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Journal of the Acoustical Society of America

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May 2001

Volume 105, Issue 5, pp. 2388-L12

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Computer modeling and prediction in the design of coupled volumes for a 1000‐seat concert hall at Goshen College, Indiana (A)

Byron W. Harrison, Gary Madaras, and Robert D. Celmer

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2388-2388 (2001); (1 page)

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The effects of specific architectural parameters (volume, shape, location, aperture size, and location) on the acoustic characteristics of variable‐acoustic coupled volumes were analyzed using the computer modeling application, CATT‐Acoustic. The purpose of the study was to optimize design and determine the effectiveness of coupled volumes for use in the Goshen College Concert Hall, Goshen, Indiana. The computer modeling technique allowed specific variables to be isolated and parameters to be analyzed. Prototype models were evaluated by a visual inspection of decay curves, comparisons of T15, T30, and T60, and auralizations. The goal was to preserve the early decay rate of the main hall while achieving a slower decay rate after 750 ms. The analysis confirmed the use of computer modeling and prediction in studying the acoustic performance of coupled volume designs. Chamber volumes were found to be the most influential factor in realizing the double‐sloped decay. Shape, location, and aperture configuration had noticeable, but lesser, effects on acoustic performance than did chamber volume. To realize efficient acoustic variability for the small‐volume hall researched, the chambers required volumes that compared to large percentages of the main enclosure volume (50–75 percent).<ppx>PACS4355Fw,4355KaEPACS<ppa>

A model for acoustical evaluation of rectangular concert halls (A)

Nurgun Tamer Bayazit

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2388-2389 (2001); (2 pages)

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The studies on room acoustics have shown the importance of hall geometry on the acoustical performance perceived subjectively by the audience. The aim of this study is to examine the effects of the geometrical parameters on the objective acoustical parameters and to develop an analytical model which will help designers in the early stage of design. The acoustical parameters of the model halls designed, selected as rectangular forms with varying dimensional ratios, were calculated with the aid of the ODEON room acoustical simulation program. In order to obtain realistic results with the proposed model, the geometrical parameters of model halls were chosen so as to allow comparison with those of existing halls and the set of data used for the calculation of acoustical parameters were chosen to reflect real conditions in the best possible way. Calculated acoustical parameters were statistically analyzed to derive an analytical model based on the relationship between the hall geometry and the acoustical parameters. The geometrical parameters are fed into the model as independent variables, and the acoustical parameters are calculated on the basis of them.<ppx>PACS4355Br,4355KaEPACS<ppa>

Applications of physical and mathematical modeling in concert hall design (A)

Paul Calamia and Martha Larson

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2389-2389 (2001); (1 page)

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A variety of testing methods were employed throughout the early and mid stages of design for an eighteen‐hundred‐seat flexible‐use concert hall. Full‐scale laboratory testing, model‐scale measurements, and mathematical modeling were undertaken to provide guidance on particular design decisions. Mathematical techniques were used to guide the full‐scale laboratory testing, and full‐scale testing results were applied in the model‐scale measurements. Auralization techniques were implemented to study specific effects.   <ppx>PACS4355Br,4355Fw,4355KaEPACS<ppa>

Calculation of diffuse sound reflections in predicting room acoustics using the image method (A)

Hee‐Won Lee, Il‐Doo Ko, Se‐Jin Doo, Yang‐Ki Oh, Dae‐Up Jeong, Chul‐Min Choi, and Lai‐Hoon Kim

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2389-2389 (2001); (1 page)

