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Oct 2010

Volume 128, Issue 4, pp. EL151-2490

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Radiated signal characteristics of marine vessels in the cepstral domain for shallow underwater channel

Arnab Das, Arun Kumar, and Rajendar Bahl

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL151-EL156 (2010); (6 pages)

Online Publication Date: 10 Sep 2010

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This work examines the distribution of cepstral energy of the radiated signal of a marine vessel and the underwater channel modeled as a block-adaptive linear system. Detailed simulation analysis of the signal at the receiver of a passive sonar has led to the observation that, in the cepstral domain the radiated signal of a marine vessel largely occupies the lower cepstral indices while the underwater channel occupies the higher indices, such that for several range and depth conditions, the two can be separated out. This finding can facilitate the design of filters in the cepstral domain for reducing distortions due to the underwater channel. The work presents analytical justification and simulation studies in this regard.
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43.60.Pt Signal processing techniques for acoustic inverse problems
43.30.Jx Radiation from objects vibrating under water, acoustic and mechanical impedance
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Unwanted sounds generated with test tone presentation can spoil extended high-frequency audiometry

Kenji Kurakata, Tazu Mizunami, Kazuma Matsushita, and Kimio Shiraishi

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL157-EL162 (2010); (6 pages) | Cited 1 time

Online Publication Date: 10 Sep 2010

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Unwanted sounds from a commercially available audiometer were evaluated in terms of their effects on extended high-frequency (EHF) audiometry. Although the manufacturer reported that the audiometer conformed to relevant International Electrotechnical Commission (IEC) standards, the audiograms obtained using the audiometer were erroneous because the subjects had responded falsely to noise generated with the test tone presentation before detecting the test tone. Analyses of acoustic and electric output signals revealed that the audiometer generated most of the unwanted sounds, not the earphones that were used. Based on the measurement results, clinical implications of the measurement results are discussed for conducting more reliable EHF audiometry.
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43.80.Vj Acoustical medical instrumentation and measurement techniques
43.66.Sr Deafness, audiometry, aging effects
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End level bias on direct loudness ratings of increasing sounds

Patrick Susini, Sabine Meunier, Régis Trapeau, and Jacques Chatron

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL163-EL168 (2010); (6 pages)

Online Publication Date: 10 Sep 2010

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Three experiments on loudness of sounds with linearly increasing levels were performed: global loudness was measured using direct ratings, loudness change was measured using direct and indirect estimations. Results revealed differences between direct and indirect estimations of loudness change, indicating that the underlying perceptual phenomena are not the same. The effect of ramp size is small for the former and important for the latter. A similar trend was revealed between global loudness and direct estimations of loudness change according to the end level, suggesting they may have been confounded. Measures provided by direct estimations of loudness change are more participant-dependent.
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43.66.Cb Loudness, absolute threshold
43.66.Lj Perceptual effects of sound
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Phonemic restoration in sensorineural hearing loss does not depend on baseline speech perception scores

Deniz Başkent

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL169-EL174 (2010); (6 pages) | Cited 4 times

Online Publication Date: 10 Sep 2010

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The brain can restore missing speech segments using linguistic knowledge and context. The phonemic restoration effect is commonly quantified by the increase in intelligibility of interrupted speech when the silent gaps are filled with noise bursts. In normal hearing, the restoration effect is negatively correlated with the baseline scores with interrupted speech; listeners with poorer baseline show more benefit from restoration. Reanalyzing data from Başkent et al. [(2010). Hear. Res. 260, 54–62] , correlations with mild and moderate hearing impairment were observed to differ than with normal hearing. This analysis further shows that hearing impairment may affect top-down restoration of speech.
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43.71.Ky Speech perception by the hearing impaired
43.71.Es Vowel and consonant perception; perception of words, sentences, and fluent speech
43.66.Lj Perceptual effects of sound
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Passive cavitation mapping for localization and tracking of bubble dynamics

Miklós Gyöngy and Constantin-C. Coussios

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL175-EL180 (2010); (6 pages) | Cited 1 time

