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Feb 2000

Volume 107, Issue 2, pp. 673-L12

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Call for business meeting of the Society

Charles Schmid

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 673-673 (2000); (1 page)

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43.05.Hw Meetings
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Canadian Audiology Centre to be created

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 677-677 (2000); (1 page)

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43.10.Ce Conferences, lectures, and announcements (not of the Acoustical Society of America)
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Daniel William Martin • 1918–1999

William Cavanaugh and William Hartmann

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 679-679 (2000); (1 page)

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43.05.Sf Obituaries
01.60.+q Biographies, tributes, personal notes, and obituaries
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Transient acoustic wave propagation in rigid porous media: A time-domain approach

Z. E. A. Fellah and C. Depollier

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 683-688 (2000); (6 pages) | Cited 25 times

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Wave propagation of acoustic waves in porous media is considered. The medium is assumed to have a rigid frame, so that the propagation takes place in the air which fills the material. The Euler equation and the constitutive relation are generalized to take into account the dispersive nature of these media. It is shown that the connection between the fractional calculus and the behavior of materials with memory allows time-domain wave equations, the coefficients of which are no longer frequency dependent, to be worked out. These equations are suited for direct and inverse scattering problems, and lead to the complete determination of the porous medium parameters. © 2000 Acoustical Society of America.
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43.20.Bi Mathematical theory of wave propagation
43.20.Hq Velocity and attenuation of acoustic waves

Backscattering enhancements for tilted solid plastic cylinders in water due to the caustic merging transition: Observations and theory

Florian J. Blonigen and Philip L. Marston

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 689-698 (2000); (10 pages) | Cited 3 times

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Bulk shear and longitudinal waves give rise to important contributions to the scattering of ultrasound by tilted finite plastic and rubber cylinders in water. This occurs in situations where either the shear or longitudinal speed is less than the speed of sound in the surrounding water. At a certain critical tilt angle, large backscattering enhancements are observed for finite cylinders, where the wave vector can reverse direction upon reflection from the cylinder truncation. The scattering process is analogous to the enhancement produced by the merging of rainbow caustics of primary rainbow rays in the scattering of light by long dielectric cylinders, also known as the caustic merging transition [C. M. Mount, D. B. Thiessen, and P. L. Marston, Appl. Opt. 37, 1534–1539 (1998)]. A ray theory was developed to model the backscattering mechanism at the critical tilt angle. It employs the idea of the Bravais effective refractive index, convenient for constructing ray diagrams for the projections of rays in the base plane of the cylinder. There is general agreement between the theory and the experiment down to relatively low ultrasonic frequencies (ka as small as 10). The enhancement is the most significant backscattering contribution for a wide range of tilt angles. © 2000 Acoustical Society of America.
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43.20.Fn Scattering of acoustic waves
43.30.Ft Volume scattering
43.35.Mr Acoustics of viscoelastic materials

Acoustic scattering by a modified Werner method

Patrice Ravel and Ahmed Trad

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 699-708 (2000); (10 pages)

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A modified integral Werner method is used to calculate pressure scattered by an axisymmetric body immersed in a perfect and compressible fluid subject to a harmonic acoustic field. This integral representation is built as the sum of a potential of a simple layer and a potential of volume. It is equivalent to the exterior Helmholtz problem with Neumann boundary condition for all real wave numbers of the incident acoustic field. For elastic structure scattering problems, the modified Werner method is coupled with an elastodynamic integral formulation in order to account for the elastic contribution of the displacement field at the fluid/structure interface. The resulting system of integral equations is solved by the collocation method with a quadratic interpolation. The introduction of a weighting factor in the modified Werner method decreases the number of volume elements necessary for a good convergence of results. This approach becomes very competitive when it is compared with other integral methods that are valid for all wave numbers. A numerical comparison with an experiment on a tungsten carbide end-capped cylinder allows a glimpse of the interesting possibilities for using the coupling of the modified Werner method and the integral elastodynamic equation used in this research. © 2000 Acoustical Society of America.
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43.20.Fn Scattering of acoustic waves
43.20.Bi Mathematical theory of wave propagation
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods

New explicit solutions in acoustics of closed spaces on the basis of divergent series

M. A. Sumbatyan, A. Pompei, and M. A. Rigano

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 709-713 (2000); (5 pages) | Cited 1 time