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Numerous investigations have demonstrated that diffuse reflection is one of the most important factors in predicting room acoustics by computer simulation. Recent studies have suggested several computational algorithms in order to account for diffuse reflections in the ray‐tracing or beam‐tracing method. In this study, a computational algorithm for the calculation of diffuse sound reflections in the image method is suggested and a computer simulation system is developed based on the suggested algorithm. The methodology adopted in our computer simulation system is similar to the extended radiosity method, which was developed for computer graphics. Various descriptions of room acoustics, including spatial distribution of image sound sources, impulse responses and other commonly used room acoustical measures, can be obtained from the MLS based monaural room acoustics measurement system. The measured results in a midsized rectangular classroom with/without chairs on the floor were compared with the predicted results using the computer simulation in which diffuse reflection coefficients of the floor were varied. [Work supported by Korean Research Foundation Grant KRF‐1999‐1‐310‐004‐3.]<ppx>PACS4355KaEPACS<ppa>

Acoustic simulation and auralization in architectural design practices (A)

Quinsan Ciao (Cao)

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2389-2389 (2001); (1 page)

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This paper is dedicated to predictive relationships between space design and acoustic performance in architectural practice through auraliza‐tion. With such a relationship, design alternatives can be evaluated subjectively, as well as objectively, without being constructed. In traditional practice, designers rely on vague experiences and rough estimates to predict acoustic effects while designing architectural spaces. Acoustic analysis and predication are seen as a luxury remedy and only affordable in large‐scale theaters and concert halls. The recent available personal‐computer‐based auralization technologies brought the possibility of applying the latest science and art of architectural acoustics into design practices. Case studies presented in this paper illustrate that the auralization technology makes it possible for potential occupants as well as designers to evaluate the acoustic performance of a design by hearing it directly before a decision to build is made. They also illustrate that the auralization is a powerful tool for the general public to uncover everyday acoustic problems in common building types that have been constantly harming their well being and would otherwise be undetected. Furthermore, they demonstrate that auralization is an effective means to evaluate noise effect on subjective perception where the noise effect is signal context dependent.<ppx>PACS4355Br,4355KaEPACS<ppa>

Interactive and aliasing‐free acoustic modeling of reflections and diffractions in architectural environments (A)

Thomas Funkhouser, Nicolas Tsingos, Ingrid Carlbom, Gary Elko, Gopal Pingali, Mohan Sondhi, and Jim West

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2389-2389 (2001); (1 page)

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A primary challenge in geometrical acoustic modeling is computation of reverberation paths from sound sources fast enough for real‐time auralization. This paper describes an aliasing‐free beam tracing algorithm based on precomputed spatial subdivision and beam tree data structures that enables real‐time acoustic modeling and auralization for sound sources in interactive applications. The proposed method traces convex polyhedral beams from the location of each sound source and receiver through a precomputed spatial subdivision data structure, constructing a beam tree representing the regions of space reachable by potential sequences of transmissions, diffractions, and specular reflections at surfaces of a 3D polygonal model. By computing beam trees asynchronously (off‐line), our system can generate reverberation paths between sources and receivers at interactive rates and spatialize audio signals in real‐time as sources and receivers move under interactive user control. Unlike previous geometrical acoustic modeling work, our beam tracing method: (1) supports evaluation of early reverberated paths at interactive rates, (2) scales well for large, densely occluded architectural environments, and (3) computes paths of diffraction without aliasing using the uniform theory of diffraction. This system is being used to develop interactive applications in which a user experiences a virtual environment immersively via simultaneous auralization and visualization.<ppx>PACS4355Ka,4320El,4355BrEPACS<ppa>

Spatial and temporal variation of sperm whale (Physeter macrocephalus) codas in the northern Gulf of Mexico (A)

Trent C. Apple

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2390-2390 (2001); (1 page)

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Sperm whale (Physeter macrocephalus) codas, short rhythmic patterns of clicks, were recorded annually from a towed linear array throughout the northern Gulf of Mexico during 2‐week cruises spanning October 1991–August 1997. Codas were sampled and classified according to their temporal pattern and the number of clicks they contained. Regular codas, with equally spaced intervals between clicks, were analyzed along with irregular codas, with double intervals between the last two clicks. In this sample, short codas (less than five clicks) were more common than long codas (greater than six clicks). Preliminary analysis (n=11), using Student’s t‐test, suggests that irregular codas may not vary in the northern Gulf of Mexico. When comparing interclick intervals, irregular coda types did not differ significantly across multiple years, and no spatial variation was observed. This pattern of similarity in coda types, if upheld with additional analyses, may suggest that coda repertoires remain stable over periods of years or that a resident population of animals may inhabit this area. Further analysis, however, is required to substantiate this supposition. [Work supported by Texas A&M University at Galveston.]<ppx>PACS4380CsEPACS<ppa>

