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The aquatic world of animals is an acoustic world as sound is the most prominent sensory capacity to extract information about the environment for many aquatic species. Fish can hear particle motion, and a swim bladder potentially adds the additional capacity to sense sound pressure. Combining these capacities allows them to sense direction, distance, spectral content, and detailed temporal patterns. Both sound pressure and particle motion were recorded in a shallow part of the North Sea before and during exposure to a full-scale airgun array from an experimental seismic survey. Distinct amplitude fluctuations and directional patterns in the ambient noise were found to be fluctuating in phase with the tidal cycles and coming from distinct directions. It was speculated that the patterns may be determined by distant sources associated with large rivers and nearby beaches. Sounds of the experimental seismic survey were above the ambient conditions for particle acceleration up to 10 km from the source, at least as detectable for the measurement device, and up to 31 km for the sound pressure. These results and discussion provide a fresh perspective on the auditory world of fishes and a shift in the understanding about potential ranges over which they may have access to biologically relevant cues and be masked by anthropogenic noise.& nbsp;(C)2021 Acoustical Society of America

In vivo ultrasonic attenuation was estimated for extracranial soft tissues of two Tursiops truncatus and one Delphinapterus leucas. Backscatter data was non-invasively collected as part of routine health-based ultrasound examinations using a transducer operating in the 1.6-3.7 MHz frequency range. Data collected over the proximal mandible and temporal regions was processed to yield attenuation estimates using a reference tissue phantom whose properties had been independently determined. The estimated attenuations were at the low end of the range of literature-reported values for mammalian fatty and connective tissues. A companion model-based analysis quantified errors arising from tissue composition and sound speed uncertainties.

Pacific bluefin tuna (Thunnus orientalis) is a highly migratory, commercially valuable species potentially vulnerable to acoustic noise generated from human activities which could impact behavior and fitness. Although significant efforts have been made to understand hearing abilities of fishes, the large size and need to continuously swim for respiration have hindered investigations with tuna and other large pelagic species. In this study, Pacific bluefin tuna were trained to respond to a pure tone sound stimulus ranging 325-800 Hz and their hearing abilities quantified using a staircase psychophysical technique. Hearing was most sensitive from 400 to 500 Hz in terms of particle motion (radial acceleration -88 dB re 1 m s(-2); vertical acceleration -86 dB re 1 m s(-2)) and sound pressure (83 dB re 1 μPa). Compared to yellowfin tuna (Thunnus albacares) and kawakawa (Euthynnus affinis), Pacific bluefin tuna has a similar bandwidth of hearing and best frequency, but greater sensitivity overall. Careful calibration of the sound stimulus and experimental tank environment, as well as the adoption of behavioral methodology, demonstrates an experimental approach highly effective for the study of large fish species in the laboratory.

It is currently thought that all fishes detect acoustic particle motion, and it is therefore critical to measure this vector quantity when studying hearing, acoustic behavior, and noise impacts. Proper measurement of particle motion in any of its forms (acceleration, velocity, or displacement) is subject to a range of errors whose magnitude depends on the sound source(s) of interest and the environments in which they are observed. Particle motion measurement principles and errors have primarily been addressed in the literature for free field plane waves. However, such fields are rarely encountered in bioacoustic studies, and experiments or calculations made based on plane wave assumptions can lead to substantially erroneous measurement results and flawed study conclusions. This paper presents a unified treatment of underwater acoustic particle motion measurement by reviewing acoustic field attributes of commonly encountered source/environment scenarios, quantifying measurement error sources, and concluding with guidelines and recommendations for bioacoustic studies.

Evidence suggests that the capacity for sound source localization is common to mammals, birds, reptiles, and amphibians, but surprisingly it is not known whether fish locate sound sources in the same manner (e.g., combining binaural and monaural cues) or what computational strategies they use for successful source localization. Directional hearing and sound source localization in fishes continues to be important topics in neuroethology and in the hearing sciences, but the empirical and theoretical work on these topics have been contradictory and obscure for decades. This chapter reviews the previous behavioral work on directional hearing and sound source localization in fishes including the most recent experiments on sound source localization by the plainfin midshipman fish (Porichthys notatus), which has proven to be an exceptional species for fish studies of sound localization. In addition, the theoretical models of directional hearing and sound source localization for fishes are reviewed including a new model that uses a time-averaged intensity approach for source localization that has wide applicability with regard to source type, acoustic environment, and time waveform.

