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Apr 1991

Volume 89, Issue 4B, pp. 1851-2015

Page 1 of 27 Pages Next Page | Jump to Page
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back to top Session 1OC: Acoustical Oceanography: Open Workshop on Gassy Seafloor Sediment: Field Measurements
Invited Papers
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Interpretation of shallow gas‐charged sediments on seismic records (A)

Martin Hovland

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1851-1851 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Porewater and gas seepages through the seafloor have become the target of frontier research over the last decade. Such seepages occur worldwide and at water depths to over 5000 m. The seep locations are often identified by use of high‐resolution shallow seismic systems. Data examples and case studies from the North Sea, the Skagerrak, offshore Mid Norway, and the Persian and Mexican Gulfs are presented and discussed. Such terms as “acoustic turbidity,” “acoustic voids,” “enhanced reflectors,” and “wipe out zones” will also be discussed.
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Anomalous acoustic behavior exhibited by gas‐rich sediments of the subaqueous Mississippi Delta (A)

Anthony R. Tinkle, Jeffrey A. May, Charles A. Meeder, and Kenneth R. Wener

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1851-1851 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Sediments of the subaqueous Mississippi River Delta contain high concentrations of free gas, as manifested by anomalous physical and acoustic behavior. Much of the near‐delta offshore area is a seismic “no‐data zone,” impenetrable to conventional profiling techniques. Specialized techniques developed for direct measurement of sediment acoustic properties reveal that velocities of less than 1000 ft/s (300 m/s) routinely occur in these muds, and acoustic energy at frequencies greater than 100 Hz is almost completely absorbed by only a few tens of feet of very gassy sediment. Severity of the acoustic anomalies typically decreases with depth beneath the seafloor, with the most severe anomalies generally occurring within the first 200 ft subbottom. Trapping of acoustic energy within these near‐bottom, gas‐charged layers is also observed. The nature and severity of the acoustic anomalies correlate well with both sediment physical properties, and the unstable system of collapse and mudflow features that typify the seafloor morphology of the deltaic no‐data zone.
Contributed Papers
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Field experiences in the direct measurement of anomalous acoustic behavior of gas‐rich sediments (A)

Anthony R. Tinkle, Charles A. Meeder, and Kenneth R. Wener

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1852-1852 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Much of the seafloor in the immediate vicinity of the Mississippi Delta consists of sedimentary material containing high concentrations of free gas. Specialized techniques were developed for direct measurement of the acoustic properties of these gas‐charged sediments, to sub‐bottom depths of the order of 300 ft (100 m). These techniques included seafloor receiving arrays, modified checkshot profiles conducted in conventional engineering borings, and a seafloor‐penetrating, hydrophone‐instrumented rod used in the study of the shallowest sub‐bottom layers. Collection of corresponding sediment samples was carefully integrated with these seismic experiments, to better determine the geologic nature and mechanical properties of the gas‐charged materials. Final data acquisition practice was influenced both by the unusual physical properties of the materials being investigated, and the presence of strong and rapidly changing currents from the Mississippi River. Results of these in situ measurements demonstrate the widespread occurrence of anomalously low acoustic velocities (less than 1000 ft/s, or 300 m/s) and related extreme attenuation of higher frequency energy (greater than 100 Hz).
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Effects of sediment gas on chirp sonar reflection profiles (A)

Steven G. Schock, Lester R. LeBlanc, and Larry A. Mayer

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1852-1852 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Chirp sonar reflection profiles of gassy and gas‐free seabeds are quantitatively compared to show the effects of sediment gas on normal incidence backscattering measurements. Acoustic FM pulses that sweep over the band of 2 to 10 kHz are generated by the chirp sonar and compressed using a correlation filter to generate a bandlimited impulse response of the seabed. The amplitude spectrum of the ideal wavelet (the unattenuated, compressed FM pulse) and the spectrum of wavelets backscattered from gassy sediments are used to investigate the frequency dependence of backscattering from gassy seabeds. [Work supported by ONR.]
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Sound propagation in a shallow fresh‐water aquaculture pond (A)