Analysis of frequency structure of sperm whale clicks as a function of dive depth and animal orientation (A)

Aaron M. Thode, David K. Mellinger, Sarah Stienessen, Anthony Martinez, and Keith Mullin

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2390-2390 (2001); (1 page)

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During overnight tracking of a pod of sperm whales in the Gulf of Mexico in July 2000, the NOAA ship GORDON GUNTER recorded their characteristic ‘‘click’’ sounds on a five‐element towed hydrophone array. Multiple reflections from the surface and ocean bottom were also recorded. Analysis of the arrival times and bearings of the reflections allowed the computation of three dimensional dive profiles for several animals. By assuming the orientation of the animal was aligned with its velocity, the relative orientation of the animals relative to the array could also be estimated. A visual examination of the frequency content of the received clicks versus dive time suggested that the double resonances in the 1.2 and 2.2 kHz band increased 20%–30% during depth changes of 1000 m. In this presentation the possible relationship between click structure and whale depth and orientation is rigorously analyzed, and the observed relationships are compared with predictions from various sound production and resonator models, in an attempt to gain insight into the sound production mechanism. [Work supported by Minerals Management Service, National Marine Fisheries Service, and ONR.]<ppx>PACS4380‐nEPACS<ppa>

An ‘‘acoustic niche’’ for Antarctic killer whale and leopard seal sounds (A)

Julia A. Mossbridge and Jeanette A. Thomas

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2390-2390 (2001); (1 page)

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It is not known whether species that share the same acoustic environment develop an ‘‘acoustic niche’’ to reduce competition. To address this issue, spectrograms were used to analyze underwater killer whale (Orcinus orca) and leopard seal (Hydrurga leptonyx) sounds from recordings made in December and January 1977/1978 in Antarctica. Forty‐seven leopard seal sounds were found in December recordings; no leopard seal sounds were found in January recordings. FM points were defined as the frequency within a sound at which a detectable change in the frequency slope occurred. The FM‐point distribution for December killer whale sounds showed a gap between 2500 and 3750 Hz, which is near the frequency range of the most common leopard seal sound (2777–3802 Hz). No gap was found in January’s FM‐point distribution. The FM‐point distributions differed significantly between months, while the number of FM points per sound and the duration of killer whale sounds did not differ between months. December killer whale sounds did contain frequencies between 2500 and 3750 Hz, but frequency modulations were uncommon in this frequency range. It seems that killer whales may have used frequency modulation to adapt their acoustic niche when leopard seal sounds were present. [Work supported by NSF.]<ppx>PACS4380Ka,4380NdEPACS<ppa>

The underwater whistle repertoire of wild false killer whales (Pseudorca crassidens) (A)

A. E. Nester, J. A. Thomas, and A. Acevedo‐Guiterrez

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2390-2390 (2001); (1 page)

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There is limited information on the underwater repertoire of false killer whales (Pseudorca crassidens). Underwater recordings of feeding and traveling whales were made near Isla de Coco, Costa Rica in January and February 1994. This study classified the whistle repertoire, analyzed acoustic characteristics, examined differences associated with feeding and traveling whales, and examined whistles in varying noise environments. Whistles were categorized by contour into eight types; 197 were during feeding and 172 during traveling. Whistles were 1220 to 17<th>891 Hz, had harmonics with intervals between 1797 and 13<th>125 Hz, and were 0.03 to 2.07 s long. Some differences in whistles from traveling and feeding whales were found. Cluster analysis, principle component analysis, and stepwise discriminant function were used to classify whistle types. Ambient noise was measured at 1 and 2 kHz in recordings of seven different ambient environments (over a 25‐dB range). Quieter environments were associated with feeding whales. When ambient noise was high, whales shifted the frequency of whistles higher and increased the duration and total number of whistles in a series. Results provide evidence for whales adapting frequency and time structure of their whistles in different noise environments.<ppx>PACS4380KaEPACS<ppa>