Researchers often perform hearing studies on fish in small tanks. The acoustic field in such a tank is considerably different from the acoustic field that occurs in the animal’s natural environment. The significance of these differences is magnified by the nature of the fish’s auditory system where either acoustic pressure (a scalar), acoustic particle velocity (a vector), or both may serve as the stimulus. It is essential for the underwater acoustician to understand the acoustics of small tanks to be able to carry out valid auditory research in the laboratory and to properly compare and interpret the results of others.

In this paper, we discuss the issues encountered when trying to perform hearing experiments in water-filled tanks that are several meters in lateral extent, typically large in terms of the size of the animals under study but not necessarily so with respect to the wavelengths of interest. This paper presents measurements of pressure and particle motion fields in these “large” tanks. The observed characteristics and complexities are discussed in reference to their potential impact on the planning and interpretation of hearing experiments.

Researchers often perform hearing studies on fish in small tanks. The acoustic field in such a tank is considerably different from the acoustic field that occurs in the animal's natural environment. The significance of these differences is magnified by the nature of the fish's auditory system where either acoustic pressure (a scalar), acoustic particle velocity (a vector), or both may serve as the stimulus. It is essential for the underwater acoustician to understand the acoustics of small tanks to be able to carry out valid auditory research in the laboratory and to properly compare and interpret the results of others.

In this paper, we discuss the issues encountered when trying to perform hearing experiments in water-filled tanks that are several meters in lateral extent, typically large in terms of the size of the animals under study but not necessarily so with respect to the wavelengths of interest. This paper presents measurements of pressure and particle motion fields in these "large" tanks. The observed characteristics and complexities are discussed in reference to their potential impact on the planning and interpretation of hearing experiments.

Evidence suggests that the capacity for sound source localization is common to mammals, birds, reptiles, and amphibians, but surprisingly it is not known whether fish locate sound sources in the same manner (e.g., combining binaural and monaural cues) or what computational strategies they use for successful source localization. Directional hearing and sound source localization in fishes continues to be important topics in neuroethology and in the hearing sciences, but the empirical and theoretical work on these topics have been contradictory and obscure for decades. This chapter reviews the previous behavioral work on directional hearing and sound source localization in fishes including the most recent experiments on sound source localization by the plainfin midshipman fish (Porichthys notatus), which has proven to be an exceptional species for fish studies of sound localization. In addition, the theoretical models of directional hearing and sound source localization for fishes are reviewed including a new model that uses a time-averaged intensity approach for source localization that has wide applicability with regard to source type, acoustic environment, and time waveform.

All fish can detect the three vector components of particle motion, enabling them to determine source bearing, but particle motion alone cannot resolve which direction is towards the source and which direction is away (i.e., there is a 180° ambiguity). However, fish with swim bladders can also detect acoustic pressure which enables such fish to resolve this 180° ambiguity. Proposed processing algorithms, though, often employ unrealistic restrictions such as sinusoidal time dependence and free-field conditions. We propose a robust algorithm for determining the unambiguous source direction by a fish that can detect acoustic pressure. The method is based on the time-averaged acoustic intensity vector (the time-integral of the product of pressure and particle velocity). This non-oscillatory quantity points in the direction of energy flow and hence directly away from the source. The algorithm has the advantage of being applicable to signals with any time dependence, in any propagation environment. For sharks, which do not have swim bladders, we hypothesize that the vertical component of particle acceleration can be substituted for pressure, provided that the shark is sufficiently close to the water surface. We test the validity of this hypothesis by determining the direction to the source for reef noise, a biologically relevant signal, and boat noise in a range-varying shallow water environment.All fish can detect the three vector components of particle motion, enabling them to determine source bearing, but particle motion alone cannot resolve which direction is towards the source and which direction is away (i.e., there is a 180° ambiguity). However, fish with swim bladders can also detect acoustic pressure which enables such fish to resolve this 180° ambiguity. Proposed processing algorithms, though, often employ unrealistic restrictions such as sinusoidal time dependence and free-field conditions. We propose a robust algorithm for determining the unambiguous source direction by a fish that can detect acoustic pressure. The method is based on the time-averaged acoustic intensity vector (the time-integral of the product of pressure and particle velocity). This non-oscillatory quantity points in the direction of energy flow and hence directly away from the source. The algorithm has the advantage of being applicable to signals with any time dependence, in any propagation environment. For sharks, ...

