bluuebunny/crossref_metadata_2025 huggingface dataset:
Initial tests in AT&T Bell Labs’ Varechoic Chamber
Authors: Ward William C. , Elko Gary W. , Kubli Robert A. , McDougald W. Craig
Published: October 2005

Results are available for the first measurements in the recently completed Varechoic chamber, a digitally controllable variable acoustics facility at AT&T Bell Laboratories in Murray Hill, NJ. The 118 m3 (6.7×6.1×2.9 m) room has 368 independently actuated surfaces in the walls, ceiling, and floor. The room boundary conditions can be completely changed in a fraction of a second. This chamber is a unique facility for acoustical measurements, testing of electroacoustic and signal processing systems, perception tests, or for recording. The panels from which it is constructed could be applied in multipurpose rooms, recording studios, or performance spaces. Reverberation time in the 750-Hz octave band is variable from 0.1–1.6 s; this talk will present full 1/3-oct decay rates of the room. Some interesting impulse responses, and comparisons with image model calculations, will also be given.
Effects of boat noise on the acoustic behavior of humpback whales
Authors: Norris Thomas F.
Published: October 2005

The effects of boat noise on cetacean acoustic behavior are not well understood. To examine these, real sources of boat noise were experimentally introduced to singing humpback whales (Megapteranovaeangliae). Humpback whales were chosen as subjects because they sing long songs that are easy to record. Also, they are often distributed in nearshore environments with heavy boat traffic. Songs from nine animals were analyzed (n=9). Ten variables describing time and frequency characteristics of humpback song signals and the structure of song patterns were compared before and during exposure to boat noise. Means of two variables (unit duration and phrase duration) were significantly less during boat passes than during control periods. Means of eight other variables were not significantly different. The statistical power of detecting a difference between the means was ≳90% for all variables describing frequency characteristics of songs. Because the durations of some variables were shortened, these results indicate that boat noise might affect humpback whale singing behavior. However, power analyses indicate that frequency structure is probably not affected. The significance of these effects concerning the behavioral biology of humpback whales is uncertain at this time.
Acoustic backscattering by the anisotropic and dipped structure of velocity fluctuations within the bottom sediments
Authors: Yamamoto Tokuo
Published: October 2005

The three-dimensional power spectra of the velocity fluctuations within the seabed measured by high-frequency (1–50 kHz) crosswell acoustic tomography experiments indicate that the velocity fluctuation spectra are anisotropic in general and are often dipped; i.e., the major and minor axes of anisotropy are tilted from the vertical and the horizontal direction (Yamamoto, this meeting). The aspect ratio of the horizontal scale to the vertical scale ranges from 4 to 10 in the shallow-water sediments. The angle of tilt, called dip, is found as large as 30 deg. The intensity of the fluctuation spectrum depends on the sediment type. These parameters of the three-dimensional power spectrum affect the scattering of acoustic waves. An analytical solution to acoustic wave scattering by a dipped anisotropic 3-D velocity fluctuations in the sediments has been obtained. The strong dependence of acoustic backscattering on the grazing and azimuthal angle observed by Jackson and Briggs (1993) is excellently predicted when the realistic anisotropy and dip structures of the velocity fluctuations are incorporated in this analytical model of scattering by sediment volume fluctuation. [Work supported by ONR.]
Influence of sediment transport events upon bottom backscattering, Eckernfoerde Bay
Authors: Jackson Darrell , Wright L. D.
Published: October 2005

In Spring 1993, oceanographic and acoustic apparatus were deployed in Eckernfoerde Bay, Baltic Sea. An instrumented tetrapod recorded physical data, revealing turbidity events associated with resonant internal waves. Acoustic bottom backscattering data at 40 kHz were acquired simultaneously and show features that correlate with the turbidity events. Such a correlation is surprising as estimates of shear velocity suggest that bottom stress never reached the critical magnitude necessary to resuspend sediment; rather the turbidity events appear to involve advection from a distant location. Correlations are evident in comparisons of acoustically derived time series for change in temperature and change in echo character with time series from current meters, optical backscattering sensors, and thermistors. [Work supported by the NRL CBBL Special Research Program.]
Low-frequency hearing in California sea lions and harbor seals
Authors: Kastak David , Schusterman Ronald J.
Published: October 2005

Studies on pure-tone detection thresholds were conducted on two female California sea lions and on a harbor seal. The older sea lion and the harbor seal were trained to wear custom-fitted headphones in order to determine minimum audible pressures in a binaural listening task. All three animals were trained to respond to underwater signals at frequencies ranging from 100–1600 Hz at a depth of about 1.5 m. Results were very reliable, owing to a combination of psychophysical threshold determining measures. Sensitivity to low-frequency sounds by both species were 25–30 dB better underwater than in air. The low-frequency hearing of the harbor seal was 2–25 dB better than the older seal lion. At 100 Hz, the sensitivity of the harbor seal was 17 dB superior to that of the younger seal ion, and 23 dB superior to the older seal lion. Results at low frequencies support the notion that the harbor seal (phocid) ear is more water adapted than the sea lion (otariid) ear.
Acoustically enhanced bubble growth at low frequencies and its implications for human diver and marine mammal safety
Authors: Crum Lawrence A. , Mao Yi
Published: October 2005