Online Publication Date: 10 Sep 2010

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multimedia

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Current acoustic techniques for studying cavitation dynamics are only readily applicable to single-bubble activity, while optical methods can only be used in transparent media. However, multi-bubble cavitation often occurs in opaque media such as biological tissue. Here, the signals received passively by each of the 64 channels of a diagnostic ultrasound array are used to localize and separate emissions from several bubble clusters cavitating in agar gel, thereby providing a method of observing cavitation dynamics. The method has a high spatiotemporal resolution and is applicable to cavitation in opaque media.
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43.60.Fg Acoustic array systems and processing, beam-forming
43.20.Fn Scattering of acoustic waves
43.25.Vt Intense sound sources
43.80.Ev Acoustical measurement methods in biological systems and media
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Long range transmission loss of broadband seismic pulses in the Arctic under ice-free conditions

Aaron Thode, Katherine H. Kim, Charles R. Greene, Jr., and Ethan Roth

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL181-EL187 (2010); (7 pages)

Online Publication Date: 10 Sep 2010

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In 2008 the Louis S. St-Laurent (LSSL) surveyed deep Arctic waters using a three-airgun seismic source. Signals from the seismic survey were detected between 400 km and 1300 km range on a directional autonomous acoustic recorder deployed in water 53 m deep off the Alaskan North Slope. Observations of received signal levels between 10–450 Hz versus LSSL range roughly fit a cylindrical transmission loss model plus 0.01 dB/km attenuation in deep ice-free waters, and fit previous empirical models in ice-covered waters. The transition between ice-free and ice-covered propagation conditions shifted 200 km closer to the recorder during the survey.
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43.30.Qd Global scale acoustics; ocean basin thermometry, transbasin acoustics
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Fishbone-like instability in a looped-tube thermoacoustic engine

Zhibin Yu, Artur J. Jaworski, and Abdulrahman S. Abduljalil

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL188-EL194 (2010); (7 pages)

Online Publication Date: 10 Sep 2010

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Quasi-periodic bursts of acoustic oscillations were observed during the start-up process in a looped-tube thermoacoustic engine. The acoustic oscillations have a constant frequency of 111 Hz, while the bursts have “quasi-periods” in the order of 14–25 s. The quasi-periodic bursts show a new mode of amplitude growth in this thermoacoustic engine. The envelope of the acoustic oscillations has a fishbone-like shape. The nature of the observed fishbone-like instabilities suggests a strong interaction between the acoustic and temperature field.
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43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
43.25.Ts Nonlinear acoustical and dynamical systems
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Manipulation of microparticles using phase-controllable ultrasonic standing waves

C. R. P. Courtney, C.-K. Ong, B. W. Drinkwater, P. D. Wilcox, C. Demore, S. Cochran, P. Glynne-Jones, and M. Hill

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL195-EL199 (2010); (5 pages) | Cited 5 times

Online Publication Date: 10 Sep 2010

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A method of manipulating microparticles in a liquid using ultrasound is proposed and demonstrated. An ultrasonic standing wave with nodal planes whose positions are controllable by varying the relative phase of two applied sinusoidal signals is generated using a pair of acoustically matched piezoelectric transducers. The resulting acoustic radiation force is used to trap micron scale particles at a series of arbitrary positions (determined by the relative phase) and then move them in a controlled manner. This method is demonstrated experimentally and 5 μm polystyrene particles are trapped and moved in one dimension through 140 μm.
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43.25.Qp Radiation pressure
43.38.Ar Transducing principles, materials, and structures: general
43.20.Ks Standing waves, resonance, normal modes
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Short-term adaptation to accented English by younger and older adults

Sandra Gordon-Salant, Grace H. Yeni-Komshian, Peter J. Fitzgibbons, and Jaclyn Schurman

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. EL200-EL204 (2010); (5 pages)

Online Publication Date: 10 Sep 2010

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This study examined the effects of age and hearing loss on short-term adaptation to accented speech. Data from younger and older listeners in a prior investigation [ Gordon-Salant et al. (2010). J. Acoust. Soc. Am. 128, 444–455 ] were re-analyzed to examine changes in recognition over four administrations of equivalent lists of English stimuli recorded by native speakers of Spanish and English. Results showed improvement in recognition scores over four list administrations for the accented stimuli but not for the native English stimuli. Group effects emerged but were not involved in any interactions, suggesting that short-term adaptation to accented speech is preserved with aging and with hearing loss.
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43.71.Lz Speech perception by the aging
43.71.Ky Speech perception by the hearing impaired
43.71.Bp Perception of voice and talker characteristics
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Constraining the minute amount of audible energy radiated from binary collisions of light plastic spheres in conditions of incomplete angular coverage of the measured pressure