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A new approach to the acoustics of closed spaces is developed that involves solutions for polygonal shapes in explicit form. It is shown that exact solutions can be constructed for polygonal geometries where all the interior angles are equal to π/n (n is an integer). It is stated that the set of such polygons consists of the rectangle (known result) and three types of triangles. Some new explicit formulas are obtained for the eigenfrequencies of the triangles. It is demonstrated that the proposed technique also permits an exact representation of the impulse response function for the geometries described. © 2000 Acoustical Society of America.
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43.20.Ks Standing waves, resonance, normal modes

Acoustics of a flanged cylindrical pipe using singular basis functions

N. Amir, H. Matzner, and S. Shtrikman

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 714-724 (2000); (11 pages) | Cited 2 times

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The problem of acoustic radiation from a cylindrical pipe with an infinite flange has been discussed in a number of papers. The most common approach is to decompose the field inside the pipe over a basis of Bessel functions. A very large number of basis functions is usually required, with a large degree of ripple appearing as an artifact in the solution. In this paper it is shown that a close analysis of the velocity field near the corner yields a new family of functions, which are called “edge functions.” Using this set of functions as test functions and applying the moment method on the boundary between the waveguide and free space, a solution is obtained with greatly improved convergence properties and no ripple. © 2000 Acoustical Society of America.
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43.20.Mv Waveguides, wave propagation in tubes and ducts
43.20.Bi Mathematical theory of wave propagation

Intensity streamlines and vorticity streamlines in three-dimensional sound fields

Jiang Zhe

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 725-730 (2000); (6 pages) | Cited 1 time

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The properties of intensity streamlines and vorticity streamlines are discussed in this paper. It is found that the properties in three-dimensional sound fields are different from the properties in two-dimensional sound fields. The integral behavior of intensity streamlines is that the beginning and the end are attached to a sound source surface or that the beginning is on the sound source surface and the end extends into the infinite. For the vorticity streamlines, the integral behavior is that it is a closed curve or that the beginning and the end are attached to the sound source surface. Three examples are given for intensity and vorticity streamlines.© 2000 Acoustical Society of America.
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43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods
43.20.Bi Mathematical theory of wave propagation
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Modified impulse method for the measurement of the frequency response of acoustic filters to weakly nonlinear transient excitations

F. Payri, J. M. Desantes, and A. Broatch

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 731-738 (2000); (8 pages) | Cited 2 times

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In this paper, a modified impulse method is proposed which allows the determination of the influence of the excitation characteristics on acoustic filter performance. Issues related to nonlinear propagation, namely wave steepening and wave interactions, have been addressed in an approximate way, validated against one-dimensional unsteady nonlinear flow calculations. The results obtained for expansion chambers and extended duct resonators indicate that the amplitude threshold for the onset of nonlinear phenomena is related to the geometry considered. © 2000 Acoustical Society of America.
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43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves
43.25.Ed Effect of nonlinearity on velocity and attenuation
43.25.Zx Measurement methods and instrumentation for nonlinear acoustics
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Matched-field processing using measured replica fields

Laurie T. Fialkowski, Michael D. Collins, W. A. Kuperman, John S. Perkins, Lesley J. Kelly, Ashley Larsson, John A. Fawcett, and Lindsay H. Hall

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 739-746 (2000); (8 pages) | Cited 5 times

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An approach for avoiding the problem of environmental uncertainty is tested using data from the TESPEX experiments. Acoustic data basing is an alternative to the difficult task of characterizing the environment by performing direct measurements and solving inverse problems. A source is towed throughout the region of interest to obtain a database of the acoustic field on an array of receivers. With this approach, there is no need to determine environmental parameters or solve the wave equation. Replica fields from an acoustic database are used to perform environmental source tracking [J. Acoust. Soc. Am. 94, 3335–3341 (1993)], which exploits environmental complexity and source motion. © 2000 Acoustical Society of America.
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43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect

Predicting acoustic effects of internal waves from the basic climatology of the world ocean

Kimberly J. Noble and Stanley M. Flatté

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 747-757 (2000); (11 pages) | Cited 3 times

