bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of the low-frequency sound produced by 2-D and 3-D breaking waves.
Authors: Loewen M. R. , Melville W. K.
Published: October 2005

Laboratory measurements of the sound beneath both 2-D and 3-D breaking waves are described. Sound at frequencies as low as 20 Hz was observed beneath the 2-D breakers and the mean-square acoustic pressure in the frequency band from 20 Hz–1 kHz correlated strongly with the wave slope and dissipation. The sound spectra observed beneath the 3-D breakers showed significant increases in level across the entire bandwidth from 10 Hz to 20 kHz and the spectra sloped at −5 to −6 dB per octave at frequencies greater than 1 kHz. The mean-square acoustic pressure in the frequency band from 10 to 150 Hz correlated with the wave amplitude similar to the results obtained in the two-dimensional breaking experiments. Large-amplitude low-frequency spectral peaks from 15 to 275 Hz were observed in the spectra of the sound from the 2-D and 3-D breaking waves. It is postulated that these low-frequency signals are due to the collective oscillation of the bubble clouds. Void fraction measurements were available for six of the breaking events (Lamarre and Melville, 1992, this meeting) and therefore the geometry and sound speed inside the bubble clouds were known. Using this information estimates of the resonant frequencies of the bubble clouds were made and these estimates were found to agree closely with the observed frequencies. The close agreement supports the hypothesis that the observed low-frequency signals were produced by the collective oscillation of the bubble clouds. [Work supported by ONR (Ocean Acoustics).]
Active structural acoustic control with piezoelectric actuators and PVDF acoustic near-field sensors.
Authors: Clark Robert L. , Fuller Chris R.
Published: October 2005

Acoustic near-field sensors were designed from polyvinylidene fluoride (PVDF) and were implemented in the acoustic near field of a baffled simply supported plate as error sensors for active control of structure-borne sound. Sensor position was determined from nonlinear optimal design techniques as were the optimal positions of the piezoelectric control actuators. Both analytical and experimental results were obtained and compared to evaluate the sensor design as well as the control implementation for off-resonance excitation of the plate. Microphones were implemented as error sensors in the narrow-band cost function of the LMS control algorithm to provide a basis for comparison with the acoustic near-field sensors. The acoustic response of the structure was attenuated by 20 dB when implementing the acoustic near-field sensors, which was the same level of attenuation observed when implementing microphone error sensors in the radiated field. [Work supported by ONR/DARPA.]
Comparison of diffraction effects from an unfocused and focused circular transducer.
Authors: Jiang Peng , Apfel Robert E.
Published: October 2005

A simple theoretical model is used to study the diffraction effect of a pistonlike transmitter as well as a focused transmitter for receivers of different diameter in a transmission system used to measure the acoustic properties of a liquid, e.g., sound speed and attenuation. Numerical calculations show that, as one would expect, the diffraction effect from a focused transducer is smaller than that from an unfocused transmitter, and when the diameter of the receiver is large enough (more than twice that of the transmitter), the received pressure from a focused transmitter has approximately a plane wave character, e.g., constant amplitude and linear phase over a large range of the radiation field. The same cannot be said for an unfocused transducer due to phase problems. Results of experiments are presented in support of the theoretical study. [Work supported by the National Institutes of Health through Grant 5R01CA39374.]
Enhanced backscattering from bubble cloud distributions on the ocean surface.
Authors: Prosperetti Andrea , Sarkar Kausik
Published: October 2005

It has been shown in earlier studies [Prosperetti, Lu, and Kim; Sarkar and Prosperetti, both submitted to J. Acoust. Soc. Am.] that bubble clouds produced by breaking waves at the ocean’s surface can explain the unexpectedly high backscattering levels observed experimentally by Chapman and Harris [J. Acoust. Soc. Am. 34, 1592 (1962)] and others at low grazing angles. Gas volume fractions of the order of 1%, linear dimensions of the order of 1 m, and surface coverage of the order of 1% (the latter of which agrees with the experimentally measured values for 10 m/s winds) are sufficient to give an excellent match of the data as a function of frequency in the range 0.1–2 kHz and wind speeds from 5 to 25 m/s. In the previous work the clouds were treated as independent scatterers. In the present work the previous results are refined to include lowest-order multiple scattering effects along the lines of Foldy [Phys. Rev. 67, 107 (1945)] and Biot [J. Acoust. Soc. Am. 44, 1616 (1968)]. [Work supported by ONR.]
The proposed Knight Inlet surface backscattering experiment.
Authors: Ewart T. , Farmer D. , Henyey F. , Jackson D. , Kaczkowski P. , Olson L. , Thorsos E.
Published: October 2005