Whistle repertoire of Pacific white‐sided dolphins (Lagenorhynchus obliquidens) at the John G. Shedd Aquarium. (A)

Julie L. Whitten and Jeanette A. Thomas

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2391-2391 (2001); (1 page)

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Whistles of Pacific white‐sided dolphins (Lagenorhynchus obliquidens) at the John G. Shedd Aquarium were studied to determine: (1) whistles associated with a particular individual, (2) if whistles of isolated dolphins differed from whistles from dolphins in a group, (3) time and/or frequency variables important in classification, and (4) whistle types. Individual dolphin whistles were recorded in an isolated pool using a hydrophone and a Telex recorder. Whistles were recorded from dolphins in a group setting using the same hydrophone, but with a Marantz cassette recorder. Whistles were digitized using Canary software. Frequency and time measurements were taken from power spectra and spectrograms. ANOVA, principle component analysis (PCA), discriminate analysis (DFA), and cluster analysis were performed. Whistles were classified visually into six types. There were no significant differences between whistles from isolated or social dolphins. PCA showed duration of the series, number of whistle in the series, maximum frequency, minimum frequency, and peak frequency were important in classification. DFA showed dominant bandwidth, duration of first whistle, harmonic structure, maximum frequency, and peak frequency were important in categorizing types. Using the variables in PCA, cluster analysis separated the whistles into different types than those classified visually. This study provides no evidence of individualized signature whistles.<ppx>PACS4380KaEPACS<ppa>

Separation of the acoustic ‘‘sound space’’ by nearshore fishes in the Great Barrier Reef, Australia (A)

Robert D. McCauley and Douglas H. Cato

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2391-2391 (2001); (1 page)

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Long‐term sea noise recordings were made off Cowley Beach. Four fish sources dominated ambient noise. Each displayed unique patterns in calling: character, habits, timing, and spatial extent. Two sources, a knocking and drumming, were up to 2 s long, always occurred as distinct calls, had spectral peaks over 350–450 Hz and had respective mean squared pressure (msp) source levels of 132 and 149 dB re 1 μPa @ 1 m. Call rates to 120/10 min occurred during the Austral summer, the highest calling period for all sources. The other sources, a ‘‘trumpet’’ and ‘‘pop,’’ occurred in choruses and, respectively, were 100–220 ms and <20 ms long; had spectral peaks over 800–1500 Hz and 400–700 Hz; and had source levels of 150 and 141 dB re 1 μPa @ 1 m (msp). Trumpet choruses were up to 2 km across and detectable to 8 km from chorus center in water <10 m deep. Popping choruses were dispersed over larger regions. It was possible that in nearshore waters all sources occurred within a 24‐h period. Sources avoided competition for the acoustic ‘‘space,’’ partly by offsetting spectral peaks and geographic separation, but primarily by separating the time of chorus or maximum call‐rate.<ppx>PACS4380KaEPACS<ppa>

Analysis of surface vocalizations from mother and pup Weddell seals (Leptonychotes weddellii) (A)

Beth A. Noe and Jeanette Thomas

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2391-2391 (2001); (1 page)