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Internal waves of a given strength will produce acoustic effects that vary from water mass to water mass. Presented here is a means of predicting the strength of acoustic fluctuations due to internal waves, given the basic climatology, that is, measurements of depth, temperature, and salinity of an oceanic region. An acoustic fluctuation strength parameter F is defined as the ratio of the fractional potential sound-speed change to the fractional potential-density change. Here F is calculated at three depth levels (275, 550, and 850 m), on a one-degree grid of latitude and longitude, using NODC/OCL’s World Ocean Atlas 1994. Representative values of F are presented for 15 upper water masses that range from F = 5 in the North Pacific to F = 34 in the North Atlantic, with a typical value for most of the upper waters being F = 15. Results for two depth levels within 12 intermediate water masses range from F = 7 in the North Pacific to F = 62 in the North Atlantic, with a typical value of F = 20, although there is considerable variation. In general, F exhibits higher values in the Atlantic Basin than in the Indian or Pacific, and has a maximum at 550 m. The main use of F will be the prediction of travel-time fluctuations in acoustic propagation experiments, which will be proportional to the value of F, given a universal strength of internal waves. © 2000 Acoustical Society of America.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean

Long time-base observations of surf noise

Grant B. Deane

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 758-770 (2000); (13 pages) | Cited 5 times

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A year of surf noise observations in the very near shore region of La Jolla Shores beach are presented. Ambient sound levels and surface wave height were recorded for 9 min every hour from July 1997 through June 1998 at a monitoring station located 360 m seaward of the beach in 8-m deep water. Sound segments that were dominated by the noise from breaking surf formed the basis of a correlation analysis between surf noise level and wave height, wave period, wind speed, and mean water depth. The analysis shows that surf noise is primarily determined by wave height, and scales approximately with the wave height squared. The surface wave energy flux onto the beach also scales with wave height squared, leading to the conclusion that the conversion of the mechanical energy of the surface wave field into noise energy is approximately constant. In fact, the ratio of noise energy to surface wave energy flux varies by up to a factor of 3 over the range of energy fluxes considered (100–3000 W per m). © 2000 Acoustical Society of America.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography

Collective oscillations of fresh and salt water bubble plumes

Gregory J. Orris and Michael Nicholas

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 771-787 (2000); (17 pages) | Cited 1 time

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Bubble plumes of various void fractions and sizes were produced by varying the flow velocity of a water jet impinging normally on a water surface. The bubbles entrained at the surface were carried downwards by the fluid flow to depths ranging from 33 to 65 cm, and formed roughly cylindrical plumes with diameters ranging from 12 to 27 cm. The acoustic emissions from the plumes were recorded onto digital audio tape using a hydrophone placed outside the cloud at distances ranging from 50 cm to 16.0 m. Closeup video images of the individual bubbles within the plume were also taken in order to gain knowledge of the bubble size distributions. The experiments were performed in both fresh-water and salt-water environments. The fresh-water clouds emitted sounds with a modal structure that was significantly different from that produced by the salt-water clouds. Furthermore, the smaller bubbles present in the salt-water clouds have a fundamental effect on the amplification of turbulence noise, generating sound at significant levels for frequencies up to several hundred Hertz. © 2000 Acoustical Society of America.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect
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Ultrasonic absorption in critical binary mixture of perfluoromethylcyclohexane and carbon tetrachloride

Issam R. Abdelraziq

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 788-792 (2000); (5 pages) | Cited 1 time

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The results of ultrasonic absorption and velocity measurements for the system perfluoromethylcyclohexane-carbon tetrachloride are presented. In addition, viscosity measurements were made. Ultrasonic absorption at 5, 7, 10, 15, 21, and 25 MHz, above critical temperature Tc, is analyzed using the dynamic scaling theory of Ferrell and Bhattacharjee. The values of α/f2 vs f−1.06 show a good agreement with the theory. The experimental values of α/αc for the binary mixture are compared to the scaling function F(ω∗). © 2000 Acoustical Society of America.
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43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions

Comparison between the dispersion curves calculated in complex frequency and the minima of the reflection coefficients for an embedded layer

A. Bernard, M. Deschamps, and M. J. S. Lowe

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 793-800 (2000); (8 pages) | Cited 4 times