An experiment is planned for the 93–95 time frame to measure surface backscattering at 400 Hz using a ‘‘pencil beam’’ source and receiver currently under development. The acoustics experiment will be conducted simultaneously with surface zone environmental measurements (notably, surface height statistics and bubble densities) contributed by various scientists in the air–sea acoustics program. Initial operation in the fjord allows the equipment to be installed at low wind speeds, and the apparatus can be connected by cable to the protected waters of a small bay. In this way operation of the equipment over a wide variety of wind conditions can be achieved safely prior to open ocean deployment. (High winds are common in the confined channel of the fjord.) As the principal goal, the apparatus will be used to determine the regime where rough surface scattering theory does not adequately describe the observations, and to determine the dominant mechanisms. In this scattering regime the measurements will focus on the relative importance of the various surface zone phenomena (bubbles in their different incarnations) predicted to yield an enhanced surface backscattering signature. The rationale and design of the experiment will be discussed. The experiment will provide the experience required before open ocean measurements can be undertaken.
Wave breaking, turbulence, and related similarity laws−A review of wind-wave research at Tohoku University.
Authors: Toba Y. , Kawamura H. , Ebuchi N.
Published: October 2005

Research on wind waves made by our group at Tohoku University is reviewed with emphasis on wave breaking, turbulence, and the underlying similarity laws. Detailed dynamical processes related to wind waves, including ordered motions and turbulence in the air and water, have been investigated in a tank, using quantitative flow visualization techniques combined with the direct measurement of turbulent flows. Conspicuous events, such as airflow separation, formation of a high-vorticity layer at the wave crest, wave breaking, and bursting in air and water are found in the vicinity of the water surface. They are related to one another to explain momentum transfer from the airflow and turbulence generation, which may be associated with sound generation at the water surface. Similarity laws exist, which correspond to the relationship between waves and turbulence below and above the interface, describing the gross nature of the wind-wave field. A parameter u2*/νσp is introduced to describe overall processes of wind-wave breaking, whitecapping, and drop production. It is suggested that this parameter may describe also the overall intensity of bubble formation in the sea.
Bubble plume echoes from convergence zone ranges.
Authors: McDonald B. E. , Orris G. J. , Collins Michael D.
Published: October 2005

Surface reverberation from bubble plumes at long ranges (one or two convergence zones) is modeled by combining a Born approximation scatter model [McDonald, J. Acoust. Soc. Am. 89, 617–622 (1990)], which has given agreement with surface backscatter data at close ranges (much less than a convergence zone), and an efficient method for coupling the scattering and propagation aspects of the problem [Collins and Werby, J. Acoust. Soc. Am. 85, 1895–1902 (1989)]. A signal from a point source near the surface is propagated through a realistically refractive ocean to convergence, where it incoherently insonifies a set of bubble plumes. These are represented as vertically aligned cylindrical patches containing a sound-speed defect that diminishes exponentially with depth. For low frequencies (on the order of 100 Hz), the hypothesis that backscatter is due almost entirely to deeply entrained bubbles, likely resulting from Langmuir circulation cells, is examined.
High-frequency surface scattering phenomena in high sea states.
Authors: Commander Kerry W.
Published: October 2005

The determination of acoustic scattering from a rough sea surface has received considerable attention in recent years. Knowledge of whitecap characteristics [J. Wu, IEEE J. Oceanic Eng. 17, 150–158 (1992)] has led to a better understanding of surface scattering phenomena [D. F. McCammon and S. T. McDaniel, IEEE J. Oceanic Eng. 15, 95–100 (1990)]. For this study a substantial amount of backscattering data was acquired with autonomous underwater vehicles running between 50 and 100 ft of the surface. These vehicles used a short pulse length sonar operating at 215 kHz. The use of a high-frequency sonar allows deep penetration of densely populated bubble plumes. Because of the high repetition rate, good spatial variation of backscattering strength from breaking waves has been obtained. Previous measurements by other scientists of bubble distribution functions in breaking waves allows an approximate conversion of scattering strength at the sonar frequency to most frequencies of interest. The database here consists of measurements made in deep water environments with seas ranging from sea state 0 to 3. The spatial variation of scattering strength due to the sea surface and bubble plumes from breaking waves is discussed as a function of wind speed.
Short pulse acoustic transmission through marine microbubbles.
Authors: Cowley Alastair , Pace Nicholas G. , Campbell Tony
Published: October 2005

When a bubble is excited at or near its natural frequency it is a very effective scatterer and absorber of sound. In fact, at resonance, scattering and absorption cross sections are of the order of 1000 times greater than the geometric cross section for typical marine microbubbles. As with any resonant system, a bubble takes a finite albeit short time to ring up to steady state oscillation and continues to oscillate for a finite time after the driving force ceases. A number of studies have been carried out to attempt to describe how acoustic absorption and backscatter depend upon the duration of the driving force. Most notable of these are the backscatter measurement for short pulses in near surface seawater [Akulichev etal., Sov. Phys. Acoust. 23, 177–180 (1986)] and the forward transmission laboratory measurements for short pulse lengths at 120 kHz [H. R. Suiter, J. Acoust. Soc. Am. 91, 1383–1387 (1992)]. Akulichev observed a decrease in acoustic backscatter for short pulses at three discrete frequencies, whereas Suiter found no corresponding enhancement in forward transmission at 120 kHz. This paper details a comprehensive study that measured acoustic attenuation through a well defined bubble cloud over the frequency range 20–200 kHz and for pulse lengths from 20 cycles down to a single cycle using a parametric transmitter. The experiment simulated the conditions for which a decrease in attenuation with decreasing pulse length might have been expected. No effect was observed for two different but well defined bubble distributions. [Work supported by DRA Maritime.]
Low-frequency backscatter from dense submerged bubble clouds.
Authors: Roy Ronald A. , Carey William , Nicholas Michael , Crum Lawrence A. , Schindall Jeff
Published: October 2005