A joint China-U.S. underwater acoustics experiment was conducted in the Yellow Sea with a very flat bottom and a strong and sharp thermocline. Broadband explosive sources were deployed both above and below the thermocline along two radial lines up to 57.2 km and a quarter circle with a radius of 34 km. Two inversion schemes are used to obtain the seabottom sound speed. One is based on extracting normal mode depth functions from the cross-spectral density matrix. The other is based on the best match between the calculated and measured modal arrival times for different frequencies. The inverted seabottom sound speed is used as a constraint condition to extract the seabottom sound attenuation by three methods. The first method involves measuring the attenuation coefficients of normal modes. In the second method, the seabottom sound attenuation is estimated by minimizing the difference between the theoretical and measured modal amplitude ratios. The third method is based on finding the best match between the measured and modeled transmission losses (TLs). The resultant seabottom attenuation, averaged over three independent methods, can be expressed as alpha=(0.33+/-0.02)f(1.86+/-0.04)(dB/m kHz) over a frequency range of 80-1000 Hz.

In the Shallow Water’ 06 experiment, two L-shape arrays (ARRAY52 and ARRAY32) were deployed. The vertical line array (VLA) components of both ARRAY52 and ARRAY32 were exactly perpendicular to the direction of sound propagation. The horizontal line array (HLA) component of ARRAY52 was exactly perpendicular to the direction of sound propagation. The HLA component of ARRAY32 was exactly parallel to the direction of sound propagation. This configuration offered an opportunity to simultaneously analyze the three dimensional (3-D) spatial coherences: vertical, transverse horizontal and longitudinal horizontal. When the source and the receivers were below the thermocline, both the vertical and longitudinal horizontal coherence lengths in units of wavelength increased with increasing range and frequency. When the source was within the thermocline, the transverse horizontal coherence length in units of wavelength decreased with frequency and exhibited weak range dependence.

The results of flow visualization and particle image velocimetry (PIV) studies of steady streaming flows near oscillating spheroids (aspect ratios of 0.75, 1, and 1.33) are reported for normalized amplitudes of 0.025–0.15 and Reynolds numbers of 27–89. The oscillations are described by either a single-frequency sine wave or the sum of two such sine waves with different frequencies. For normalized amplitudes of 5% or less, the PIV data suggest that steady streaming flow fields superpose. In other words, the time-independent portion of the velocity field created by forcing the spheroid simultaneously at two different frequencies is found to be the sum of the two steady streaming velocity fields created by forcing the body individually at each of the two frequencies.

Experimental studies of steady streaming flows are presented for spheroids of various aspect ratios oscillating in a viscous fluid along their axis of symmetry at Reynolds numbers up to 150 and normalized oscillation amplitudes up to 1.2. The steady streaming component of the flow is visualized using phase-locked particle pathline images along the central plane of the spheroid. The extent of the inner region of the flow, characterized by the distance to the stagnation points, appears to depend on a length scale based on the spheroid aspect ratio and equivalent radius. Spheroids were also oscillated at a nonzero angle with respect to their axis of symmetry. The angular displacement of the stagnation points on the surface of these obliquely oscillated spheroids is related to the aspect ratio and the angle of oscillation.