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The antarctic Weddell seal (Leptonychotes weddellii) is a vociferous species with an extensive underwater repertoire, but little is documented about their airborne sounds. Weddell seals return to traditional pupping colonies, are approached easily, and recordings of individual seals are available from a tagging study conducted since the early 1970’s. Therefore, this study on airborne vocalizations of Weddell seal mothers and their pups provides a unique opportunity for detailed analysis of calls. During 1976, 1977, 1979, and 1996, surface vocalizations of mother and pup Weddell seals were recorded. Analysis variables included beginning, ending, maximum, and minimum frequency, fundamental harmonic interval, components per call, intercall interval, and duration. This study determined that surface calls of Weddell seal pups changed as the pup grew older. Although pups within an age bracket showed little variability in call structure, adult vocalizations varied among individuals. Relatedness of mothers and pups could not be concluded solely by the call variables examined. Further studies need to be conducted to determine the extent of the change that pup vocalizations undergo as they mature and to identify the acoustic variables influencing mother/pup vocalizations. [Acknowledgments: Dr. Jeanette Thomas, Dr. Douglas Quin, Bart Jones.]<ppx>PACS4380‐nEPACS<ppa>

Patterns of echolocation in a group of captive bottlenose dolphins in Roatan, Honduras (A)

Heather L. Heinemeyer and J. A. Thomas

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2391-2391 (2001); (1 page)

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Patterns of echolocation in a group of bottlenose dolphins (Tursiops truncates) at the Institute for Marine Sciences in Roatan, Honduras were examined. While much is known about echolocation, the role of echolocation in the context of dolphin social or communicatory behavior has been, for the most part, speculative (Xitco, 1996). Two females and two males were paired and presented a variety of novel targets in the water. Approximately equal numbers of each pair combination were conducted over 22 sessions consisting of a 5‐min no‐target trial followed by a 5‐min target trial. Data were obtained via an underwater video camera and by an underwater observer. Number of click trains, silent passes, passes by both dolphins, passes by a clicking dolphin, and times a dolphin approach and click on the target first were analyzed. Clearly, dolphins paid attention to the targets. The number of trains and passes significantly increased during target trials. Females clicked more often during target trials and were first to approach and click at the target. Target type had no effect on echolocation behavior. This study should be repeated at other facilities and with other dolphins to determine whether gender differences are real or due to small sample sizes.<ppx>PACS4380KaEPACS<ppa>

Blue whale calling off the Western Australian coast (A)

Robert D. McCauley, Curt Jenner, John L. Bannister, and Douglas H. Cato

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2391-2391 (2001); (1 page)

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Throughout 2000, aerial surveys, boat‐based studies and acoustical censusing was carried out to search for blue whales in the Rottnest trench, west of Perth. Historical records indicated a Western Australian population of blue whales, while a preliminary boat survey in 1994 suggested a Rottnest trench aggregation. This was confirmed in 2000, with 17 blue whales sighted. Of 5000 acoustic records almost all contained blue whale calling, some having up to nine distinct callers. Although of a different format, the calls had a similar character to those described from other populations. A call was composed of three signals, each respectively 44–45 s, 20–23 s and 20–25 s in length, separated by 5–10 s and 23 s with the sequence repeated every 78 s. Each component was dominated by constant or slowly up‐sweeping amplitude‐modulated tones over 18–26 Hz, with harmonics evident up to 100 Hz and a secondary pulsed source of frequency 65–66 Hz present. Components had different source levels. Several other baleen whale calls were also common. The low frequency (<100 Hz) sea noise spectra made over 33.5 days off Rottnest was dominated by whale calling.<ppx>PACS4380KaEPACS<ppa>

An improved contour extractor for bottlenose dolphin whistles (A)

Ryuji Suzuki and John R. Buck

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2392-2392 (2001); (1 page)

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The time‐frequency contours of bottlenose dolphin (Tursiops truncatus) whistles are commonly extracted features for the classification and comparison of these signals. We have previously developed a method to extract the whistles’ fundamental frequency contours [J. Acoust. Soc. Am. 108, 2635 (2000)] by sequentially tracking the contour. These contours are often extracted from recorded rather than real‐time signals, which allows us to read ahead or access the ‘‘future’’ signal. This talk extends our previous work to propose a technique to estimate the fundamental frequency at each time from the ‘‘future’’ values of the recorded signal in addition to the past and present signals. This technique is similar to the Kalman smoother except that the states are probability densities, and it uses a probabilistic state transition model with a Bayesian projection to produce the maximum a posteriori probability estimate of the frequency contour based on all recorded data, not just the present and past signal. Experimental evaluation indicates that this method greatly improves the reliability of contour extraction from noisy signals, with considerable resistance to strong harmonics, some dropouts and echolocation clicks, when compared to our previous technique. A huge improvement is achieved from the technique proposed by Buck and Tyack [J. Acoust. Soc. Am. 94, 2497–2506 (1993)].<ppx>PACS4380KaEPACS<ppa>