The frequency-dependent characteristics of the acoustic backscatter from submerged bubble clouds was studied using both conventional and parametric sound sources. The clouds, all of which possessed a characteristic length scale of 0.5 m, a void fraction of order 10−3, and a mean bubble radius of approximately 1.5 mm, were generated 39.2 m beneath the surface of a freshwater lake (Lake Seneca, NY) using an array of hypodermic needles driven with pulsed flow of compressed air. Backscattering target strengths were measured for frequencies ranging from 300 Hz to 14 kHz. Results obtained using conventional and parametric sources were comparable, with target strengths falling in the +1 to −10-dB range. Several peaks were observed in the target strength versus frequency data. Low-frequency (less than 1 kHz) results exhibit elevated target strengths throughout, with evidence of a broad peak at approximately 500 Hz. Results obtained over the entire range of frequencies will be compared with theoretical predictions based on coherent, resonance scattering (≳1 kHz) and coherent, off-resonance scattering (<1 kHz) and collective bubble oscillations (<1 kHz). [Work supported by ONR, ONT, AEAS, and NUSC/IR.]
Using acoustic scattering (high frequency) to predict acoustic scattering (low frequency).
Authors: Henyey Frank S. , Farmer David , Vagle Svein
Published: October 2005

Models of low-frequency reverberation from tenuous bubble clouds have made assumptions about the geometry of the clouds. Some of those assumptions are replaced with properties remotely measured using high-frequency scattering. This approach is applied to the measurements made during the CST-7 experiment, which is the first experiment including both high- and low-frequency measurements. Also discussed is the use of ocean information obtained from the high-frequency scattering in addition to the geometry of the clouds to help infer low-frequency reverberation characteristics. [Work supported by the Office of Naval Research.]
A critique on the relationship between backscattering and environmental measurements.
Authors: Goodman R. R.
Published: October 2005

In the paper by Gilbert that was presented earlier in this session it was shown that, by the use of the first-order Born approximation and a plausible assumption about the vertical distribution of bubbles, backscattering strength measurements in several experiments yield horizontal wave-number power spectra for bubble densities of the form P(Kh)= AK−βh. The values of β were found to lie approximately between 3 and 4. Since a given value of β requires that the oceanographic processes produce a bubble distribution that obeys, on the average, a specific scaling invariance, it is interesting, then, to see if there are any correlations between the oceanographic/atmospheric observations and the values of β for the experiments that were analyzed. It is the purpose of this paper to examine the results of each of the experiments from the standpoint of acoustics and environmental observations in order to seek ‘‘clues’’ to the correlation of β with oceanographic/atmospheric observations. It will be clear that none of the experiments contained measurements that are sufficient to yield specific connections between the environmental measurements and the acoustics.
Starting transients of flue organ pipes in relation to pressure rise time
Authors: Nolle A. W. , Finch T. L.
Published: October 2005

A wind supply with adjustable pressure rise time is used to study the initial transients of adjustable stopped and open flue pipes which approximate classic voicing. The precursor, at several times the fundamental frequency, and a subsequent oscillation dominated by the next mode above the fundamental, are strongest when the pressure rise time is somewhere between 1 and 10 fundamental periods. These features may vanish for much longer or shorter rise times. Decreasing the inharmonicity of the resonances increases the next-mode content in the attack, as does reducing the cutup. External sound at the fundamental frequency, applied either at the pipe mouth or through the air supply, suppresses both the precursor and the subsequent next-mode burst. The precursor does not involve a longitudinal pipe resonance. Pipe frequencies were 198–256 Hz. Air supply pressure was 530 Pa. A computer simulation, based on an oscillator model with three resonant modes and time-delayed nonlinear feedback, incorporating ideas of N. H. Fletcher and others, accounts for major features of the experimental results. The suppression of the initial next-mode burst when the pressure rises slowly is caused by the extra time spent in a pressure range where the feedback loop gain (particularly the phase) for the next mode does not support self-oscillation. With sufficiently fast pressure rise, impulsive excitation favoring the fundamental enables it to dominate the attack, preventing the next-mode burst.
Perception of structured phantom targets in the echolocating bat, M
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Authors: Schmidt Sabine
Published: October 2005