Joe R. Zagar and Kenneth E. Gilbert

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1852-1852 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Measurements made as early as 1943 have shown that gassy freshwater sediments can be excellent reflectors of sound and behave acoustically as pressure‐release surfaces. A simple propagation model is presented, based on bottom‐reflection coefficients, that indicates one should expect strict mode propagation and sharp cutoffs in a horizontally stratified environment having gassy sediments. Continuous‐wave measurements taken in a commercial aquaculture pond are presented and compared to the propagation model. The data show that, between 600 Hz and 3.0 kHz, a gassy sediment layer does behave as a nearly pressure‐release surface. A simple mode‐inversion technique is used to determine bottom‐reflection coefficients for the sediments in the pond. Typical values for the bottom‐reflection coefficient are found to be in the range of −0.85 to −0.95. These values, as well as the estimates of the sediment sound speed, are in excellent agreement with those found by others. Additional measurements taken in depths as shallow as 7 cm are presented. These additional data suggest that the acoustic properties of the aquaculture pond that was studied are typical of all shallow water ponds possessing gassy sediments. [Work supported by ONR and USDA.]
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Gas bubbles in ocean sediments and high‐frequency acoustic backscattering strength (A)

Nicholas P. Chotiros and Frank A. Boyle

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1852-1852 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The influence of trapped gas bubbles on the high‐frequency acoustic bottom backscattering strength of ocean sediments will be examined. Statistical trends in the extant backscatter database at shallow grazing angles suggest that gas bubbles may be a dominant factor. The effect may be modeled in terms of an increase in the volume scattering strength of the sediment. Results from model and laboratory studies will be presented. [Work supported by ONR under NOARL management.]
Invited Papers
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Near‐surface gas hydrates in deep‐sea sediments (A)

Roger D. Flood, Patricia L. Manley, and Mary I. Scranton

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1852-1853 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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Recent studies of depositional processes in deep‐sea continental margin and abyssal settings show that early diagenesis of organic‐rich sediments can significantly alter physical properties in the upper 5–50 + m of the sediment column. Apparently, in situ methane production leads to sediment methane levels that may be high enough to allow hydrates to form while high CO2 levels may cause localized carbonate precipitation. These diagenetic processes can give rise to a series of near‐surface reflecting horizons (and associated physical properties) that are not related to primary depositional processes. Where best studied (in the Argentine Basin at 5000‐ to 5500‐m water depth [P. L. Manley and R. D. Flood, Deep‐Sea Res. 36, 611–623 (1989)]), sediment velocities as low as 1.35–1.42 km/s overlie velocities as high as 1.8–3.0 km/s within 30 m of the sediment surface. While the high velocities appear to be related to near‐surface hydrate formation, the origin of the low velocities is less well understood. Similar reflecting sequences have been observed in other areas, particularly in the North Atlantic.
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Seismic reflection velocity study of a gas‐hydrate zone on the continental slope offshore South Carolina (A)

Thomas H. Shipley, Warren Wood, and Paul L. Stoffa

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1853-1853 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The acoustical and physical significance of bottom‐simulating seismic reflections (BSR's) remains an observational challenge to geophysical methods. A common depth point (CDP) seismic reflection profile using a 240‐channel, 6000‐m array with a 177‐liter (5‐ to 60‐Hz) source was collected along the continental rise off the eastern U.S. where a BSR reflection is observed along a small portion of the line at about 3200‐m depth. These data provide some velocity estimates in the vicinity of the BSR. The CDP data were transformed to the domain of vertical delay time and horizontal ray parameter for velocity analysis purposes. Even so, the resulting velocity profiles have limited vertical resolution (about 200 m) due to the distribution of interpreted sedimentary reflections used in the vertical delay time velocity analysis. Even with this admittedly low vertical resolution, the velocity above the BSR is at least 2000 m/s in an approximately 200‐m zone, while the predicted velocity based on the extrapolation of regional gradients indicates that normal sediments should have a velocity of about 1850 m/s. A velocity of 2000 m/s suggests on average about a 50% substitution of hydrate in the pore spaces but the actual vertical concentration gradient is not constrained. This velocity anomaly also extends into other areas just above the theoretical phase boundary position, but where there is no detectable BSR. Beneath the BSR, even with the relatively low vertical resolution, a velocity decrease to about 1700 m/s is detected. This low velocity is observed only in zones with a detectable BSR. It is not observed beneath the theoretical phase boundary position elsewhere. This suggests that the origin of the BSR is not a simple boundary between hydrated and nonhydrated, normal sediments below. Initial investigations of amplitudes indicate significant increase in amplitude with offset. Full waveform, offset modeling of the data is underway.
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Estimation of amounts of gas hydrate in marine sediments using amplitude reduction of seismic reflections (A)