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Analytical solutions of Lamb functions for symmetric and antisymmetric elastodynamic modes propagating within a solid layer embedded in an infinite medium are presented. Alternative theoretical analyses of such modes are performed, first in terms of the usual approach of harmonic heterogeneous plane waves (real frequency and complex slowness) and then in terms of transient homogeneous plane waves (complex frequency and real slowness). An example structure of a 0.1-mm-thick “alpha case” (an oxygen-rich phase of titanium that is relatively stiff) plate embedded in titanium is used for the study. A large difference between the usual dispersion curves calculated in real frequency and complex slowness and those calculated in complex frequency and real slowness is shown. Thus the choice between a spatial and a temporal parameter to describe the imaginary part of the guided waves is shown to be significant. The minima and the zeros of the longitudinal and shear plane-wave reflection coefficients are calculated and are compared with the dispersion curves. It is found that they do not match with the dispersion curves for complex slowness, but they do agree quite well with the dispersion curves for complex frequency. This implies that the complex frequency approach is better suited for the comparison of the modal properties with near-field reflection measurements. © 2000 Acoustical Society of America.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants

Measurement of acoustic dispersion using both transmitted and reflected pulses

Ping He

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 801-807 (2000); (7 pages) | Cited 4 times

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Traditional broadband transmission method for measuring acoustic dispersion requires the measurements of the sound speed in water, the thickness of the specimen, and the phase spectra of two transmitted ultrasound pulses. When the sound speed in the specimen is significantly different from that in water, the overall uncertainty of the dispersion measurement is generally dominated by the uncertainty of the thickness measurement. In this paper, a new water immersion method for measuring dispersion is proposed which eliminates the need for thickness measurement and the associated uncertainty. In addition to recording the two transmitted pulses, the new method requires recording two reflected pulses, one from the front surface and one from the back surface of the specimen. The phase velocity as well as the thickness of the specimen can be determined from the phase spectra of the four pulses. Theoretical analysis and experimental results from three specimens demonstrate the advantages of this new method. © 2000 Acoustical Society of America.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Yb Ultrasonic instrumentation and measurement techniques

The effect of gas loading on the RUS spectra of spheres

R. S. Sorbello, J. Feller, Moisés Levy, D. G. Isaak, J. D. Carnes, and O. L. Anderson

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 808-818 (2000); (11 pages) | Cited 2 times

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Resonant Ultrasound Spectroscopy (RUS) of a spherical sample in a pressurizing gas atmosphere was investigated experimentally and theoretically. Measurements were made on a fused silica sphere in He, Ar, and N2 gases up to pressures of 120 bar. The pressure-dependent shift in the resonant frequency, Δf, and the Q-factor were measured for the S00, S11, and T02 modes. A theoretical model based on acoustic radiation impedance was used to calculate Δf and the radiation-resistance component, Qr, of the Q-factor. Agreement between theory and experiment was good for Qr, but there were discrepancies for Δf. It was found that the theoretical Δf due to gas-loading effects associated with acoustic radiation was very small and consistent with the observed dependence on pressure and gas species for the T02 mode but not for the S00 and S11 modes. We conclude that the T02 mode is the most reliable of these modes to use in measuring third-order elastic constants by RUS. © 2000 Acoustical Society of America.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants

Using phase space diagrams to interpret multiple frequency drive sonoluminescence

Jeffrey A. Ketterling and Robert E. Apfel

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 819-826 (2000); (8 pages) | Cited 5 times

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The recent experimental results of J. Holzfuss, M. Rüggeberg, and R. Mettin [Phys. Rev. Lett. 81, 1961 (1998)] in which a second harmonic drive system was used to generate sonoluminescence (SL) have been analyzed in the context of the dissociation hypothesis (DH) of D. Lohse and S. Hilgenfeldt [J. Chem. Phys. 107, 6986 (1997)]. The second harmonic introduces two more variables that are under experimental control: a phase and an additional pressure term to the acoustic drive pressure. Diffusive equilibrium curves for a fixed gas concentration were calculated as was the Mach criterion. A phase space diagram was constructed to permit the prediction of regions of stable SL, unstable SL, stable non-SL, and unstable non-SL. These were compared to Holzfuss' experimental observations, and excellent quantitative agreement was seen. The results provide further evidence that the underlying assumptions of DH are sound. They also indicate the utility of DH for determining appropriate experimental conditions to achieve SL and for optimizing an experimental system. © 2000 Acoustical Society of America.  
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43.35.Ei Acoustic cavitation in liquids
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Use of electrodynamic drivers in thermoacoustic refrigerators