Bats can discriminate among objects with different texture using echolocation. The mechanisms involved in texture discrimination are unknown. In real echoes reflected from three-dimensional objects, the spatial structure of the target creates a specific temporal and spectral pattern. In the experiments described here, simply structured phantom targets mimicking echoes reflected from an object with two parallel planes were generated by adding two differentially delayed copies of the bat’s vocalizations. The discrimination performance of Megaderma lyra was studied in a two-alternative, forced-choice experiment using phantom targets with different internal delays and relative amplitudes of the two copies. When the reference target to which the bats were trained was presented, targets differing in internal delay by about 1 μs were discriminated. However, discrimination performance was not a globally monotonic function of the internal delay of the unrewarded target. When both targets presented in a trial differed from the reference, the bats still preferred one of them. Changes in overall level of the phantom target echo had little effect on performance; however, performance dropped considerably if only one of the two copies in an echo was attenuated. A model framework is introduced to account for these results. Models based on frequency processing yield better approximations than does a model based on time differences. The model based on spectral correlation gives a unified description of all the data. Although time domain models for texture discrimination cannot be generally refuted, the results presented here show that the discrimination of target surface structure can be explained as a discrimination of echo spectra.
Comparison of the acoustic properties of six popular stethoscopes
Authors: Abella Manuel , Formolo John , Penney David G.
Published: October 2005

This study evaluated stethoscope acoustics by using a sound frequency generator and an active artificial ear. Six popular, currently available stethoscopes were compared in their various modes involving bells, diaphragms, etc.: Littmann Classic II, Littmann Cardiology II, Littmann Master Cardiology, Hewlett-Packard Rappaport-Sprague, Tycos Harvey Triple Head, and Allen Medical Series 5A RPS Binaural. The transfer function was measured from 37.5–1000 Hz, the range where nearly all heart and lung sounds are found. Sound in the low-frequency range (37.5–112.5 Hz) was in most cases amplified by the bells and attenuated by the diaphragms; however, there were no significant differences. Both bells and diaphragms attenuated sound transmission in the high range, and this increased with frequency. The Tycos Harvey Triple Head ribbed diaphragm attenuated sound transmission to a significantly greater extent than the other diaphragms (P<0.01). The results show that the bell and diaphragm for a given stethoscope usually have different transmission characteristics, particularly at low frequencies. The Littmann Classic II is an exception. The Hewlett–Packard and Tycos Harvey stethoscopes showed the greatest differences in low frequency response between the bell and the diaphragm. While the differences found in sound transmission between stethoscopes were in most cases small, the Littmann Cardiology II, bell and diaphragm, appears to possess the best overall performance by this study design.
Across frequency integration in a model of lateralization
Authors: Shackleton Trevor M. , Meddis Ray , Hewitt Michael J.
Published: October 2005

A computational model of binaural lateralization is described. An accurate model of the auditory periphery feeds a tonotopically organized multichannel cross-correlation mechanism. Lateralization predictions are made on the basis of the integrated activity across frequency channels. The model explicitly weights cross-correlation peaks closer to the center preferentially, and effectively weights information that is consistent across frequencies more heavily because they have a greater impact in the across frequency integration. This model is complementary to the weighted-image model of Stern et al. [J. Acoust. Soc. Am. 84, 156–165 (1988)], although the model described in this paper is physiologically more plausible, is simpler, and is more versatile in the range of input stimuli that are possible.
Fluctuations of high-frequency shallow-water seafloor reverberation
Authors: Stanic S. , Kennedy E.
Published: October 2005

High-frequency shallow-water reverberation statistics were measured from a seafloor that was covered by a thick layer of coarse shells. The reverberation statistics are presented as a function of frequency (20–180 kHz), grazing angle (8.8°–28°), and source beamwidths (1.2°–16°). In most cases the reverberation statistics did not follow a Rayleigh fading model. The model dependence of the reverberation statistics ranged from near Gaussian (high grazing angle) to beyond log normal (low grazing angle). The results suggest that small scale variations in bottom scattering properties and system beamwidth will significantly change the model dependence of the reverberation statistics.
Performance evaluation of frequency diverse Bayesian ultrasonic flaw detection
Authors: Saniie Jafar , Wang Tao , Jin Xiaome
Published: October 2005

The performance of an ultrasonic flaw detection system is valued by the success of differentiating the flaw echoes from those echoes scattered by microstructures (e.g., grains in polycrystalline metals). Microstructure noise (clutter) is stationary and will only vary when the frequency of the ultrasonic beam is changed. In this paper, frequency diverse microstructure information is obtained through the split-spectrum processing of broadband ultrasonic backscattered echoes. The statistical difference between clutter and flaw echoes over different frequency bands has led to the development of a Bayes classifier that is quadratic and can incorporate the correlation properties of scattered echoes. The performance of the Bayes classifier has been examined for both experimental and computer simulated data, and compared to other commonly used techniques such as mean, minimum, median, and polarity detectors. The Bayes classifier shows superior performance, and the flaw-to-clutter visibility has been improved in the range of 5–15 dB when the measured flaw-to-clutter ratio is 0 dB or less. These results confirm the feasibility of grain noise suppression by frequency diversity with a subsequent statistical processor for flaw detection.
Comparison of frequency selectivity and consonant recognition among hearing-impaired and masked normal-hearing listeners
Authors: Dubno Judy R. , Schaefer Amy B.
Published: October 2005