Andi Petculescu and Joshua Riner

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1575-1577 (2010); (3 pages)

Online Publication Date: 18 Oct 2010

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Usually, the energy released as air-coupled sound following a collision is dismissed as negligible. The goal of this Letter is to quantify the value of this small but measurable quantity, since it can be useful to impact studies. Measurements of sound radiation from binary collisions of polypropylene balls were performed in order to constrain the fraction of incident energy radiated as sound in air. In the experiments, one ball is released from rest, directly above a stationary target ball. The transient acoustic waveforms are detected by a microphone rotated about the impact point at a radius of 10 cm. The sound pressure was measured as a function of the polar angle θ (the azimuthal symmetry of the problem was verified by rotating the microphone in the horizontal plane). The angular pattern has two main lobes that are asymmetric with respect to the impact plane. This asymmetry is ascribable to interference and/or scattering effects. Gaps in the acoustic measurements at the “poles” (i.e., around 0° and 180°) pose a challenge similar to that of extrapolating the cosmic microwave background in the galactic “cut.” The data was continued in the gaps by polynomial interpolation rather than least-squares fitting, a choice dictated by the accuracy of the reconstructed pattern. The acoustic energy radiated during the impact, estimated by multiplying the collision time by the sound intensity integrated over a spherical surface centered at the impact point, is calculated as four orders of magnitude smaller than the incident energy (0.23 μJ versus 1.6 mJ).
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43.40.Kd Impact and impact reduction, mechanical transients
43.20.Px Transient radiation and scattering

Detection of modulation of a 4-kHz carrier

Neal F. Viemeister, Mark A. Stellmack, and Andrew J. Byrne

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1578-1581 (2010); (4 pages)

Online Publication Date: 18 Oct 2010

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To better understand the processing of complex high-frequency sounds, modulation-detection thresholds were measured for sinusoidal frequency modulation (SFM), quasi-frequency modulation (QFM), sinusoidal amplitude modulation (SAM), and random-phase FM (RPFM). At the lowest modulation frequency (5 Hz) modulation thresholds expressed as AM depth were similar for RPFM, SAM and QFM suggesting the predominance of envelope cues. At the higher modulation frequencies (20 and 40 Hz) thresholds expressed as total frequency excursions were similar for SFM and QFM suggesting a common mechanism, one perhaps based on single-channel FM-to-AM conversion or on a multi-channel place mechanism. The fact that the nominal envelopes of SFM and QFM are different (SFM has a flat envelope), seems to preclude processing based on the envelope of the external stimulus. Also, given the 4-kHz carrier and the similarity to previously published results obtained with a 1-kHz carrier, processing based on temporally-coded fine structure for all four types of modulation appears unlikely.
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43.66.Nm Phase effects
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music

Radiofrequency electrode vibration-induced shear wave imaging for tissue modulus estimation: A simulation study

Shyam Bharat and Tomy Varghese

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1582-1585 (2010); (4 pages) | Cited 1 time

Online Publication Date: 18 Oct 2010

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Quasi-static electrode displacement elastography, used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney, is extended in this paper to dynamic vibrational perturbations of the ablation electrode. Propagation of the resulting shear waves into adjoining regions of tissue can be tracked and the shear wave velocity used to quantify the shear (and thereby Young’s) modulus of tissue. The algorithm used utilizes the time-to-peak displacement data (obtained from finite element analyses) to calculate the speed of shear wave propagation in the material. The simulation results presented illustrate the feasibility of estimating the Young’s modulus of tissue and is promising for characterizing the stiffness of radiofrequency-ablated thermal lesions and surrounding normal tissue.
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43.35.Yb Ultrasonic instrumentation and measurement techniques
43.60.Mn Adaptive processing

On the limitation of a diffusion equation model for acoustic predictions of rooms with homogeneous dimensions

José Escolano, Juan M. Navarro, and José J. López

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1586-1589 (2010); (4 pages) | Cited 2 times