Computations are made of the conditions necessary to obtain bubble growth by rectified diffusion under a variety of conditions associated with low-frequency sound propagation in the ocean. The complex issue of microbubble nuclei stabilization is treated by assuming either a sufficient level of supersaturation to stabilize the initial bubble size, or by examining a microbubble nucleus with zero surface tension. The bubble growth rates and thresholds are obtained for a range of SPLs (re: 1 μPa) from 150–220 dB, for initial bubble radii from 1–10 μm, and for levels of the dissolved gas concentration from 100% to 223% of saturation. It was determined that for the range of conditions examined, it was necessary to utilize three different formulations of the equations for bubble growth. The results of these calculations and assumptions indicate that for SPLs in excess of 210 dB, significant bubble growth can be expected to occur, and divers and marine mammals exposed to these conditions could be at risk. For SPLs below about 190 dB, however, significant bubble growth is unexpected. [Work supported in part by NSMRL.]
Time scales for sonoluminescence
Authors: Weninger Keith , Hiller Robert , Putterman Seth , Barber Bradley P.
Published: October 2005

The establishment of stable sonoluminescence from a single trapped bubble of air in water requires more than 5 s. During this time the bubble goes through a transition period (about 1 s long) that is characterized by an emitted intensity which is over ten times smaller than the steady state. Pure noble gas bubbles turn on to their steady state values on a much shorter time scale (say less than 0.2 s). During the transient period light from an air bubble is weaker than light from an Argon bubble but in the steady state the air bubble is brighter. In view of the long time scale required for the establishment of sonoluminescence from a single bubble of air it is concluded that this is a fundamentally different phenomenon from the transient multibubble sonoluminescence that has been studied since its discovery in 1934. [Work supported by the U.S. DOE Division of Advanced Energy Projects.]
Searching for an isotope effect in sonoluminescence
Authors: Hiller Robert A. , Putterman Seth
Published: October 2005

The only pure liquids in which sonoluminescence from a single stable bubble has been observed are water and heavy water. With regard to the content of the trapped bubble there are a number of gases which yield light. Helium is particularly interesting because its spectrum is strongly peaked in the far ultraviolet. In order to learn about the mechanism responsible for sonoluminescence, the search is on for differences between the spectra of He4 and He3 bubbles in water and heavy water. Other isotope pairs to be compared include hydrogen and deuterium. [Research supported by the U.S. DOE Division of Advanced Energy Projects.]
Sonoluminescence from large plasma-induced vapor bubbles
Authors: Hess Gary R. , Loree Ellis L. , Wieting James D. , White Forest E. , Young Chris M.
Published: October 2005

The light emitted during the collapse of large vapor bubbles has been measured. The emission lasts for tens of microseconds and is so intense it must be filtered, under our experimental conditions, to record the complete time-dependent behavior. The majority of the energy appears to be emitted well below 630 nm. Such a long duration emission appears to be more consistent with adiabatic heating than shock phenomena. The temporal shape of the emission compares well with the temporal shape of the pressure produced prior to and after collapse, as well. Such bubbles promote study of the collapse processes in regions of parameter space which are highly accessible. [Work supported by the U.S. Navy.]
Experimental determinations of transient conditions during cavitation
Authors: Suslick Kenneth S. , Kemper Kathleen A.
Published: October 2005

The collapse of cavitation bubbles generates intense local energy release, resulting in extreme local heating, high local pressures, and high energy chemistry. Determining the local conditions formed during cavitation, however, has proved to be a difficult problem. In our work, sonochemical reactions and the sonoluminescence that results have been used as quantitative probes of both temperature and pressure during cavitation. The effective temperatures reached during cavitation can be probed by measurement of the relative rates of a series of chemical reactions whose temperature dependence is already known. Comparative rate thermometry of multibubble cavitation (immersion horn at 20 kHz and ≊30 W/cm2) yields effective temperatures of ≊5200 K in hydrocarbons [K. S. Suslick, Science 247, 1439 (1990)]. Consistent with this, sonoluminescence in these liquids closely resembles flame emission from excited states of C2; rotational and vibrational fine structure permits a spectroscopic determination of the emission temperature of C2 excited states at 5080±160 K [E. B. Flint and K. S. Suslick, Science 253, 1397 (1991)]. Sonoluminescence from excited state metal atoms is produced from sonolysis of organometallic compounds, for which the linewidth determines the collisional lifetimes of emitting atoms and the effective local pressures. For excited state Cr atoms produced from Cr(CO)6, emission lifetime is only 0.20 ps, corresponding to local densities of ≊0.15 g/cm3 or pressures of ≊1.5 kBar at 5000 K. [Supported by NSF.]
Hydrodynamic simulations of bubble collapse and picosecond sonoluminescence
Authors: Moss William C. , Clarke Douglas B. , White John W. , Young David A.
Published: October 2005