Modeling of sound propagation in sandy ocean sediments (A)

Nicholas P. Chotiros

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2392-2392 (2001); (1 page)

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A physically accurate model of sound reflection and penetration into ocean sediments is the objective. Sandy sediments are of particular interest because current models are not compatible with acoustic measurements. The discrepancies of the fluid and elastic solid approximations are clearly demonstrable. The Biot theory has the potential to satisfy the objective. Stolls formulation requires 13 input parameters, which may be divided into three groups according to the accuracy with which they are known. The first group consists of tabulated physical constants; the second is less precisely known, and the third is not measurable. An inversion procedure was devised to estimate the immeasurable group from simple acoustic measurements — reflection loss, compressional and shear wave speeds, and attenuations. The imprecisely known group was handled in a probabilistic manner. The inversion results for water‐saturated sand, using published laboratory and at‐sea measurements, show a definite incompatibility between model and measurements. To find a solution, a couple of hypotheses were considered: (1) the inclusion of some proportion of the pore fluid within the frame and (2) the relaxation of the uniform, elastic frame assumption. The latter proved to be a viable solution. [Work supported by ONR, Ocean Acoustics.]<ppx>PACS4330Cq,4330MaEPACS<ppa>

WARBLE: A high resolution approach to subbottom geoacoustic inversion (A)

Charles W. Holland and John Osler

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2392-2393 (2001); (2 pages)

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Subbottom geoacoustic information supports a broad range of activities serving military, scientific, and commercial needs. Since direct sampling (i.e., coring) of the seabed is costly and time‐consuming, numerous acoustic inverse methods have been developed. One of the most serious weaknesses, which is suffered by all the inversion methods, is a uniqueness problem, wherein multiple solutions fit the data equally well. An associated problem is that some methods have no means to determine the number of sediment layers in the solution space. Our WARBLE (wide‐angle reflection and bottom loss experiment) method addresses these problems by conducting the inversion in both the space‐time and the space‐frequency domains in order to maximize the amount of independent data. The space‐time data provide the number of important layers as well as the layer thicknesses, and interval sound speeds. The space‐frequency domain data provide sound velocity, density, and attenuation for each layer. High‐resolution results can be obtained with this method, resulting in layer thickness of order several tens of centimeters. Penetration depends upon sediment type and water depth but is typically 50–150 m. The inversion method will be illustrated with several shallow‐water measurement/inversion results, including comparisons with core data.   <ppx>PACS4330Pc,4330MaEPACS<ppa>

Quantitative characterization of the uppermost seabed stratum: Some results from the ISACS project (A)

Andrea Caiti, Jens M. Hovem, and Bjarte Berntsen

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2393-2393 (2001); (1 page)

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ISACS is a research initiative funded by the European Union in the years 1996–1999. The objective was to prove the feasibility of seafloor characterization by exploitation and integration of data from commercially available sonar equipment. The activities have focused on analysis of side‐scan, multibeam and parametric sonar data, and have resulted in processing methods for classification, visualization, and quantitative characterization of the uppermost seabed stratum. Among these, two approaches have been implemented as stand‐alone software packages for identification of bottom roughness, impedance and volume inhomogeneities by inversion of parametric sonar data. The first package, sirob, relies on a model‐based approach in which the forward model is the prediction code boris. Bottom parameters are identified by minimizing the data‐prediction discrepancy in a generalized time‐frequency domain. The second package, farim, uses the coherent part of the backscattered time signal. The frequency shift in the data is used for finding the roughness, and roughness estimate is used to correct the impedance estimate found from the returned echo. farim is simpler and faster than sirob, but it determines a subset of the parameters identified by sirob. Results obtained on field data show good agreement among both methods and available ground truth.   <ppx>PACS4330Ma,4330PcEPACS<ppa>