Frequency selectivity and consonant recognition were determined for normal-hearing and hearing-impaired listeners using techniques that facilitate comparisons of performance among listeners whose absolute thresholds vary in magnitude and configuration. First, for each of six subjects with cochlear hearing loss, masked thresholds in notched noise and narrow-band-noise maskers were obtained and compared to results for three normal-hearing listeners whose thresholds were precisely matched to the impaired listeners’ by masking with spectrally shaped broadband noise. Second, for hearing-impaired listeners and their masked normal-hearing controls, measurements of consonant recognition were obtained at several speech-presentation levels selected on the basis of articulation-index predictions to assure equal speech-spectrum audibility across listeners. The results suggest that frequency selectivity is poorer for hearing-impaired listeners than for masked normal-hearing listeners, even when thresholds among subjects are equated, but the deviation from normal frequency selectivity is smaller than estimated from comparisons with normal-hearing listeners in quiet. Critical ratios for hearing-impaired listeners are equivalent to normal. Although frequency selectivity is reduced, there is no consistent difference in consonant recognition between hearing-impaired subjects and masked normal-hearing subjects, when performance is assessed under conditions that assure equal speech-spectrum audibility across subjects.
Modeling air-to-water sound transmission using standard numerical codes of underwater acoustics
Authors: Chapman David M. F. , Thomson D. J. , Ellis Dale D.
Published: October 2005

Recently, it was shown that a normal mode underwater acoustic propagation code can be modified to model air-to-water sound transmission simply by altering the mode excitation coefficients [D. M. F. Chapman and P. D. Ward, J. Acoust. Soc. Am. 87, 601–618 (1990)]. Herein, it is shown that if the height of the source above the surface is sufficiently small, then the air-to-water transmission problem can be modeled by replacing the source in air with a source in water at a depth d≪λ below the surface, where λ is the acoustic wavelength in water. For a distant receiver in water, both the true source in air and the effective source in water exhibit nearly identical dipole radiation patterns, although the source strengths are different. The correct transmission loss from the source in air can be recovered by adding the quantity 20 log10(kad), where ka is the wave number in air. Using this approach, numerical results for three standard underwater acoustic models (normal mode, multipath expansion, parabolic equation) are compared to benchmark results provided by the SAFARI model for a generic air-to-water transmission example.
Sound scattering by a cylindrical shell reinforced by lengthwise ribs and walls
Authors: Klauson Aleksander , Metsaveer Jaan
Published: October 2005

An analytical solution is derived for the acoustic response of submerged thin-walled ring cylindrical shell containing lengthwise stiffening members: internal stringers and walls. On the basis of the analysis of the acoustic pressure versus time diagrams the stiffener-borne wave-generation mechanisms are traced. Shown is that the shell/stiffener junctions act as additional entry and exit points of circumferential waves circulating in the shell and the fluid. The stiffening members cause transformations of circumferential waves from one propagation type to another.
Time optimal active damping of a cantilever beam.
Authors: Syang Yichiang , Busch-Vishniac Ilene J.
Published: October 2005

Active damping control of spatially distributed systems has typically been accomplished using a number of discrete sensors and actuators. However, use of discrete elements introduces the problem of actuator/observer spillover that can be quite a problem for large lightly damped structures, such as might be used in space applications. While some researchers have studied distributed sensing and actuation, their application to active damping of structures has been limited primarily to a Liapunov energy minimization approach, in which the control strategy is derived by making the power always flow out of the vibrating structure. In this work modal sensors and actuators such as described by Lee and Moon [J. Acoust. Soc. Am. 85, 2432–2439 (1989)] are used in the design of an active damping system for a cantilever beam. The modal sensors and actuators control the lowest modes of the system and produce no spillover. A time optimal control strategy, which enables one to minimize the time it takes to make the vibration cease, is used. This control method, while more difficult to implement than Liapunov control algorithms, directly addresses the desired objective of damping the vibration as fast as possible.
Computation of the signal distortion of reflections from specular reflectors by impulse ray modeling
Authors: Linssen Frans , Hoeks Arnold
Published: October 2005

The shape of an ultrasound pulse that is reflected by a specular reflector will be distorted because of differences in the length of the path traveled by the acoustical waves between different points on the transducer surface to different points on the reflector surface. This effect can be studied by calculating the radiation coupling function for the combination of a transducer and a reflector. The calculation is performed with a numerical method, basically a modification of ray models used in the calculation of heat transfer by radiation. The signal distortion can be described as a linear filtering process. Its transfer function elucidates the distortion of an emitted waveform. By way of example, the transfer function is calculated for various positions and diameters of a cylindrical reflector in front of a focused transducer. It appears that severe signal distortions can be expected in practical situations.
Radiation of a guided wave at the end of the cylindrical shell.
Authors: Léon Fernand , Décultot Dominique , Lecroq Florence , Maze Gérard
Published: October 2005