Ray Scott Wakeland

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 827-832 (2000); (6 pages) | Cited 12 times

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Some issues involved in matching electrodynamic drivers to thermoacoustic refrigerators are examined using an equivalent circuit model. Conclusions are that the driver should be chosen to have a large product (Bl)2/(ReRm); the suspension stiffness should be chosen to make the combined impedance of the mechanical and acoustical parts of the system entirely real at the operating frequency; and the piston area should be selected to maximize electroacoustic efficiency, or other desired parameter, by matching the acoustic load to the optimum mechanical load for the particular driver. Alternately, if the piston area is fixed, the operating frequency can be adjusted to make this same match. © 2000 Acoustical Society of America.
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43.38.Dv Electromagnetic and electrodynamic transducers
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect

Acoustical performance of an electrostrictive polymer film loudspeaker

Richard Heydt, Ron Pelrine, Jose Joseph, Joseph Eckerle, and Roy Kornbluh

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 833-839 (2000); (7 pages) | Cited 6 times

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A new type of loudspeaker that generates sound by means of the electrostrictive response of a thin polymer film is described. Electrostrictive polymer film (EPF) loudspeakers are constructed with inexpensive, lightweight materials and have a very low profile. The films are typically silicone and are coated with compliant electrodes to allow large film deformations. Acoustical frequency response measurements from 5×5 cm (planar dimensions) prototype EPF loudspeakers are presented. Measurements of harmonic distortion are also shown, along with results demonstrating reduced harmonic distortion achieved with square-root wave shaping. Applications of EPF loudspeakers include active noise control and general-purpose flat-panel loudspeakers. © 2000 Acoustical Society of America.
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43.38.Ja Loudspeakers and horns, practical sound sources
43.58.Vb Calibration of acoustical devices and systems
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Hybrid tool for quickly estimating the radiated acoustic power from a vibrating structure in a multiple-source environment

Olivier Beslin, Olivier Foin, and Jean Nicolas

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 840-850 (2000); (11 pages)

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This paper presents a new hybrid method for predicting overestimating and underestimating indicators of the acoustic power radiated by a vibrating surface even in the presence of other surrounding acoustic sources. This method is applicable to plates or low curvature surfaces radiating in open acoustic fields. The method is hybrid in the sense that the vibration field is measured and the parietal pressure field is predicted considering two extreme academic cases “baffled” and “unbaffled.” Many simplifications are made and justified in order to save running time. The method is successfully validated in comparison with experimental results on both laboratory and real life structures. This method has led to a quick tool, allowing one to obtain a good approximation of the radiated power in a few minutes. It provides a natural extension of a classical analyzer for vibroacoustics engineering. © 2000 Acoustical Society of America.
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43.50.Cb Noise spectra, determination of sound power
43.50.Yw Instrumentation and techniques for noise measurement and analysis
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Rotating machinery dynamics simulation. I. Rigid systems with ball bearing nonlinearities and outer ring ovality under rotating unbalance excitation

Fawzi M. A. El-Saeidy

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 851-860 (2000); (10 pages)