Numerical hydrodynamic simulations of the growth and collapse of a 10-μm air bubble in water were performed. Both the air and the water are treated as compressible fluids. The calculations show that the collapse is nearly isentropic until the final 10 ns, after which a strong spherically converging shock wave evolves and creates enormous temperatures and pressures in the inner 0.02 μm of the bubble. The reflection of the shock from the center of the bubble produces a diverging shock wave that quenches the high temperatures (≳30 eV) and pressures in less than 10 ps (FWHM). The picosecond pulse widths are due primarily to spherical convergence/divergence and nonlinear stiffening of the air equation of state that occurs at high pressures. The peak temperature at the center of the bubble is affected strongly by the ionization model used for the air. The results are consistent with recent measurements of sonoluminescence that had optical pulse widths less than 50 ps and 30-mW peak radiated power in the visible. [This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.]
Is sonoluminescence collision-induced emission?
Authors: Frommhold Lothar , Atchley Anthony A.
Published: October 2005

An estimate is attempted of the collision-induced emission (CIE) intensity and spectral profile in the visible and near UV region of the spectrum of N2–χ pairs, where χ represents another N2 molecule or an argon atom, etc., for conditions that correspond to shock waves believed to exist in sonoluminescence experiments. Calculated profiles consist of superimposed high overtone bands and resemble measured profiles of sonoluminescence spectra. The intensities calculated on the basis of a few, simple assumptions concerning the induced dipole surface compare favorably with measurements. The agreement obtained suggests that collision-induced emission is a viable or even an attractive alternative to bremsstrahlung to explain sonoluminescence. According to the theory presented, the CIE source is optically thin so that the spectral emission profile is not related to Planck’s radiation law.
Energy flux vector field in an ensonified elastodynamic sphere
Authors: Dean Cleon E. , Werby Michael F.
Published: October 2005

Recent interest has been shown in the energy flux density inside an ensonified elastodynamic scatterer. A spherical model was chosen as the simplest and most general geometry to analyze. To properly model the energy flow in an elastodynamic medium, it is necessary to employ the elastodynamic equivalent of the Poynting vector. Plots of the vector field of the elastodynamic intensity are presented with an eye toward the classification of the type of resonance behavior displayed. Analogies with other resonance phenomena in related geometries suggest the possibility of fruitful comparison with resonance modes in shells and plates. [Work in this area supported by ONR/NRL and by ASEE/ONT Summer Fellowship Program.]
Acoustic scattering from a fluid-loaded elastic plate with a distributed mass inhomogeneity
Authors: Cuschieri J. M. , Feit D.
Published: October 2005

The scattering of a plane acoustic wave by a fluid-loaded, thin, elastic plate, of infinite extent, with a distributed inhomogeneity is investigated by solving the equation of motion in the wave-number domain using the hybrid numerical analytical approach [J. Acoust. Soc. Am. 95, 1998–2005 (1994)]. The presence of the distributed inhomogeneity in the equation of motion, when expressed in the wave-number domain, results in a Fredholm integral equation of the third kind. By substituting for the product of the response and the plate characteristic equation, the Fredholm integral of the third kind is reduced to a Fredholm integral of the second kind. The plate response in the wave-number domain is obtained from the solution of the Fredholm integral. Inverse Fourier transforming the wave-number domain response function gives the spatial domain solution for the response. The hybrid approach is used to perform the inverse Fourier transform. The scattered pressure is obtained in a similar manner. Response and scattered pressure results for distributed mass inhomogeneities, with different distribution functions, are presented and compared to the results for a line inhomogeneity. [Work sponsored by ONR.]
Approximate closed-form analysis of modal coupling for radiation and scattering from fluid loaded structures
Authors: Franzoni Linda P. , Bliss Donald B.
Published: October 2005

Modal analysis is often used in problems involving radiation and scattering from fluid loaded structures. The modal formulation leads to a coupled system of equations for the modal amplitudes. Typically, structural modes are coupled through the effects of the fluid. In practice, modal coupling is sometimes assumed unimportant, enabling modal coefficients to be solved independently in closed form. This approach is particularly appealing when subsequent analytical treatment of expressions is desirable. The validity of this approach, including the order of the error involved in neglecting coupling is discussed. It is shown that the principal effects of weak modal coupling can be incorporated in a simple way without solving the fully coupled system. An approximate closed-form solution for weakly coupled systems of equations is developed. The approximation is found to work well, even in systems where coupling is fairly strong. The approximate form gives physical insight into the dominant effects of modal coupling, and shows that neglecting coupling may lead to significant errors. The effects of coupling are assessed by solution of model problem involving acoustic scattering from a fluid loaded plate. Solutions are compared for three methods: fully coupled, uncoupled, and the approximate analysis. [Work supported by ONR.]
Practical issues in the use of a frequency-domain delay estimator for microphone-array applications
Authors: Adcock John , DiBiase Joe , Brandstein Michael , Silverman Harvey F.
Published: October 2005