Nonlinear acoustic properties of a composite medium, consisting of hollow microspheres suspended in Castor Oil, at elevated hydrostatic pressure are experimentally investigated. The acoustic nonlinear parameter B∕A of the medium is found to be highly dependent upon hydrostatic pressure. B∕A varies from a small negative value near ambient pressure to a very large negative value (around −6000) in the vicinity of 7×104 Pascals, above ambient pressure. With a further increase in hydrostatic pressure the magnitude of B∕A decreases passing through zero and finally becomes positive. Finite amplitude wave propagation in this medium at low hydrostatic pressures is characterized by waveform steepening in the backward direction leading to rarefactive shockwaves and, at high hydrostatic pressures, by steepening in the forward direction leading to compressive shockwaves. Estimates of B∕A are obtained from both the measurement of thermodynamic properties and from waveform distortion during propagation.

Reverberation interferes with the ability to understand speech in rooms. Overlap-masking explains this degradation by assuming reverberant phonemes endure in time and mask subsequent reverberant phonemes. Most listeners benefit from binaural listening when reverberation exists, indicating that the listener's binaural system processes the two channels to reduce the reverberation. This paper investigates the hypothesis that the binaural word intelligibility advantage found in reverberation is a result of binaural overlap-masking release with the reverberation acting as masking noise. The tests utilize phonetically balanced word lists (ANSI-S3.2 1989), that are presented diotically and binaurally with recorded reverberation and reverberation-like noise. A small room, 62 m3, reverberates the words. These are recorded using two microphones without additional noise sources. The reverberation-like noise is a modified form of these recordings and has a similar spectral content. It does not contain binaural localization cues due to a phase randomization procedure. Listening to the reverberant words binaurally improves the intelligibility by 6.0% over diotic listening. The binaural intelligibility advantage for reverberation-like noise is only 2.6%. This indicates that binaural overlap-masking release is insufficient to explain the entire binaural word intelligibility advantage in reverberation.

Optimal array-processing techniques in the ocean often require knowledge of the spatial coherence of the reverberation. A mathematical model is derived for the reverberation vertical coherence (RVC) in shallow water (SW). A method for analysis of RVC data is introduced. Measured reverberation cross-correlation coefficients as a function of time and frequency, obtained during the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, are reported. SW reverberation from a single shot provides a continuous spatial sampling of the surrounding sound field up to several tens of kilometers and holds valuable information on the geoacoustic properties of the sea floor over this distance. SW reverberation data can, therefore, be used as the basis for a quick and inexpensive method for geoacoustic inversion and has the obvious advantage that acquiring the data in situ requires only a single platform. This paper considers the use of the vertical coherence of the reverberation as the starting point for such an inversion. Sound speed and attenuation in the sea bottom at the ASIAEX site are obtained over a frequency range of 100-1500 Hz by finding values that provide the best match between the measured and predicted RVC.

Time reversal permits acoustic focusing and beam forming in inhomogeneous and/or high-scattering environments. A volumetric array geometry can suppress back lobes and can fit a large, powerful array of elements into small spaces, like the free-water spaces on submarines. This research investigates applying the time-reversal method to an underwater acoustic volume array. The experiments evaluate the focusing performance of a 27-element volume array when different scattering structures are present within the volume of the array. The array is arranged in a 3x3x3 cubic matrix configuration with 18.75-cm vertical and horizontal element spacing. The system utilizes second-derivative Gaussian pulses to focus on a point 30 cm from the array. Results include a comparison between time-reversal focusing and standard focusing, an evaluation of the volume array's ability to suppress back lobes, and an analysis of how different scattering environments affect focal region size. Potential underwater applications for a volume array using time reversal include acoustic imaging, naval mine hunting, sonar, and underwater communications.

The goal of this project is to develop and demonstrate a system for non-invasive in vivo measurement of the complex elastic moduli (stiffnesses and loss factors) of cetacean head soft tissues. This system is ultimately intended to provide a portable diagnostic capability for use in stranded animal assessments.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

Chapters cannot be read stand-alone. Please see complete SpringerBrief at: http://link.springer.com/book/10.1007/978-3-319-06659-2.

