussed, and it is shown that further improvement in signal‐to‐noise ratio is achievable, in principle, for a spatial periodicity tuned to the optimum temporal periodicity of the repetition rate of the pulsed laser.
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43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect

The properties of the estimation error of sound power measurement using sound intensity techniques

Mei Q. Wu and Malcolm J. Crocker

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1182-1190 (1989); (9 pages)

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In this article, a general expression was derived for the sound intensity normal to a plane measurement surface caused by a plane distribution of monopole sources. A program was developed to calculate the ratio between the approximate sound power estimated from a finite number of measurements of sound intensity on a plane surface and the exact power passing through the same measurement area. The ratio, which is considered the estimation error of the sound power measurement, is affected by many factors such as the number of measurements, the distance between the source and the measurement surface, the locations of the sources with respect to the measurement points and the types of sources. The effects of these factors on the estimation error were investigated, and some error curves were plotted. By examining the error curves, it was found: (1) when the measurement surface has unit area, the estimation error decreases with an increase in the measurement distance and an increase in the number of sound intensity measurements; and (2) when the measurement surface is a surface enclosing the source, the estimation error depends on the total number of sound intensity measurements over the surface, and does not depend on the measurement distance or the number of measurements over each unit area. Some experimental tests were carried out to check the error curves for the case when the sound source is a monopole. The experimental results agreed well with the theoretical predictions.
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43.50.Cb Noise spectra, determination of sound power
43.50.Yw Instrumentation and techniques for noise measurement and analysis
43.58.Fm Sound level meters, level recorders, sound pressure, particle velocity, and sound intensity measurements, meters, and controllers

An acoustic head simulator for hearing protector evaluation. I: Design and construction

Hans Kunov and Christian Giguère

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1191-1196 (1989); (6 pages) | Cited 1 time

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As an alternative to subjective methods, an acoustic head simulator was constructed for hearing protector evaluation. The primary purpose of the device is for hearing protector testing and research under high‐level steady‐state and impulse noise environments. The design is based on the KEMAR manikin and therefore approximates the physical dimensions and the acoustical eardrum impedance of the median human adult. The head simulator includes a mechanical reproduction of the human circumaural and intraaural tissues with a silicone rubber material. A compliant head–neck system was constructed to approximate the vibrational characteristics of the human head in a sound field in order to simulate the inertia effect of earmuffs. The bone‐conducted sounds are not mechanically reproduced in the design. Applications for the device are reported in a companion article [C. Giguère and H. Kunov, J. Acoust. Soc. Am. 85, 1197–1205 (1989)].
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43.50.Hg Noise control at the ear
43.50.Pn Impulse noise and noise due to impact
43.66.Vt Hearing protection
43.66.Yw Instruments and methods related to hearing and its measurement

An acoustic head simulator for hearing protector evaluation. II: Measurements in steady‐state and impulse noise environments

Christian Giguère and Hans Kunov

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1197-1205 (1989); (9 pages) | Cited 1 time

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The attenuation characteristics of hearing protection devices (HPDs) were measured using a modular acoustic head simulator. The effect in changes in the head configuration was assessed in a steady‐state diffuse sound field. The use of artificial circumaural skin had a relatively small influence on the insertion loss of earmuffs (max. 6–7 dB at low frequencies). This contrasts to the very large effects found for the artificial intraaural skin on the insertion loss of earplugs (in excess of 40 dB at low frequencies for some devices). Results were also compared with real‐ear attenuation at threshold (REAT) data (ANSI S3.19‐1974). In general, there is good agreement between the two methods, especially for earmuffs. Design improvements are proposed for earplugs. The result of an exploratory study aimed at measuring the complex (amplitude and phase) insertion loss of HPDs using an impulse noise source are also reported.
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43.50.Hg Noise control at the ear
43.50.Pn Impulse noise and noise due to impact
43.66.Vt Hearing protection
43.66.Yw Instruments and methods related to hearing and its measurement

Calculation of free‐field deviation in an anechoic room

Ji‐qing Wang and Biao Cai

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1206-1212 (1989); (7 pages) | Cited 3 times

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After a review of various calculation methods currently available for free‐field deviation (in dB) in an anechoic room, a more precise and rational calculation of the sound field is presented in this article by solving wave equations including the interference of first reflections only. In the case of a pure tone, the phase factor plays an important role in the calculation; therefore, the calculated free‐field deviation (in dB) also depends on the source position, measuring direction and distance from the source, and the frequency used in the test in addition to other factors such as the dimensions of the room and sound absorption characteristics of all of the boundary surfaces. For a broadband noise, calculation becomes simpler as the phase factor can be ignored, and a general equation of such a calculation is given for any source position and different combinations of boundary absorption. The results of the calculation can be shown in a diagram with the aid of a microcomputer, as a map of free‐field range within a given maximum deviation. This is very useful as a guideline to design and to use an anechoic room.
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43.55.Br Room acoustics: theory and experiment; reverberation, normal modes, diffusion, transient and steady-state response
43.55.Pe Anechoic chamber design, wedges
43.20.Ks Standing waves, resonance, normal modes

Early lateral reflections in some modern concert halls

Lothar Cremer

J. Acoust. Soc. Am. Volume 85, Issue 3, pp. 1213-1225 (1989); (13 pages)

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This article describes some of the means by which early lateral reflections (ELR) of sound have been provided in recent concert halls in which the author has been involved as acoustical consultant. Special problems are encountered nowadays. In earlier times, the disposition of audience seating in acoustical design was only a question of access: aisles, staircases, etc. Now, special account of the seating arrangement must be taken in order to provide suitable lateral sound reflections; and, particularly in large halls, this requires some departure from traditional seating plans.
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43.55.Fw Auditorium and enclosure design
43.55.Gx Studies of existing auditoria and enclosures
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