A frequency-domain delay estimator has been used as the basis of a microphone-array talker location and beamforming system [M. S. Brandstein and H. F. Silverman, Techn. Rep. LEMS-116 (1993)]. While the estimator has advantages over previously employed correlation-based delay estimation methods [H. F. Silverman and S. E. Kirtman, Comput. Speech Lang. 6, 129–152 (1990)], including a shorter analysis window and greater accuracy at lower computational cost, it has the disadvantage that since delays between microphone pairs are estimated independently of one another, there is nothing to ensure that a set of estimated delays corresponds to a single location. This not only introduces errors in talker location but degrades the performance of the beamformer. A method for delay estimation and talker location with a microphone array is described that preserves the low computational complexity and rapid tracking ability of the frequency-domain delay estimator, while improving the coherence and stability of the estimated delays and derived source locations. Experimental results using data from a real 16-element array are presented to demonstrate the performance of the algorithms. [Early work principally funded by DARPA/NSF Grant IRI-8901882, and current work by NSF Grant No. 9314625.]
A constant-directivity beamforming microphone array
Authors: Elko Gary W. , Chou Thomas C. , Lustberg Robert J. , Goodwin Michael M.
Published: October 2005

The quality of audio teleconferencing in large rooms and noisy environments can be increased with the use of steerable directional microphone arrays. A minimum bandwidth of 4 oct is required to faithfully transmit the speech signal. In a typical teleconferencing arrangement, only discrete angular directions are of interest and therefore the microphone steering directions are quantized. A standard delay-sum beamformer can result in noticeable frequency response changes as the talker moves between these steering locations. In an effort to mitigate this problem, a broadband constant-directivity beamformer has been designed and constructed. A few of the algorithms developed in this work will be discussed and compared to existing techniques. Basically, the solution revolves around the design of FIR filters that are inserted in the delay-sum beamformer after each element. A constant-beamwidth 4 oct steerable linear array microphone using directional elements will be described. A real-time implementation utilizing multiple AT&T DSP3210 digital signal processors is also described.
Hands-free mobile telephony by means of an adaptive microphone array
Authors: Nordholm Sven , Claesson Ingvar , Nordebo Sven , Dahl Mattias
Published: October 2005

By employing speech generation models and new algorithms more and more apriori information about speech signals is utilized in speech recognition and speech coding. A fair signal-to-noise ratio is therefore required to ensure that the apriori information is correct. This implies a need for noise reduction under adverse conditions, such as hands-free operation of telephones in the car compartment or speech recognition in cars [S. Nordholm etal., ‘‘Adaptive Array Noise Suppression of Handsfree Speaker Input in Cars,’’ IEEE Trans. Veh. Tech. 42, 514–518 (1993)]. The paper presents two adaptive microphone array schemes, aimed for this situation. The first, denoted spatial filtering generalized sidelobe canceller (SFGSC), gives good noise suppression with little distortion of the speech but requires careful calibration. The second, denoted adaptive microphone array employing calibration signals recorded on-site (AMAEC), facilitates a simple built-in calibration. It is beneficial from a user point of view to use a calibration signal recorded on site eliminating amplifier tuning and microphone selection. The calibration can be done within 60 s. The AMAEC calibrates the array to the speakers’ location, microphone positions and lobe gains, amplifiers, and to the acoustic environment in the car. No apriori information about signal statistics or array geometry is utilized. [Work supported by Nutek.]
Robust hands-free speech recognition
Authors: Lin Qiguang , Che Chi Wei , Flanagan James
Published: October 2005

When speech recognition technology moves from the laboratory to real-world applications, there is increasing need for robustness. This paper describes a system of microphone arrays and neural networks (MANN) for robust hands-free speech recognition. MANN has the advantage that existing speech recognition systems can directly be deployed in practical adverse environments where distant-talking sound pickup is required. No retraining nor modification of the recognizers is necessary. MANN consists of two synergistic components: (1) signal enhancement by microphone arrays and (2) feature adaptation by neural network computing. High-quality sound capture by the microphone array enables successful feature adaptation by the neural network to mitigate environmental interference. Through neural network computation, a matched training and testing condition is approximated which typically elevates performance of speech recognition. Both computer-simulated and real-room speech input are used to evaluate the capability of MANN. Measurements of isolated-word recognition in noisy, reverberant, and distant-talking conditions show that MANN leads to a word recognition accuracy which is within 4%–6% of that obtained under a close-talking condition in quiet.
Wideband microphone array for hearing aid preprocessing
Authors: Yao Kung , Soli Sigfrid D. , Korompis Dan
Published: October 2005