Online Publication Date: 18 Oct 2010

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In recent years a model for predicting sound fields in enclosures has been proposed, based on the mathematical theory of diffusion. This model is held to be valid for predicting the late reverberation component of the impulse response, on the basis that sufficient reflection events must occur [ Valeau et al., J. Acoust. Soc. Am. 119, 1504–1513 (2006) ]. The present work determines numerically the extent of reflections necessary for the solution of the diffusion equation model to be accurate in quasi-cubic rooms. Some preliminary numerical experiments have been carried out to determine after how many mean-free times of the impulse response, which is obtained by a geometrical-acoustic approach, gives a similar result to the solution obtained from a diffusion equation model.
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43.55.Br Room acoustics: theory and experiment; reverberation, normal modes, diffusion, transient and steady-state response
43.55.Ka Computer simulation of acoustics in enclosures, modeling
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Efficient evaluation of edge diffraction integrals using the numerical method of steepest descent

Andreas Asheim and U. Peter Svensson

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1590-1597 (2010); (8 pages)

Online Publication Date: 18 Oct 2010

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For the problem of edge diffraction from an edge of finite length a frequency-domain solution, obtained from an analytical time-domain solution, has been presented by Svensson et al. [Acta. Acust. Acust. 95, 568–572] . This formulation takes the form of a Fourier-type integral whose evaluation is expensive in the high frequency range. This paper demonstrates that by using tailored highly oscillatory quadrature methods based on asymptotic properties of the integral, accurate approximations in the high frequency case can be obtained with little computational effort.
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43.20.El Reflection, refraction, diffraction of acoustic waves

Cancellation of spurious arrivals in Green’s function retrieval of multiple scattered waves

Roel Snieder and Clement Fleury

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1598-1605 (2010); (8 pages) | Cited 5 times

Online Publication Date: 18 Oct 2010

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The Green’s function for wave propagation can be extracted by cross-correlating field fluctuations excited on a closed surface that surrounds the employed receivers. This study treats an acoustic multiple scattering medium with discrete scatterers and shows that for a given source the cross-correlation of waves propagating along most combinations of scattering paths gives unphysical arrivals. Because theory predicts that the true Green’s function is retrieved, such unphysical arrivals must cancel after integration over all sources. This cancellation occurs because the scattering amplitude of each scatterer satisfies the generalized optical theorem. The cross-correlation of scattered waves with themselves does not lead to the correct retrieval of scattered waves, because the cross-terms between the direct and scattered waves is essential.
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43.20.Fn Scattering of acoustic waves
43.20.Bi Mathematical theory of wave propagation
43.30.Ft Volume scattering

Acoustic metafluids made from three acoustic fluids

Andrew N. Norris and Adam J. Nagy

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1606-1616 (2010); (11 pages) | Cited 4 times

Online Publication Date: 18 Oct 2010

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Significant reduction in target strength and radiation signature can be achieved by surrounding an object with multiple concentric layers comprised of three acoustic fluids. The idea is to make a finely layered shell with the thickness of each layer defined by a unique transformation rule. The shell has the effect of steering incident acoustic energy around the structure, and conversely, reducing the radiation strength. The overall effectiveness and the precise form of the layering depends upon the densities and compressibilities of the three fluids. Nearly optimal results are obtained if one fluid has density equal to the background fluid, while the other two densities are much greater and much less than the background values. Optimal choices for the compressibilities are also found. Simulations in 2D and 3D illustrate effectiveness of the three fluid shell. The limited range of acoustic metafluids that are possible using only two fluid constituents is also discussed.
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43.20.Fn Scattering of acoustic waves
43.40.Sk Inverse problems in structural acoustics and vibration
43.20.Tb Interaction of vibrating structures with surrounding medium
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Computation of the acoustic radiation force using the finite-difference time-domain method

Feiyan Cai, Long Meng, Chunxiang Jiang, Yu Pan, and Hairong Zheng

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1617-1622 (2010); (6 pages) | Cited 3 times

Online Publication Date: 18 Oct 2010

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The computational details related to calculating the acoustic radiation force on an object using a 2-D grid finite-difference time-domain method (FDTD) are presented. The method is based on propagating the stress and velocity fields through the grid and determining the energy flow with and without the object. The axial and radial acoustic radiation forces predicted by FDTD method are in excellent agreement with the results obtained by analytical evaluation of the scattering method. In particular, the results indicate that it is possible to trap the steel cylinder in the radial direction by optimizing the width of Gaussian source and the operation frequency. As the sizes of the relating objects are smaller than or comparable to wavelength, the algorithm presented here can be easily extended to 3-D and include torque computation algorithms, thus providing a highly flexible and universally usable computation engine.
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43.25.Qp Radiation pressure
43.35.Ty Other physical effects of sound