Techniques for estimating sediment properties from chirp sonar data (A)

Steven G. Schock

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2393-2393 (2001); (1 page)

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The chirp sonar is a towed wideband FM reflection profiler developed to collect normal incidence reflection data suitable for sediment property inversions; the sonar also generates detailed reflection profiles of the seabed. The chirp sonar system transmits FM pulses with high time‐bandwidth products to attain high pulse energy; consequently, the acoustic data have a high signal‐to‐ambient‐noise ratio after correlation processing. The vertical resolution of the images is about 1 cm when transducer bandwidth is approximately 40 kHz. Data processing methods have been developed to automatically map the locations of sediment layer interfaces, and to estimate compressional wave attenuation, acoustic impedance, and compressional wave velocity of sediment layers using normal incidence FM reflection data. The height of sand ripples and the phase dispersion of compressional waves can also be measured from the acoustic imagery. Field data collected by other investigators during the High Frequency Acoustics DRI experiments off Fort Walton Beach were used to verify the acoustic property estimates. Comparisons show that chirp sonar estimates of acoustic properties agree with direct measurements of the properties based on in situ acoustic probe data and sediment core data collected by investigators from Naval Research Laboratory, Stennis Space Center, and APL, Washington State.<ppx>PACS4330MaEPACS<ppa>

Matched‐field geoacoustic inversion in a range‐varying waveguide (A)

Chris A. Gillard, David J. Thomson, Gordon R. Ebbeson, and Garry J. Heard

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2393-2393 (2001); (1 page)

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In recent years, geoacoustic inversion methods based on matched‐field processing concepts have been devised for inferring the sub‐bottom properties of shallow‐water waveguides using measurements of acoustic signals received on hydrophone arrays. The development of these matched‐field inversion (MFI) procedures is driven by two goals: (1) to obtain an accurate geophysical description of the morphology of the sub‐bottom structure, and (2) to characterize the acoustic response of the sub‐bottom so that its effect on sound propagation can be predicted. Knowledge of (2) can improve the effective ranges for using sound in underwater communication and/or source localization applications. Of practical interest is the question: How crude can our estimates of the sub‐bottom structure be and still allow successful acoustic source localization using model‐based signal processing methods? In this paper, a hybrid simplex simulated annealing MFI code [M. R. Fallat and S. E. Dosso, J. Acoust. Soc. Am. 105, 3219–3230 (1999)] is combined with a high‐order parabolic equation (PE) model and applied to geoacoustic inversion in a shallow‐water waveguide whose sub‐bottom properties vary with range. In particular, we investigate simple sub‐bottom characterizations for some test cases considered recently at the Inversion Techniques Workshop held in Gulfport, MS. a)Presently on exchange from the Defence Science and Technology Organisation, Australia.<ppx>PACS4330PcEPACS<ppa>

Range‐dependent geoacoustic inversion (A)

Anna‐Liesa Lapinski, Stan Dosso, and Ross Chapman

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2393-2393 (2001); (1 page)

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This paper considers acoustic inversion for a minimum‐structure geoacoustic model in a range‐dependent environment. The general survey configuration consists of recording acoustic fields at a vertical array of sensors due to a number of sources distributed in range along a track. The unknown model consists of the sound speeds and thicknesses of sub‐bottom layers, which vary with range in an arbitrary manner, and geometric parameters of the experiment. The goal is to determine a range‐dependent geoacoustic model with the least structure that is consistent with the resolving power of the data. An under‐parametrized approach is developed, which consists of solving the inverse problem a number of times, each time increasing the range variability allowed in the model. The optimal parametrization is subsequently obtained by examining the data mismatch and a norm of the model structure as a function of the number of parameters. The individual inversions are performed using an adaptive hybrid inversion algorithm applied to all data simultaneously.<ppx>PACS4330Pc,4360PtEPACS<ppa>