Speech communication in environments with low signal/noise ratios (SNRs) is a primary complaint of the hearing impaired. Microphone beam formation techniques provide an effective approach to improving SNR in these environments. A novel, fixed microphone array is being developed with user-controlled mainlobe spatial look direction and attenuation band(s), and with a flat frequency response over the speech bandwidth. The array of R microphones and L taps per microphone maximizes energy concentration over a spatial look region and frequency band, subject to spatial and frequency constraints. Constrained maximization of w*Aw/w*Bw is required, where A and B are matrices specifying spatial and frequency factors, and w is the RL dimensional weight vector. The constraining subspace is specified by the array values, derivative values, and spatial directional constraints; w is obtained as the solution of a tractable unconstrained full-rank lower dimensional generalized eigenvalue problem. Numerical and simulation results for different values of R and L and for different bandwidths will be reported, as well as results of preliminary listening tests with normally hearing and hearing impaired individuals. The feasibility of real-time acoustic beamformers with arrays for hearing aids, and the advantages of this scheme over conventional adaptive schemes will also be discussed.
Noise regulations in Mexico
Authors: Elizondo Fernando J.
Published: October 2005

An overview of the regulations on noise in Mexico is presented. Included are historic development, classification, categorization, contents, and sources. Information on how to update and obtain the laws and regulations is also mentioned.
Noise levels in Mexican industry
Authors: Ortiz Garcia Juan Antonio
Published: October 2005

Since 1985, labor regulations on noise exposure have existed, and as in other countries, the quantity employed is the A-weighted sound-pressure level. Maximum allowable exposure levels were taken from ISO-1999-1975. According to local statistics of professional illness, hearing loss occupies second place in importance. The measurement results of A-weighted sound-pressure levels in different types of Mexican industries, from 1990 to 1993, are described. These measurements were made in concordance with official methodology. The results showed that less than 50% of a sampled population stayed in a noisy ambient (continuous or fluctuating) with levels higher than 90 dBA. It is considered that A weighted sound-pressure level is not enough to properly describe noise at the workplace; due to high level values usually encountered at low frequencies, which, even though they are not normally audible, may cause workers’ low performance, beside other biological and psychological effects. Frequency analysis results of the same industries and how they are employed for the calculation of other quantities, like loudness and noisiness, are also described.
Sonoluminescence
Authors: Crum Lawrence A. , Cordry Sean
Published: October 2005

In 1896, Henri Becquerel discovered that a uranium salt could darken a photographic plate, and from this effect went on to discover radioactivity. In 1934, Frenzel and Schultes discovered that acoustic waves, generated in a water bath, could also darken a photographic plate. They attributed this effect to luminescence from the sound field, a phenomenon that would later be known as sonoluminescence. A sound field by itself does not cause luminescence; it is through the mechanism of acoustic cavitation, whereby microscopic gas bubbles are caused to oscillate so violently that a remarkable energy concentration occurs. Under certain conditions, the cavitation event is transitory, and self-destructs within a few acoustic cycles; under other conditions, a stable bubble will luminesce for extended periods of time. This initial presentation will serve as a general review of the phenomenon of acoustic cavitation and demonstrate how bubble dynamics leads to many of the observed effects. Later papers in this session will provide more detail of various aspects of the general phenomenon. [Work supported in part by the Office of Naval Research.]
The investigation of millimeter scale heterogeneity in Coastal Benthic Boundary Layer sediments using microresistivity and x-ray imaging of ‘‘diver’’ cores
Authors: Jackson Peter D. , Briggs Kevin B. , Flint Robert , Lovell Michael A. , Harvey Peter K.
Published: October 2005

A micro-resistivity imaging technique, developed for use on 70-mm ‘‘half-round’’ slabbed core has been adapted for use with larger ‘‘diver’’ cores, 33 mm thick, 350 mm wide, and 440 mm long. This resistivity technique is shown to have a resolution of 5 mm and to operate in a manner which is complimentary to x-ray photography. Theoretical and practical examples are presented showing responses to individual heterogeneity such as shells. The benefits of 3-D investigations are explored via numerical models are shown to be beneficial even in cores such as those above having a ‘‘flat’’ aspect ratio. The microresistivity method is shown to be particularly sensitive to layered structures and the presence of shells within the very high porosity surface layers in Eckernforde Bay (89% porosity 0–10 cm below sea floor). [Work supported by U.S. Navy.]
Correlation functions for sediment acoustic properties
Authors: Briggs Kevin B.
Published: October 2005

Models for sediment volume scattering require knowledge of the correlation functions for sediment sound velocity and density (or porosity). High-resolution vertical correlation functions of these properties have been estimated using sediment porosity and compressional wave velocity data measured vertically at regular, closely spaced intervals from diver-collected core samples. Because the data series are truncated owing to a limited core length, the correlation functions were estimated using a first-order autoregressive model and autocorrelations were calculated with Burg’s algorithm. Correlation length estimates were from a variety of shallow-water sites. Relationships between grain size and correlation length are discussed.
Microstructural modeling of clay sediments in Eckernfoerde Bay
Authors: Kolle Jack J. , Mueller Alan C. , Dvorkin Jack
Published: October 2005