William P. Dillon, Myung W. Lee, Kristen Fehlhaber, and Deborah R. Hutchinson

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1853-1853 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Gas hydrates (solid, crystalline water‐gas mixtures) exist in sediments just below the seafloor. In seismic profiles, hydrate cementation creates zones of increased velocity and reduced amplitudes of stratal reflections (blanking). By using sediment velocities (estimated by an inversion method), known sediment porosity, and pure hydrate velocity, the amount of hydrate in the highest velocity and most intensively blanked sediments off the southeastern U.S. is calculated; this represents maximum hydrate cementation. To create seismic models of the range of possible blanking effects, ordinary, nonhydrated sediments across a reflecting boundary (caused by a porosity change) are “replaced” with this maximum‐hydrate end member in various proportions. Three classes of blanking are defined; class boundaries represent a change in reflection amplitude by a factor of 2, and the classes are relatable to the amounts of hydrate in bulk sediment. In order to estimate the amount of hydrate, these classes are mapped in a grid of reflection profiles processed to preserve relative amplitude, and the first semiquantitative estimate of gas hydrates in deep‐sea sediments is produced. [Work supported by DOE.]
Contributed Paper
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Deep‐towed acoustic measurements of gas hydrates (A)

Mary M. Rowe and Joseph F. Gettrust

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1854-1854 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Acoustic data were collected using the Naval Oceanographic and Atmospheric Research Laboratory's (NOARL's) Deep Towed Acoustical/Geophysical System (DTAGS) at the northern end of the Blake Outer Ridge. A bottom‐simulating reflector (BSR) is observed in these data at the same depth that it is found in surface‐tow multichannel seismic sections from approximately 150 km to the east, near Deep Sea Drilling Project (DSDP) sites 102, 104, and 533. Analysis of the high‐resolution (∼ 10 m) DTAGS data confirms that gas hydrate as well as unfrozen gas are concentrated within these sediments. Distinctive properties of the acoustic signal are combined with sediment sound speeds derived from these data to estimate the distribution and extent of the gas hydrate and regions of unfrozen gas. Results of analysis of DTAGS data extend findings from DSDP Leg 11, DSDP sites on the Middle America Trench, and the Black Sea. These data provide a means for determining mechanisms for the concentration of methane gas and the formation of gas hydrate. [Work supported by the Office of Naval Technology.]
Poster Paper
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Biogenic gas in shallow water sediments as indicated by high‐resolution seismic records (A)

Douglas N. Lambert

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1854-1854 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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A 15‐kHz narrow‐beam (12°) seismic system, designated the Acoustic Seafloor Classification System (ASCS), has been used to identify areas of shallow sea floor gassy sediments. The ASCS was designed to remotely classify seafloor sediments by quantitatively and qualitatively measuring the echo return amplitude and pulse character in ten adjustable time windows that correspond to depth increments in the sediment. In addition, the ASCS produces a very high‐resolution analog seismic record of the upper few meters of the seafloor on which the amplitude of the echo return (echo strength) for each of the depth increments is also indicated. This unique combination of data provides an excellent means of identifying sediments containing biogenic gas. Examples of identified gassy sediments shown on ASCS records range from a “speckled” appearance indicating random and isolated small gas concentrations to “white outs” that indicate large gas concentrations that completely attenuate the acoustic signal. Other records show highly reflective sediment layering that is due to a concentration of gas along bedding planes alternating with large pockets of 1ow‐reflectivity, degassed sediment that appears to have been “homogenized” by gas movement. These degassed sediment pockets are occasionally accompanied by vertical gas escape conduits seen in the sediment and by the presence of vertical reflectors in the water column indicative of escaping gas bubbles.
back to top Session 1ID: Tutorial on Nonlinear Acoustics
Invited Paper
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Nonlinear acoustics (A)

Robert T. Beyer

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1855-1855 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Nonlinear acoustics (NLA), within the framework of physical acoustics, has come of age over the past 30 years, yet new phenomena continue to be observed, so that the subject is not yet exhausted. The session will begin with a review of the mathematics of describing NLA (part 1). The analysis of one‐dimensional phenomena will follow, plus the use of Burgers' equation (part 2). Data on the nonlinear parameter (B/A) will be reviewed (part 3). The physical aspects of nonlinear phenomena in two and three dimensions will be explored, and approximate methods of calculating the mutual effect of diffraction and nonlinearity will be described (part 4). The effects of the interaction of two sound beams will be considered (part 5). Applications will be treated. These include the development of parametric array sonar, surface wave devices, lithotripsy, and other medical applications (part 6). Such frontier topics as solitons and chaos will be touched on (part 7).
back to top Session 2AA: Architectural Acoustics: Newer Measurement Procedures in Auditoria I
Invited Papers
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Ten years of newer auditorium acoustics measurements (A)