Stability of the parametrically excited damped inverted pendulum: Theory and experiment

Randy M. Carbo, Robert W. M. Smith, and Matthew E. Poese

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1623-1631 (2010); (9 pages)

Online Publication Date: 18 Oct 2010

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The parametrically driven, damped, inverted pendulum can be dynamically stabilized in particular regions of the parameter space. The impact of damping on dynamic stabilization can be stabilizing or destabilizing depending on the location in parameter space (i.e., drive frequency and amplitude). Floquet analysis and numerical simulations were used to determine the stable regions. An experiment was conducted that verifies the model. Physical explanations and simple bounding approximations are provided to summarize findings. The utility of the highly damped pendulum results are illustrated by drawing the analogy to dynamic stabilization of the Rayleigh-Taylor instability: it permits ready demonstration that dynamic stabilization is impossible in that system absent surface tension.
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43.25.Ts Nonlinear acoustical and dynamical systems
43.20.Wd Analogies
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The extended Fourier pseudospectral time-domain method for atmospheric sound propagation

Maarten Hornikx, Roger Waxler, and Jens Forssén

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1632-1646 (2010); (15 pages) | Cited 3 times

Online Publication Date: 18 Oct 2010

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An extended Fourier pseudospectral time-domain (PSTD) method is presented to model atmospheric sound propagation by solving the linearized Euler equations. In this method, evaluation of spatial derivatives is based on an eigenfunction expansion. Evaluation on a spatial grid requires only two spatial points per wavelength. Time iteration is done using a low-storage optimized six-stage Runge-Kutta method. This method is applied to two-dimensional non-moving media models, one with screens and one for an urban canyon, with generally high accuracy in both amplitude and phase. For a moving atmosphere, accurate results have been obtained in models with both a uniform and a logarithmic wind velocity profile over a rigid ground surface and in the presence of a screen. The method has also been validated for three-dimensional sound propagation over a screen. For that application, the developed method is in the order of 100 times faster than the second-order-accurate FDTD solution to the linearized Euler equations. The method is found to be well suited for atmospheric sound propagation simulations where effects of complex meteorology and straight rigid boundary surfaces are to be investigated.
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43.28.Js Numerical models for outdoor propagation
43.20.El Reflection, refraction, diffraction of acoustic waves
43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors
43.28.En Interaction of sound with ground surfaces, ground cover and topography, acoustic impedance of outdoor surfaces

Measured wavenumber: Frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Blandine Arguillat, Denis Ricot, Christophe Bailly, and Gilles Robert

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1647-1655 (2010); (9 pages) | Cited 1 time

Online Publication Date: 18 Oct 2010

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Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.
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43.28.Ra Generation of sound by fluid flow, aerodynamic sound and turbulence

An iterative algorithm for computing aeroacoustic integrals with application to the analysis of free shear flow noise

Florent Margnat and Véronique Fortuné

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1656-1667 (2010); (12 pages)

Online Publication Date: 18 Oct 2010

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An iterative algorithm is developed for the computation of aeroacoustic integrals in the time domain. It is specially designed for the generation of acoustic images, thus giving access to the wavefront pattern radiated by an unsteady flow when large size source fields are considered. It is based on an iterative selection of source-observer pairs involved in the radiation process at a given time-step. It is written as an advanced-time approach, allowing easy connection with flow simulation tools. Its efficiency is related to the fraction of an observer grid step that a sound-wave covers during one time step. Test computations were performed, showing the CPU-time to be 30 to 50 times smaller than with a classical non-iterative procedure. The algorithm is applied to compute the sound radiated by a spatially evolving mixing-layer flow: it is used to compute and visualize contributions to the acoustic field from the different terms obtained by a decomposition of the Lighthill source term.
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43.28.Ra Generation of sound by fluid flow, aerodynamic sound and turbulence
43.20.Wd Analogies

Investigation of higher spanwise Helmholtz resonance modes in slender covered cavities

A. T. de Jong and H. Bijl

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1668-1678 (2010); (11 pages) | Cited 1 time