A finite-element microstructure analysis code was used to model the geoacoustic properties of shallow marine clay sediments in Eckernfoerde Bay which has been the subject of an intensive geoacoustic and geotechnical study (Coastal Benthic Boundary Layer—Special Research Program). Uniaxial strain and pure shear were applied to two-phase microstructure models with generalized, periodic plane-strain boundary conditions to generate anisotropic bulk and shear moduli. Analyses were carried out on two length scales: clay microfabric (μm) and gas-filled inclusions (mm). The clay microstructure was modeled as a random lattice of Illite platelets with a density and porosity equivalent to average values observed in shallow Eckernfoerde Bay sediments. Saturated and dry framework moduli were calculated and the effective medium velocities were compared with observed velocities. The analysis suggests that shear wave propagation in these sediments is not supported by the framework structure but by differential compression of fluid-filled pores. Radiographic images of pressurized cores containing gas-filled inclusions with dimensions of a few millimeters were also analyzed. The analysis indicates a shear wave velocity anomaly related to the inclusion structure. The calculated effective medium velocities are compared with analytical models of spheroidal inclusions. [Work supported by ONR.]
Effect of forward-scattered signal on beam noise estimation in shallow water
Authors: Lee Yung P. , Hanna John
Published: October 2005

In deterministic situations, adaptive processors resolve strong coherent interferers and eliminate their contribution at locations away from them. But, in operation, it is found that the beam noise away from strong interferers is affected by their presence. Stochastic mechanisms involved with the interferer’s signal are assumed to be responsible for this behavior. A comparison of ocean data with a simulation has been made to illustrate the effect of the forward-scattered signal from the ocean surface on adaptive beam noise estimation in shallow water. The stochastic forward-scattering mechanism is dependent on the environment and its effect is apparently influenced by undersampling in the spatial aperture of the measurement array. The dependency of beam noise on waveheight and bathymetry have been observed.
Adaptation of genetic-algorithm search for matched-field inversion of ocean bottom compressional wave speed profiles
Authors: Hayward Thomas J. , Yang T. C.
Published: October 2005

Matched-field inversion for ocean bottom compressional wave speeds using a synthetic horizontal aperture has been demonstrated previously in simulations and data analyses employing optimization by simulated annealing [M. Collins, W. Kuperman, and H. Schmidt, J. Acoust. Soc. Am. 92, 2770 (1992)] and genetic-algorithm search [P. Gerstoft, J. Acoust. Soc. Am. 95, 770 (1994)]. The first reference noted a lack of sensitivity of the matched-field ambiguity function to zero-mean functional variations of the sound-speed profile. This lack of sensitivity sometimes results in slow convergence of the search algorithm when applied to profiles with several unknown parameters. This problem is observed in simulations using genetic algorithm search. Improved convergence of the genetic algorithm is then achieved by using some nonstandard procedures, including autologous gene exchanges, to generate new solutions. The simulations indicate the practical feasibility of inversion for deep-bottom (up to 500 m) compressional wave speeds at a shallow-water site using a single receiver and coherent synthetic horizontal aperture processing of a towed-source signal.
Optimizing shallow-water sound-speed estimation using parameter resolution bounds
Authors: Makris Nicholas C.
Published: October 2005

A technique is under development to optimize experimental design for estimation of 3-D sound-speed structure by inversion of acoustic data. The motivation is to take advantage of apriori knowledge of invariant environmental parameters to estimate the minimum number of sensors necessary and their optimal deployment geometry for a well constrained inversion. First, static environmental information, such as bathymetry, geoacoustic parameters of the sediment, and mean sound-speed structure of the water column, is input to an appropriate range-dependent acoustic model for a given sensor deployment geometry. Next, a theoretical lower bound on estimation error for the water-column sound-speed structure is obtained via the Cramer–Rao bound. The deployment geometry is then perturbed until the estimation error is within acceptable bounds for oceanographic and acoustic modeling. However, the choice of sound-speed parametrization can also severely affect the accuracy of an inversion. For example, an empirical orthogonal function (EOF) representation typically has higher resolution for fewer parameters than a discrete cell representation. But this is at the cost of more limiting assumptions. These issues are addressed by computing the theoretical lower bound on estimation error for discrete cell, EOF, and Fourier internal wave representations of 3-D sound-speed structure.
Coherent acoustic bubble sizing in the ocean
Authors: Choi B. K. , Lee H. R. , Yoon S. W.
Published: October 2005

Bubbles are well known to induce variations of acoustic characteristics in water. The bubble size distribution gives significant information for understanding underwater sound propagation and acoustic roles of bubbles in the ocean. Since the conventional acoustic bubble sizing method only considered sound attenuation due to bubbles around resonance frequencies in a bubbly mixture, a bubble size distribution was incoherently estimated. In the present coherent acoustic bubble sizing method, sound speed as well as sound attenuation are considered to have a coherent estimation of bubble size distribution in the bubbly mixture. The present coherent method is proved to describe a bubble size distribution more accurately than the conventional incoherent one. [Work supported by the Korea Science and Engineering Foundation.]
Acoustic characterization of laboratory breaking waves in fresh and salt water
Authors: Kolaini Ali R.
Published: October 2005