Range‐dependent seabed characterization by inversion of acoustic data from a towed receiver array (A)

Martin Siderius, Peter L. Nielsen, and Peter Gerstoft

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2394-2394 (2001); (1 page)

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The MAPEX2000 experiments were conducted by the SACLANT Undersea Research Centre in the Mediterranean Sea in March 2000 to determine the performance of acoustic inversion for seabed properties using a towed, horizontal receiver array. Towed systems are advantageous because they are easily deployed from a ship and the moving platform offers the possibility of estimating spatially variable (range‐dependent) seabed properties. Previous research has successfully applied matched field processing (MFP) geoacoustic inversion techniques to measured acoustic data, however, in nearly all cases the inverted data were collected on moored, vertical receiver arrays. In the MAPEX2000 experiments acoustic data were collected on both towed and moored vertical receiver arrays. Results will be presented showing that seabed properties (e.g., seabed sound speed, sediment layer thickness and attenuation constant) can be extracted using MFP inversion of acoustic measurements from a towed array of receivers. These seabed properties agree with those inverted using data received simultaneously on a vertical array. These findings imply that a practical technique can be developed to map range‐dependent seabed properties over large areas using a towed acoustic system. An example of such a range‐dependent inversion is given for two sites from the MAPEX2000 experiments.<ppx>PACS4330PcEPACS<ppa>

Parameter coupling in broadband geoacoustic inversion (A)

John S. Perkins, Michael D. Collins, Dalcio K. Dacol, Laurie T. Fialkowski, and Joseph F. Lingevitch

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2394-2394 (2001); (1 page)

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For many geoacoustic inverse problems involving a single frequency data set, the search parameters are often strongly coupled to one another. This may mean that individual parameters are difficult to determine, but combinations of parameters can be resolved. In terms of the optimization process, the parameter landscape that is being searched has multidimensional valleys that hinder location of a global minimum. A coordinate rotation technique has been developed [J. Acoust. Soc. Am. 98, 1637–1644 (1995)] to speed the search for optimal parameters. This technique also gives valuable information about the parameter hierarchy and parameter couplings. One approach to decoupling the parameters and obtaining realistic physical parameter values is to process broadband data or multifrequency data. A new fast forward modeling technique [J. Acoust. Soc. Am. 106, 1727–1731 (1999)] makes it possible to consider broadband data for geoacoustic inversion. The coordinate rotation technique is combined with fast forward modeling to study parameter couplings for broadband and multifrequency problems. [Work supported by ONR.]<ppx>PACS4330PcEPACS<ppa>

Geoacoustic inversion hybrid model (A)

Mark S. Haire and David P. Knobles

J. Acoust. Soc. Am. Volume 105, Issue 5, pp. 2394-2394 (2001); (1 page)

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Simulated annealing (SA) or genetic algorithms are accepted optimization methods for geoacoustic inversion to obtain seabed parameters from measured acoustic data. These techniques permit optimizations where numerous local minima are present, but are computationally intensive. Gradient methods, such as the Levenberg–Marquardt (LM) approach, are extremely efficient at finding local minima, but typically are unable to locate the global minimum. To improve the computation speed and efficiency of SA, a ‘‘hybrid’’ algorithm combining SA and LM has been developed. The hybrid approach applies a LM algorithm after each temperature cycle of SA if an improved cost function value has been obtained during the cycle. The hybrid technique takes advantage of the fact that SA often examines a point that is in the proximity of the global minimum when the temperature is high. Factors of 3–8 increases in speed over SA alone are obtained with simulated data. Results will be presented from the application of the hybrid model to experimental data collected during the Area Characterization Test‐I in the Gulf of Mexico. [Work supported by ONR.] <ppx>PACS4330Wi,4330PcEPACS<ppa>
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