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The radial clearance in rolling bearing systems, required to compensate for dimensional changes associated with thermal expansion of the various parts during operation, may cause dimensional attrition and comprise bearing life, if unloaded operation occurs and balls skid [D. Childs and D. Moyer, ASME J. Eng. Gas Turb. Power 107, 152–159 (1985)]. Also, it can cause jumps in the response to unbalance excitation. These undesirable effects may be eliminated by introducing two or more loops into one of the bearing races so that at least two points of the ring circumference provide a positive zero clearance [D. Childs, Handbook of Rotordynamics, edited by F. Ehrich (McGraw-Hill, NY, 1992)]. The deviation of the outer ring with two loops, known as ovality, is one of the bearing distributed defects. Although this class of imperfections has received much work, none of the available studies has simulated the effect of the outer ring ovality on the dynamic behavior of rotating machinery under rotating unbalance with consideration of ball bearing nonlinearities, shaft elasticity, and speed of rotation. To fill this gap, the equations of motion of a rotor–ball bearing system are formulated using finite-elements (FE) discretization and Lagrange’s equations. The analyses are specialized to a rigid-rotor system, by retaining the rigid body modes only in the FE solution. Samples of the results are presented in both time domain and frequency domain for a system with and without outer ring ovality. It is found that with ideal bearings (no ovality), the vibration spectrum is qualitatively and quantitatively the same in both the horizontal and vertical directions. When the ring ovality is introduced, however, the spectrum in both orthogonal planes is no longer similar. And magnitude of the bearing load has increased in the form of repeated random impacts, between balls and rings, in the horizontal direction (direction of maximum clearance) compared to a continuous contact along the vertical direction (direction of positive zero clearance). This underlines the importance of the vibration measuring probe’s direction, with respect to the outer ring axes, to capture impact-induced vibrations. Moreover, when the harmonic excitation is increased for a system with ideal bearings, the spectral peaks above forcing frequency have shifted to a higher-frequency region, indicating some sort of a hard spring mechanism inherent in the system. Another observation, is that for the same external excitation, vibration amplitude at forcing frequency in the bearing force spectrum is the same for systems with or without outer ring ovality. © 2000 Acoustical Society of America.  
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43.40.At Experimental and theoretical studies of vibrating systems

Reverberation time and maximum background-noise level for classrooms from a comparative study of speech intelligibility metrics

Sylvio R. Bistafa and John S. Bradley

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 861-875 (2000); (15 pages) | Cited 15 times

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Speech intelligibility metrics that take into account sound reflections in the room and the background noise have been compared, assuming diffuse sound field. Under this assumption, sound decays exponentially with a decay constant inversely proportional to reverberation time. Analytical formulas were obtained for each speech intelligibility metric providing a common basis for comparison. These formulas were applied to three sizes of rectangular classrooms. The sound source was the human voice without amplification, and background noise was taken into account by a noise-to-signal ratio. Correlations between the metrics and speech intelligibility are presented and applied to the classrooms under study. Relationships between some speech intelligibility metrics were also established. For each noise-to-signal ratio, the value of each speech intelligibility metric is maximized for a specific reverberation time. For quiet classrooms, the reverberation time that maximizes these speech intelligibility metrics is between 0.1 and 0.3 s. Speech intelligibility of 100% is possible with reverberation times up to 0.4–0.5 s and this is the recommended range. The study suggests “ideal” and “acceptable” maximum background-noise level for classrooms of 25 and 20 dB, respectively, below the voice level at 1 m in front of the talker. © 2000 Acoustical Society of America.
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43.55.Hy Subjective effects in room acoustics, speech in rooms
43.55.Fw Auditorium and enclosure design
43.71.Gv Measures of speech perception (intelligibility and quality)
43.55.Br Room acoustics: theory and experiment; reverberation, normal modes, diffusion, transient and steady-state response

Broadband control of plate radiation using a piezoelectric, double-amplifier active-skin and structural acoustic sensing

Brody D. Johnson and Chris R. Fuller

J. Acoust. Soc. Am. Volume 107, Issue 2, pp. 876-884 (2000); (9 pages) | Cited 1 time

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The potential of a piezoelectric, double-amplifier active-skin with structural acoustic sensing (SAS) is demonstrated for the reduction of broadband acoustic radiation from a clamped, aluminum plate. The active-skin is a continuous covering of the vibrating portions of the plate with active, independently controllable piezoelectric, double-amplifier elements and is designed to affect control by altering the continuous structural radiation impedance rather than structural vibration. In simulation, acoustic models are sought for the primary and secondary sources that incorporate finite element methods. Simulation indicates that a total radiated power attenuation in excess of 10 dB may be achieved between 250 and 750 Hz with microphone error sensing, while under SAS the radiated power is reduced by nearly 8 dB in the same frequency range. In experiment, the adaptive feed forward filtered-x LMS (least mean square) algorithm, implemented on a Texas Instruments C40 DSP, was used in conjunction with the 6I6O control system. With microphone error sensing, 11.8-dB attenuation was achieved in the overall radiated power between 175 and 600 Hz, while inclusion of SAS resulted in a 7.3-dB overall power reduction in this frequency band. © 2000 Acoustical Society of America.
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43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance
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