Results of a laboratory experiment to characterize the underwater acoustic noise radiated from breaking waves in fresh and salt water are described. The underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz were reported earlier [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. (in press); 92, 2349(A) (1992)]. These waves were generated by a computer-controlled plunging-type wavemaker and propagated along a 12.7-m-long channel where they were made to break at the midsurface of a 3.6-×3.6-×2.4-m anechoic tank. Specific attention is given to a comparison of the results obtained in salt and fresh water. The sources of acoustic radiation from bubble clouds, the average sound-pressure level, bubble clouds shape, bubble concentration, and size distribution in these medium are discussed. These results may provide considerable insight into the likely source mechanisms for ocean ambient noise.
Forward-scattering simulations through ocean-ridge hydrothermal-vent diffusive layer turbulence: Feasibility of vent monitoring
Authors: Duda T. F. , Trivett D. A.
Published: October 2005

Temperature and velocity fluctuations in a bottom boundary layer down-current of Juan de Fuca Ridge hydrothermal sites were monitored in 1990 from tripods. The diffuse flow from relatively cool vents remains near the bottom, and acoustic refractive index fluctuations decay away slowly. Rytov theory-based forward-scattering simulations through structures consistent with the observations are used to verify the validity of Rytov theory, compare the usefulness of various propagation path lengths and frequencies on monitoring, and evaluate the ability of acoustic phase and amplitude recordings to monitor desired vent parameters. The Rytov theory is valid for some ranges and frequencies, but issues such as turbulence homogeneity, energy dissipation rate, boundary-layer depth, beam patterns, and sensor motion would have strong effects on the measurements.
Performance limits on acoustic thermometry of ocean climate in the presence of mesoscale sound-speed variability
Authors: Krolik Jeffrey L. , Narasimhan Sunil
Published: October 2005

The ability to measure climatic changes in ocean temperature is fundamentally limited by the presence of mesoscale variability. Because ocean acoustic propagation depends on the range-averaged sound-speed (and hence temperature) profile, long-range acoustic transmissions have been proposed as a means of filtering out mesoscale variability in order to measure a global warming related trend in mean temperature. The Cramer–Rao lower bound (CRLB) on the estimation of a change in the mean depth-dependent temperature profile is presented to determine the highest accuracy which could be achieved by acoustic thermometry. This work extends [A. B. Baggeroer, J. Acoust. Soc. Am. 95, 2850 (A) (1994)] by evaluating the CRLB for different representations of the mean depth-dependent temperature profile perturbation with different levels of apriori knowledge about the mesoscale sound-speed variability. With prior statistical knowledge of the mesoscale variability, the CRLB indicates that accurate measurement of the climate signal may be possible using a general Chebyshev polynomial representation of the mean depth-dependent temperature perturbation. [Work supported by ONR.]
High-frequency (≳1 cpd) travel time variability of long-range reciprocal acoustic transmissions in the western North Atlantic
Authors: Dushaw Brian D. , Worcester Peter F. , Cornuelle Bruce D. , Howe Bruce M.
Published: October 2005

Time series of ray travel times were obtained at 350-, 410-, and 670-km ranges in the western North Atlantic during the 1991–2 Acoustic Mid-Ocean Dynamics Experiment (AMODE). Transmissions were recorded for approximately 300 days between six transceivers in a pentagonal array. Sound-speed (current) variability is observed by calculating the sum (difference) of reciprocal travel times. The sum and difference of reciprocal travel times are high-pass filtered by removing a daily average. The barotropic-tide current is measured by the differential travel times. Both phase-locked and narrow-band internal-tide sound-speed variability, caused by the internal-tide isotach displacement, are observed by the sum travel times. The acoustic array acts as a high-directivity antenna for the incident internal tide. The observed internal tide is likely generated at the continental shelf surrounding the North American Basin, or perhaps the Mid-Atlantic Ridge. The nontidal, high-frequency variability (≳1 cpd) is due to internal-wave sound-speed and current variability.
The Kaneohe acoustic thermometry experiment and waves
Authors: Tappert Fred D. , Spiesberger John L.
Published: October 2005

A new parabolic approximation is derived and implemented that efficiently gives travel time predictions that are accurate to within a few milliseconds at a range of 1000 km and center frequency 75 Hz. Sensitivity of the full-wave travel time predictions to oceanographic and bathymetric inputs along a 3700-km track is studied by comparisons to acoustic data between a source at Kaneohe having center frequency 133 Hz and a bottom mounted receiver near the northern California coast. Path stability for a time interval of several years is also examined with the new broadband PE model, and full-wave generalizations of eigenrays, called eigentubes, are computed. [Work supported by SERDP, ARPA, and ONR.]
Change of sound ray by various equations of the sound velocity in the 621-km propagation experiment
Authors: Tsuchiya Toshio , Nakanishi Toshiyuki , Amitani Yasutaka , Ochi Hiroshi , Fujimori Hidetoshi , Hasegawa Akio
Published: October 2005