Acoustic scattering from an isotropic elastic hollow cylindrical shell of infinite length excited by an obliquely incident plane acoustic wave is investigated. The waves generated at an incident angle α on a determined area of a cylindrical shell immersed in water and filled with air, propagate in a direction parallel to the cylinder axis and re-emit their energy during the propagation at an angle α. The various velocities of propagation of helical waves allows one to record separately the resonance spectrum of each type of waves. Then, the radiation, in the far field, can be observed for each wave, at the end of a shell of semi-infinite length, with a quasiharmonic method. In this case, no phenomenon of stationary waves can occur in the cylinder length. The axial displacement is analyzed and the use of the Rayleigh integral allows one to determine a model of the pressure radiated by the circular section of the tube. The results of the theoretical calculation are in agreement with the results of experiments.
Quantification of sonic-boom distortion resulting from propagation through the Earth’s turbulent boundary layer.
Authors: Gionfriddo Thomas A.
Published: October 2005

Using digitized sonic-boom waveforms from aircraft overflights recorded by the Air Force (Edwards AFB, 1987), a method has been developed to quantify the amount of distortion present in a sonic boom, using a classic N-wave model for comparison. A statistical relation between waveform distortion and the amount of turbulence a sonic boom encounters during propagation is sought. Using the premise that energy conservation applies approximately for a sonic boom both before and after passage through the Earth’s turbulent boundary layer, a model N wave is constructed from the distorted boom signature received at the ground. The mean-squared error between the actual and constructed signatures serves as a measure of waveform distortion and is seen to correlate with the distance the wave travels through the turbulence. [Work supported by NASA-LRC.]
Three-tier computerized system using ambient noise to assess the performance of complex multielement sonar systems.
Authors: Wagstaff R. A.
Published: October 2005

Experience with both developmental and Fleet operational sonar systems suggests that it is not uncommon for a sonar to be operating far below its design performance level without the sonar operator’s knowledge that the performance is degraded. The magnitude of the degradation is often far more than an increase in gain that might be expected from a new generation improvement in that sonar. Hence, it makes sense to monitor the performance of the sonar system to detect degraded performance, to identify problems or faults when they exist, and to know when to correct the faults if it is possible to do so. Furthermore, the sonar is generally essential to an ASW mission; an assessment of its performance must be thorough and rapid. Such testing could take days, if done manually by a sonar technician. However, the task can be done more thoroughly, and in as little time as 2 or 3 min, by a smart computer program that does the analysis and decision making and uses ambient noise as the noise source. This paper discusses a three-tier comprehensive system of computer-based sonar system performance monitoring and data quality assessment algorithms. The algorithms and analysis techniques are discussed, and the results obtained on recent sea tests with vertical line arrays and towed horizontal line arrays are presented. In some cases, the sonar systems were shown to be operating at a small percentage of their theoretical or expected levels. Furthermore, if the degraded states had been known in real time, simple fixes in the field could have returned the performance of some of those systems to nearly theoretical.
Harmonic responses in a multilayered saturation porous media.
Authors: Kuo M. K. , Yeh C. S. , Liaw J. W.
Published: October 2005

The time harmonic Green’s function for a point force in a multilayered fluid-saturated porous media is derived in the context of Biot’s theory. The crucial step of the analysis is a global matrix technique, which is different from the propagator matrix approach [T. Yamamoto, J. Acoust. Soc. Am. 73, 1587–1596, 1599–1620 (1983)] and the generalized ray expansion via Gauss–Seidel matrix iterative method [R. C. Y. Chin etal., Wave Motion 7, 43–65 (1985)]. The essence of the method is the use of the normal and tangential components of solid displacement and the normal component of fluid displacement at each interface (including the outer surface) as basic unknowns in the wave-number domain. The needed continuities on displacement components across interfaces of the multilayered medium is then ensured automatically. These as yet undetermined interface displacements are finally determined from the boundary conditions at the outer surface of multilayered medium and the continuities of tractions across interfaces. The resulting linear system of equations is straightforward assembled and manifested in a global form. Finally, the wave-number integrals are evaluated numerically to yield the frequency responses. The numerical results are presented for the cases of a point source in a multilayered ocean with multilayered fluid-saturated porous seabeds.
The hazard of exposure to impulse noise as a function of frequency.
Authors: Patterson James H. , Hamernik R. P. , Ahroon W. A.
Published: October 2005