J. S. Bradley and R. E. Halliwell

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1856-1856 (1991); (1 page) | Cited 1 time

Online Publication Date: 14 Aug 2005

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This paper reviews progress in making newer types of auditorium acoustics measurements over the past 10 years. All of the newer measures require impulse responses to be obtained and the merits and limitations of various measurement approaches will be discussed including the type of source, receiver, and processing that is used. Objective descriptions and the subjective relevance of the various types of newer measures will be reviewed including both monaural and binaural quantities. Four generations of measurement systems will be described including the special problems associated with measuring inter‐aural cross correlation coefficients. Finally the questions of accuracy and repeatability will be mentioned along with some discussion of future developments and problems to be solved.
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Auditorium acoustics: What should we measure? What do we measure and what does it mean (A)

J. P. Vian and X. Pelorson

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1856-1856 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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For many years the assessment of auditorium acoustics has been made through a set of acoustical criteria whose number and kind sometimes differ substantially between groups of specialists. The situation seems quite clear for speech auditoria as much as it is confused for music auditoria. Meanwhile a growing consensus has slowly come out about a small number of quantities that are considered to explain a large amount of the variability of subjective judgments, because they correlate individually with some subjective impressions. The acoustical quality of an auditorium should be allowed to be expressed univocally by the values taken by these criteria. Unfortunately several of these criteria are not independent and very little is known about the subjective significance of the various arrangements of criteria values. Moreover all the variability observed when measuring the criteria are related not only to physical features of the auditorium but also to a large extent to the measurement technique. Different measurement techniques are discussed and fluctuations among measurement procedures, among halls and within halls are compared. It is shown how difficult it is to make a conclusion on acoustical quality of a hall by looking only at the criteria. The need for a standardized method to take measurements is stressed, as well as the need for more research work focusing on criteria, making it possible to discriminate between halls when listening judgments are different.
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Measurement of impulse response and its applications in room acoustics (A)

H. Tachibana, H. Yano, and Y. Hidaka

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1856-1857 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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Impulse response includes almost all physical information of a linear system and it is also very important in room acoustics. Fortunately, owing to the recent development of digital signal processing techniques and instrumentation, it has become possible to make a precise and convenient measurement of impulse responses in rooms. In this paper, the practical techniques for this kind of measurement including scale model studies are introduced. For the measurement in real auditoriums, a sweep pulse is radiated many times from a dodecahedral omnidirectional loudspeaker and the responses are recorded on a DAT through an omnidirectional microphone or a dummy head system. From the recorded responses, impulse responses are obtained by synchronous averaging and deconvolution techniques. In scale model experiments, impulse responses are measured by using a spark discharge source and a scale model dummy head microphone. From the impulse responses measured in such ways, not only various room acoustic quantities are derived but also the sounds including the room response can be synthesized by making convolutions between the impulse responses and arbitrary dry source signals. This technique is effectively used for subjective evaluation of room acoustics. Some examples of the experimental results for real and scale model auditoriums will be demonstrated.
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Prediction of room acoustical parameters (A)

A. C. Gade

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1857-1857 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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During a 6‐yr period, detailed room acoustical measurements have been carried out in 35 halls in Denmark and in other European countries. By subjecting these data to statistical analyses, it has been possible to confirm old and establish new relationships between design variables and the position‐averaged acoustical data. The results are presented in the form of linear, multiple regression formulas that may be used to predict the values of the newer measures of level, clarity, spaciousness, and musicians' conditions on the orchestra platform in halls with given RT and geometry.
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Subjective and objective evaluations of rooms for music (A)

Richard P. Cervone, Wei‐hwa Chiang, Gary W. Siebein, Harold W. Doddington, and Wilhelm K. Schwab