Generally, sound velocity used to be calculated according to equations by Wilson (1960), DelGrosso (1974), Chen and Millero (1977), and others, which were obtained by direct measurement in laboratories based on water depth, temperature, and salinity data collected by the XBT (expendable Bathey-tharmograph) or CTD (conductivity temperature depth) system. However, in observations such as ocean acoustic tomography (OAT) where it is necessary to make accurate measurement of the sound wave propagation time over the distance of hundreds to thousands of kilometers, even the slightest difference of the sound velocity can produce significant errors in the observatory accuracy. Thus the effect of various equations of the sound velocity on the formation of the specific sound ray in the SOFAR channel was examined by using the simulation method by the sound ray theory. The simulation results were compared with the specific sound ray data obtained in the 621-km propagation experiment in the northeast Pacific Ocean, and as a result, the equation by Chen and Millero is found to be the most consistent among the three equations.
Long-range propagation tests of a 200-Hz sound source for ocean acoustic tomography
Authors: Nakamura Toshiaki , Tsuchiya Toshio , Nakano Iwao , Nakanishi Toshiyuki , Kaihou Ieharu , Kaya Akio , Kikuchi Toshiaki
Published: October 2005

A 200-Hz low-frequency sound source for ocean acoustic tomography was developed by using a supermagnetostrictive material made of the rare-earth metal (Tb, Dy)Fe2 aimed at increasing the transmission level and the reduction of the sound source size. A sound-pressure level above 190 dB (re: 1 μPa at 1 m) with a frequency bandwidth of over 50 Hz were obtained in the depth range between 800 and 1200 m. Field tests for the long-range propagation of several hundreds of kilometers were carried out near the Japan Trench in November 1993. The sound source was moored at a depth of 1242 m and the hydrophone array was moored at a depth of 1385 m. The distance between the sound source and the hydrophone array was 621 km. Received data were recorded with a sufficient signal-to-noise ratio after achieving beamforming to discriminate the sound waves incident upon the hydrophone array from the upper and lower direction. The signals were also received by the acoustic monitoring system suspended from the ship at the distance of 772 km from the sound source. From these results, it is confirmed that this sound source has the capability of 1000-km propagation necessary for the tomography transceiver systems.
Turbofan noise research at NASA
Authors: Groeneweg John F.
Published: October 2005

Results of recent NASA research to reduce aircraft turbofan noise are described. For very high bypass ratio turbofan engines, the dominant source of engine noise is the fan. A primary mechanism of tone noise generation is the rotor blade wakes interacting with downstream stator vanes. Methods of analyzing rotor–stator tone noise generation are described and sample results are given. The analysis includes the unsteady aerodynamic response of the stators to gusts, coupling to duct modes, and finite element calculation of the far-field radiation accounting of for flow in the fan ducts. Wind tunnel tests of model fans and nacelles are described including comparisons between measured and predicted tone directivities. A novel rotating microphone technique is used to measure the acoustic modes in the fan inlet, and the results indicate that a mechanism associated with unsteady fan tip loading can be important in addition to rotor–stator interaction. Finally, concepts for active fan noise control which emphasize control at the source are addressed by an unsteady aerodynamic analysis of compliant stator vanes.
Efficient sound generation in a ducted axial fan due to a rotating wavelike flow pattern
Authors: Krane M. H. , Bent P. H. , Quinlan D. A.
Published: October 2005

Previous investigations have shown the presence of narrow-band peaks in the acoustic spectra of ducted axial fans at frequencies other than harmonics of the blade passage frequency. This phenomenon is of practical importance because these peaks are strongest when the fan is operating near its best efficiency point. Measurements of the fan flowfield were conducted in the rotating frame using hot-wires and surface-mounted hot-film sensors in order to identify the source of the narrow-band tones. These measurements have shown a rotating wavelike motion composed of several discrete modes. The wave pattern was strongest on the pressure side of the blade in the region between the tip and the midspan. One of the modes of the rotating pattern was found to be an integer multiple of the number of fan blades, resulting in efficient sound generation at that frequency. a)Present address: Acoustics Technology, McDonnell Douglas Aerospace, Mail Code 71-35, 1510 Hughes Way, Long Beach, CA 90810-1864.
A modified transfer function method for source impedance in ducts
Authors: Prasad M. G. , Kim W. S.
Published: October 2005

Experimental characterization studies of source impedance have resulted in several direct and indirect methods. The transfer function (TF) method, which is a well-known direct method, has been quite successful in measuring source impedance. However, one important drawback of the TF method is that it fails for the case of low signal-to-noise ratio in the system. The noise in the system could be due either to the operating source (to be characterized) or to the flow in the system. A novel method to overcome the problem due to low signal-to-noise ratio is presented. A modified transfer function (MTF) is determined by using an additional measurement of transfer function of the system with a very low level of signal of the secondary source used in the method. Then the two transfer functions are used to estimate the MTF. The source impedance is then calculated using the reflection coefficient which is obtained using the MTF. This new MTF method has given very good results on a model electroacoustic speaker-duct system. A modified coherency function is used for error analysis. This MTF method can be used to measure the impedance of sources such as fans and blowers used in cooling systems.