Existing criteria for safe exposure to impulse noise do not consider the frequency spectrum of an impulse as a variable in the evaluation of the hazards to the auditory system. This report presents the results of a study that was designed to determine the relative potential that impulsive energy concentrated at different frequencies has in causing auditory system trauma. One hundred and thirty (130) chinchilla, divided into 22 groups with 5 to 7 animals per group, were used in these experiments. Pre- and post-exposure audiograms were measured on each animal using avoidance conditioning procedures. Quantitative histology (cochleograms) was used to determine the extent and pattern of the sensory cell damage. The noise exposure stimuli consisted of seven different computer-generated narrow-band impulses (approximately 400-Hz bandwidth) having center frequencies located at 0.260, 0.775, 1.025, 1.350, 2.075, 2.450, and 3.550 kHz. Each narrow-band exposure stimulus was presented at two to four different intensities. An analysis of the audiometric and histological data allowed an iso-hazard weighting function to be derived. The weighting function clearly demonstrates that equivalent amounts of impulsive energy concentrated at different frequencies are not equally hazardous to the auditory system. Comparison of the derived weighting function with the A-weighting curve indicates low frequency impulses are less hazardous than predicted by A-weighted sound exposure level.
Active matched-field processing.
Authors: Perkins J. S. , Kuperman W. A. , Tinker L. E. , Murphy G. T.
Published: October 2005

In passive matched-field processing, a model is used to simulate the acoustic field propagating from an object to a receiver array (usually with vertical extent). After generating many replica fields corresponding to many object positions, one can estimate the location of the object by seeing which of these replica fields best matches the received data. In this paper, active matched-field processing is demonstrated. An acoustic model based on adiabatic normal modes is used to simulate the acoustic field on a vertical receiving array in an active scenario. That is, the acoustic energy from a point source propagates to an object in a three-dimensional environment, is scattered by the object, and is subsequently received on a vertical array of hydrophones. Just as in the passive case, this model is used to create many replica fields corresponding to many object positions, and then the position of the object is estimated by determining which of these replica fields best matches the received data. In principle, it is also possible to determine the orientation of the object.
Limitations of the hydrodynamical theory of cavitation induced sonoluminescence.
Authors: Löfstedt Ritva , Barber B. P. , Hiller R. , Putterman S.
Published: October 2005

The extremely short duration (<50 ps) of the flashes of light emitted by a sound field raises problems for the hydrodynamical description of sonoluminescence. Solutions to the Navier–Stokes equations with such a fast but short-lived adiabatic compression develop shock fronts. Even in the linear approximation a single 20-μ bubble that accelerates at this rate would radiate so much sound that its motion would dominate the Q of a 0.1-liter resonator. The extent to which such a quickly accelerating bubble can be considered to be in local equilibrium is discussed. Nevertheless, fluid mechanics provides valuable scaling laws that can be used to analytically determine the ambient radius, phase of collapse, and maximum and minimum radii of the trapped bubble’s oscillation. [Work supported by the USDOE: Office of Basic Science (theory) and Division of Advanced Energy Projects (experiment) and R. L. is an AT&T fellow.]
The influence of waveform distortion on hydrophone spatial-averaging corrections—Theory and measurement
Authors: Zeqiri Bajram , Bond Antony D.
Published: October 2005

A significant source of error in measurements of the acoustic output of medical ultrasonic equipment can arise from spatial averaging of the acoustic pressure over the active area of the hydrophone. Although criteria exist that quantify the maximum permissible effective hydrophone diameter, these are frequently violated for measurements on medical diagnostic systems, even for 0.4-mm-diam hydrophones that currently constitute the smallest commercially available device. In such circumstances, corrections have to be applied to the measured acoustical parameters. In the past, these correction procedures have been based on idealized models of the transducer pressure field distribution and have not taken account of finite amplitude effects which lead to nonlinearly distorted waveforms. This paper presents a systematic study of the influence of nonlinear distortion on spatial-averaging corrections. A 5-MHz focused transducer has been used to generate a range of acoustic waveforms suffering from varying degrees of nonlinear distortion. The important acoustical parameters have been determined using a group of hydrophones with effective diameters, determined at a frequency of 5-MHz, of between 0.3 and 4 mm. For each of the acoustical parameters, measurements were used to derive estimates of the spatial-averaging correction for the larger hydrophones used. The derived corrections were compared with existing correction procedures and the predictions of a new model which explicitly includes waveform distortion. The implications of the model to acoustic output measurements of medical ultrasonic equipment are discussed and correction curves presented which allow the magnitude of the spatial-averaging correction in many measurement situations to be estimated.
Effects of couplant mass loading on the acoustic signature V(z)
Authors: Doghmane Abdellaziz , Hadjoub Zahia , Alami Karim , Saurel Jean-Marc , Attal Jacques
Published: October 2005

Nondestructive investigations of elastic properties of materials via acoustic material signature, using reflection acoustic microscopy systems, are becoming more and more attractive. The necessity of using coupling liquids in such systems leads to the generation of generalized surface leaky waves such as Rayleigh waves. The V(z) response and hence the postion of its Fourier transform peaks change according to the liquid impedance due to a fundamental phenomenon known as mass loading. This effect, neglected up till now, has been studied in details on several materials (tungsten, stainless steel, and SiO2) with water, mercury, and other coupling liquids having different densities assuming the same velocity. For example, it is found that skimming shear velocity can be determined with a better precision when heavy coupling liquids are used. However, skimming longitudinal velocity seems to be independent of the coupling liquid density, whereas its efficiency gets larger with higher liquid densities.
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i probability source localization with array tilt uncertainty
Authors: Richardson A. M. , Nolte L. W.
Published: October 2005