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1857-1857 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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A dedicated, computer‐based analysis system was developed to perform a complete set of acoustical measurements of recent interest in full‐size rooms and in scale models of rooms. The measurements included reverberation time, early reverberation time, loudness, early to late temporal energy ratios, lateral energy fractions, interaural cross correlation, and speech transmission index among others. Measurements were made at multiple locations in ten large concert halls. Groups of listeners evaluated live music performances at three locations in each of the rooms using a seven point semantic differential rating scale. Correlation analysis and statistical modeling identified significant relationships among the qualities of the music in the room rated by the listeners with the physical measurements made in the rooms. Variations of subjective qualities were identified among the different rooms and within each of the rooms as well. The subjective qualities that contributed to overall acoustical impression were also identified. [Work supported by NSF.]
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Subjective and objective assessment of some reverberation enhancement systems (A)

Mendel Kleiner and Peter Svensson

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1857-1857 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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How much of a difference does active or passive variation of a room's acoustical conditions make? Electronic architecture and reverberation enhancement have become common in the pursuit of better acoustics in multipurpose auditoria by active means. New systems show promising results. The properties of several different installations both in the U.S. and Europe have been investigated. This has been done both objectively by measurement of omnidirectional and binaural impulse responses and subjectively by the use of binaural recording. The binaural recordings used anechoic music replayed in stereo on stage. These recordings were used in pair comparison tests to investigate the dimensions of audible difference between the halls, with and without active or passive treatment of the room response. The pair comparison tests were evaluated using multidimensional scaling. The results show that some systems are able to modify the acoustical conditions to a very large extent. Differences between various system settings may be as large as between halls. Most of the subjectively perceived differences may be explained by changes in reverberation time and lateral energy factor. [Work supported by the Swedish National Council of Building Research.]
back to top Session 2EA: Engineering Acoustics: Transducers and Arrays
Contributed Papers
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High‐power test of a barrel stave flextensional transducer (A)

Mark B. Moffett and William L. Clay, Jr.

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1858-1858 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Barrel stave flextensional transducers are potentially useful as compact, low‐frequency, high‐power projectors. An equivalent circuit model that includes a higher‐mode, extensional compliance is used to estimate the maximum radiated power. Because the mechanical quality factor, Q, is low (on the order of 3 or 4), the source level of such a projector is limited by the maximum electric field that the piezoelectric ring stack driver can safely handle without depolarization or significant dielectric losses (about 400 kV/m for Navy type III lead zirconate titanate). A barrel stave flextensional projector 18 cm long and 9 cm in diameter with a mass of 4.1 kg in air was tested to 200 psig (1.4 MPa) in the pressure vessel at NUSC's Dodge Pond Field Station. A source level of 194.7 dB//1 μPa‐m was obtained at 1.56 kHz for an applied rms voltage of 5 kV. The projector figure‐of‐merit was about 14 W/kg‐kHz‐Q, and this number would be expected to apply to a larger, lower‐frequency projector of commensurate dimensions.
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Applications of very high‐energy density electrostrictive ceramics for underwater projectors (A)

A. P. Ritter, S. M. Pilgrim, P. Kuhn, S. R. Winzer, and J. Sewell

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1858-1858 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Performance of Navy sonar transducers is limited by the inherent energy density of the driver material—especially for those size‐constrained applications that require very high source levels and/or very low frequencies. Ultimately, the maximum sound‐pressure level will be limited by the amount of power that can be generated from the ceramic driver at its maximum engineering limit of 10–15 V/mil (∼0.5 MV/m). Incremental improvements in transducer performance may be possible through design refinements, however, revolutionary large‐scale improvements require new approaches to overcome the basic PZT material limitations. Electrostrictive ceramic materials, such as the PMN‐based compositions being developed at Martin Marietta Laboratories, have energy density values an order of magnitude higher than Navy PZT's and therefore could significantly improve transducer performance (i.e., maximum attainable source level) if substituted for PZT in conventional transducer designs. Transducer model calculations for comparable PZT‐ and PMN‐driven transducers show ∼ 10‐dB gain in the transmitting voltage response for the PMN transducer. These model predictions have been verified by experimental data obtained under a joint Martin Marietta/Navy materials development program.
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Using piezoelectric film and ultrasound resonance to measure the elastic moduli of spherical ceramic particles (A)