Online Publication Date: 18 Oct 2010

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Cavity aeroacoustic noise is relevant for aerospace and automotive industries and widely investigated since the 1950s. Most investigations so far consider cavities where opening length and width are of similar scale. The present investigation focuses on a less investigated setup, namely cavities that resemble the door gaps of automobiles. These cavities are both slender (width much greater than length or depth) and partially covered. Furthermore they are under influence of a low Mach number flow with a relatively thick boundary layer. Under certain conditions, these gaps can produce tonal noise. The present investigation attempts to reveal the aeroacoustic mechanism of this tonal noise for higher resonance modes. Experiments have been conducted on a simplified geometry, where unsteady internal pressures have been measured at different spanwise locations. With increasing velocity, several resonance modes occur. In order to obtain higher mode shapes, the cavity acoustic response is simulated and compared with experiment. Using the frequency-filtered simulation pressure field, the higher modes shapes are retrieved. The mode shapes can be interpreted as the slender cavity self-organizing into separate Helmholtz resonators that interact with each other. Based on this, an analytical model is derived that shows good agreement with the simulations and experimental results.
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43.28.Ra Generation of sound by fluid flow, aerodynamic sound and turbulence
43.25.Gf Standing waves; resonance
43.28.Hr Outdoor sound sources

Analysis of the radiated information in spinning sound fields

Michael Carley

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1679-1684 (2010); (6 pages) | Cited 1 time

Online Publication Date: 18 Oct 2010

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The information content of a spinning sound field is analyzed using a combination of exact and asymptotic results, in order to set limits on how accurately source identification can be carried out. Using a transformation of the circular source to an exactly equivalent set of line source modes, given by Chebyshev polynomials, it is found that the line source modes of order greater than the source wavenumber generate exponentially small fields. Asymptotic analysis shows that the remaining, lower order, modes radiate efficiently only into a region around the source plane, with this region shrinking as the mode order is increased. The results explain the ill-conditioning of source identification methods; the successful use of low order models in active noise control; and the low radiation efficiency of subsonic jets.
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43.28.Ra Generation of sound by fluid flow, aerodynamic sound and turbulence
43.60.Pt Signal processing techniques for acoustic inverse problems
43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods
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Sound speed profile characterization by the image source method

S. Pinson and L. Guillon

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1685-1693 (2010); (9 pages)

Online Publication Date: 18 Oct 2010

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This paper presents the first results of an imaging technique that measures the geoacoustic structure of a seafloor in shallow water areas. The devices used were a broadband (100 Hz–6 kHz) acoustic source towed by a ship and a vertical array. Among all the acoustic paths existing in the water column, two are used: the direct one and the seabed-reflected one, the latter being composed of the reflections from the seafloor’s surface as well as that from each buried layer. Due to the good time resolution of the signal and to the short range configuration, the reflected signal can be modeled as a sum of contributions coming from image sources relative to the seabed layers. The seabed geometry and the sound speed profile can then be recovered with the detection and localization of these image sources. The map of the image sources is obtained by a function that combines back-propagation of signals and knowledge of the emitted pulse. The thickness and sound-speed of each layer is finally obtained by a position analysis of the image sources. The results obtained by this data-driven algorithm on both at-sea and synthetic data are satisfactory.
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43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Fg Acoustic array systems and processing, beam-forming
43.60.Rw Remote sensing methods, acoustic tomography

Multistatic detection and tracking using linear maximal sequences

W. Jobst, D. Smith, and L. Whited

J. Acoust. Soc. Am. Volume 128, Issue 4, pp. 1694-1699 (2010); (6 pages)

Online Publication Date: 18 Oct 2010

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A simulation using linear maximal sequence (m-sequence) transmissions demonstrated the potential for detecting and tracking near-surface targets. In this simulation a low power, omnidirectional source and four omnidirectional hydrophones were arbitrarily located in water approximately 4 m deep. The simulated −15 dB target traveled at 1.5 kn. Using “channel digit response” processing and “block zeroing,” the direct arrival, multipaths, clutter and reverberation were rejected. (The block zeroing process is very different from other methods typically labeled as “cancellation.”) With the improved signal-to-interference ratio, a simple probability based algorithm demonstrated tracking at source-target distances of 250 m, the maximum range investigated.
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43.30.Vh Active sonar systems
43.30.Zk Experimental modeling
43.60.Gk Space-time signal processing, other than matched field processing
43.60.Jn Source localization and parameter estimation
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