In a previous paper [J. Acoust. Soc. Am. 89, 2280 (1991)], the authors illustrated how estimation theory could be utilized to develop a localization processor that exhibited robust performance in the presence of uncertainty about the acoustic environment. This processor was termed the optimum uncertain field processor (OUFP). In this paper an OUFP is developed that is designed for use in a scenario in which there is uncertainty about the tilt angle of a vertical array. First results are presented that indicate the sensitivity of a matched-field processor to array tilt uncertainty. The uncertain field processor, on the other hand, is shown to be very robust, i.e., the processor is able to accurately locate the acoustic source even when the array tilt angle is not precisely known.
The anatomy of underwater rain noise
Authors: Medwin Herman , Nystuen Jeffrey A. , Jacobus Peter W. , Ostwald Leo H. , Snyder David E.
Published: October 2005

When rain falls onto a large body of water it produces dominating underwater sound over a broad range of audio frequencies. Laboratory studies using more than 1000 single drops, covering the complete size range of actual rain drops at their terminal speeds, have now shown that the complete underwater spectrum of rainfall sound can be dissected into the impact and microbubble sounds produced by four acoustically distinctive ranges of drop diameters D. These are defined as ‘‘minuscule’’ drops (D≤0.8 mm), ‘‘small’’ drops (0.8 mm≤D≤1.1 mm), ‘‘mid-size’’ drops (1.1≤D≤2.2 mm), and ‘‘large’’ drops (D≥2.2 mm). A minuscule raindrop produces only a very weak, almost undetectable, short duration impact noise. A small drop at terminal speed and at local, near-normal incidence, radiates measurable broadband impact sound followed by the very much stronger sound of a ‘‘type I’’ damped microbubble oscillating at frequencies near 15 kHz. A mid-size raindrop radiates only impact sound. Large raindrops, which comprise the major volume of moderate to heavy rainfall, produce an impact sound and a dominating, ‘‘type II,’’ ‘‘primary’’ oscillating microbubble of characteristic frequency 2 to 10 kHz depending on the drop diameter. Also, large drops often generate weaker sounds from ‘‘secondary’’ bubbles. The average acoustic energy spectra of large raindrops are distinctive functions of their diameters, the salinity of the surface water, and the temperature difference between the drop and the surface water. When the underwater acoustic intensity spectrum during heavy rain is calculated from the single drop acoustic energy spectra and the drop size distribution, it compares quite well with ocean measurements. The gas injection at the air–water interface is calculated from the probability of bubble formation during a heavy rainfall.
Comparison of the Rayleigh–Fourier and extinction theorem methods applied to scattering and transmission at a rough solid–solid interface
Authors: Kazandjian Luc
Published: October 2005

The Rayleigh–Fourier and extinction theorem mathematical formalisms are established and compared in the case of scattering from and transmission through a rough solid–solid interface. By using the reciprocity properties of the solution resulting from the extinction theorem formalism, it is shown that, if they have unique solutions, the two formalisms are theoretically equivalent when used to obtain the displacement fields beyond the maximum excursions of the rough interface. As a result, the perturbation approaches of the two formalisms yield identical results. From a numerical point of view, it is shown that the two methods can give close results regardless of the verification of the Rayleigh hypothesis. The closeness of the two approaches, which has already been established and numerically confirmed in the case of a fluid–solid interface, does not seem to be known or even conjectured in the literature in the case of a solid–solid interface. It is indicated that, with the mathematics included in this paper, analogous results can be obtained for electromagnetic scattering and transmission at a rough interface.
Reflection of an impulse spherical wave at a plane interface separating two fluids
Authors: Cassereau Didier , Guyomar Daniel
Published: October 2005

In this paper, the reflection of an impulse spherical wave by an infinite plane interface separating two fluids is studied. Since both propagation media have a finite acoustical impedance, the boundary conditions at the interface show that the reflected field is distorted and does not show a spherical wave front. Under some particular conditions according to the velocity ratio between the two fluids, it is possible to observe the generation of head waves and/or tail waves. A numerical method is proposed to compute the reflected field based on Fourier transforms over temporal and spatial variables. It will be shown how the reflected field can be reduced to a single integral transform, and how a closed form solution can be obtained in some particular cases. Computations of the reflected field are presented for different values of densities and velocities, illustrating the generation of head waves and tail waves in addition to the expected spherical wave front. Finally, these results are compared to those obtained with the ‘‘impulse ray modeling’’ approach [D. Guyomar and D. Cassereau, Proc. IEEE Ultrasonic Symposium, 731–734 (1987); D. Cassereau and D. Guyomar, in Proceedings of the International Symposium on Acoustic Imaging (Plenum, New York, 1987), pp. 469–481; J. Acoust. Soc. Am. 84, 1504–1516 (1988)], therefore leading to a partial and interesting justification of the ray model.