P. S. Spoor, M. J. McKenna, J. D. Maynard, and John R. Hellmann

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1858-1858 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The search for alternate sources of energy has prompted interest in small ceramic beads, called “proppants,” which were developed as a means of “propping” open cracks during the hydraulic fracturing of bedrock in the vicinity of oil wells; recently, they have been considered as possible thermal transfer media for use in solar receivers [J. R. Hellmann et al., “Evaluation of Spherical Ceramic Particles for Solar Thermal Transfer Media,” SAND86‐0981, Sandia National Laboratories, January 1987]. To monitor the effects of repeated thermal stresses on the proppants, one would like to have a reliable measure of their elastic constants; however, their spherical shape and small size (≃ 500 μm) make conventional techniques, such as pulse‐echo, inapplicable. Using a special piezoelectric film transducer and a small‐sample resonance technique [J. D. Maynard, J. Acoust. Soc. Am. Suppl. 1 85, S20 (1989)], the authors have been able to determine the elastic constants and their variation as a function of heat treatment. [Work supported by the Office of Naval Research and NSF Grant DMR 9000549.]
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Optimally formulated high efficient planar projector arrays (A)

P. M. Joseph and P. R. Saseendran Pillai

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1858-1859 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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Closely packed multi‐element transducer arrays are extensively used in underwater applications for achieving better directionality and longer transmission range. With the current tendency of extending their operation toward lower frequency and particularly near resonance, the acoustic interaction among the elements grows stronger and in turn will degrade the predicted transmitting characteristics. This troublesome effect is much alleviated in uniform planar arrays by restructuring it with the optimal interelement spacing at which the interaction force is minimum [P.M. Joseph and P. R. Saseendran Pillai, Acoust. Lett. 12 (11), 190—193 (1989)]. It has been seen from the results of computation that a further reduction in interaction can be achieved by incorporating the nonuniform array concept. A simple method for predicting the optimum configuration of element locations that reduce the interaction to a lower level, without altering the aperture dimension, is proposed in this paper. Here, the element locations are dispersed in accordance with Gaussian distributed random numbers by keeping the optimum spacing as the mean. Even though a large standard deviation value will greatly reduce interaction, a reasonable value is chosen, as large deviations produce deteriorated beams. This optimally formulated array does not exhibit any grating lobes nor does its radiation pattern differ much from the conventional λ/2 spaced and restructured arrays.
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The generation of sound by a dipole moving near the edge of a half‐plane (A)

Gerrit Schouten

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1859-1859 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The shape and the spatial amplitude distribution of the sound pulse generated by the passage of a dipole (or ring vortex) near the sharp edge of a half‐plane are computed and presented in plots. Instead of using a low‐frequency approximation to the Green's function a direct approach to the potential of a moving dipole in a branched space is used together with an image dipole. In a compressible medium (with a finite velocity of sound) the wave is physically present. In the limit of an incompressible medium the wavelength becomes infinite and the wave degenerates into a time history of the pressure variation. The incompressible pressures agree with those obtained from direct differentiations of the classical results of Sommerfeld [Proc. London Math. Soc. 30, 121–163 (1897)]. The wave results are in conflict with some results presented in the literature [T. Kambe et al., J. Fluid Mech. 155, 77–103 (1985)] obtained with a low‐frequency approach.
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Characterization of anisotropy in wood composites (A)

V. Bucur

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1859-1859 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The aim of this report is to study the anisotropic behavior of wood composites using the ultrasonic velocity method and acoustic emission method. Velocities of ultrasonic longitudinal and transversal waves were used for the estimation of five elastic constants. Complementary, stimulated acoustic emission, induced by breaking 0.5‐ram pencil lead on the surface of the specimen, was employed to measure five parameters of the acoustic emission signal (duration, counts number, energy, amplitude, and rise time). The anisotropy was estimated as the ratio of velocities, of acoustic invariants, and acoustic emission parameters.
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Flow noise induced in large arrays via the flexure of the support (A)

B. Dubus and R. E. Montgomery

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1859-1859 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The noise induced by the turbulent boundary layer in sonar arrays is usually split into a direct path where the fluctuating pressures directly excite the hydrophones after traveling through an elastomer layer backed by a rigid surface, and the indirect path where the excitation of the support by the turbulent boundary layer induces the noise in the array. To describe this last phenomena, the force‐modal transform method [M. C. Junger and D. Feit, Sound, Structures, and Their Interaction (MIT Press, Cambridge, MA)] has been extended to describe the flexure of a submerged multilayered plate and the wave‐number spectrum of the noise sensed by the array. Several configurations are analyzed showing the effect of the stiffness and the damping of the support and of the distribution of the hydrophones. From these results, some solutions to reduce the noise level are proposed. [Work partially supported by Direction des Recherches Etudes et Techniques, Paris.]
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