We investigated the roles of the swim bladder and the lateral line system in sound localization behavior by the plainfin midshipman fish (Porichthys notatus). Reproductive female midshipman underwent either surgical deflation of the swim bladder or cryoablation of the lateral line and were then tested in a monopolar sound source localization task. Fish with nominally "deflated" swim bladders performed similar to sham-deflated controls; however, post-experiment evaluation of swim bladder deflation revealed that a majority of "deflated" fish (88%, 7 of the 8 fish) that exhibited positive phonotaxis had partially inflated swim bladders. In total, 95% (21/22) of fish that localized the source had at least partially inflated swim-bladders, indicating that pressure reception is likely required for sound source localization. In lateral line experiments, no difference was observed in the proportion of females exhibiting positive phonotaxis with ablated- (37%) versus sham-ablated (47%) lateral line systems. These data suggest that the lateral line system is likely not required for sound source localization, although this system may be important for fine-tuning the approach to the sound source. We found that midshipman can solve the 180° ambiguity of source direction in the shallow water of our test tank, which is similar to their nesting environment. We also found that the potential directional cues (phase relationship between pressure and particle motion) in shallow water differs from a theoretical free-field. Therefore, the general question of how fish use acoustic pressure cues to solve the 180° ambiguity of source direction from the particle motion vector remains unresolved.

In order to produce high-amplitude, low-frequency signals, an underwater transducer must generate a relatively large volume displacement. Since water exerts a large reaction force back on the transducer, “conventional wisdom” dictates that such a transducer would have to be a high Q resonant device and thus not be broadband. However, a transducer does not have to be broadband in the conventional sense to meet the requirements of communication and sonar systems. A transducer that is capable of instantaneously switching between two discrete frequencies is adequate for communication and transmission of coded signals; one that is capable of switching among several frequencies could produce the chirp signals commonly used in active sonars. Ordinarily, a broadband transducer is needed to accomplish the frequency switching rapidly. A way around this difficulty is the “state-switched” source concept originally proposed by Walter Munk in 1980 which permits instantaneous frequency switching of a high Q resonant transducer while always maintaining the resonance state. The objective of this research has been to investigate this novel approach to the design of high-power, low-frequency, broadband transducers for use in long-range underwater communication, active sonar, and underwater research applications. This paper presents a practical realization of a “state-switched” source.

Pulse compression is normally applied only to time-invariant systems, as the variation of a system's properties during its interrogation violates assumptions of the compression process. However, there is an exact solution to the pulse-compression problem when the time variance satisfies two criteria, which are the same as those required for the operation of an ultrasonic vibrometer in the context of a tissue elastography system. One is that the variations be very small in comparison with the wavelength of the interrogating ultrasound. The other is that the bandwidth of the variations be within one Nyquist band as sampled by the periodic interrogation signal. The solution to this problem involves a step-wise interpolation of the static pulse-compression transfer function in the frequency domain. This technique, in conjunction with the selection of an appropriate interrogation signal, offers significant advantages in measurement time or measurement resolution for an ultrasonic vibrometer limited by additive noise at the receiver. The characteristics of optimal interrogation signals for this technique are the signal's crest factor, spectral energy distribution, and phasing. These relate to the intended compression pulse, the noise, and the static response of the system. The technique has been demonstrated analytically, experimentally, and with numerical models.

The acoustic startle reflex in goldfish (Carassius auratus) was studied experimentally in the large acoustic tank at Georgia Tech. Individual free swimming fish were placed in an acoustically transparent cage and positioned in the center of the tank. Their behavior was graded for a response to acoustic signals and the direction of response with respect to the sound source. A preliminary study indicated two interesting features. First, the fish was much more likely to respond in the ‘‘correct’’ direction (away from the source) when responding to subthreshold stimuli. Second, there was strong dependence of signal amplitude at threshold on fish size. For the two weight groups tested (1.6 g, n=3 and 23.6 g, n=4), the sound‐pressure level at threshold was 15 dB lower for the smaller subjects. The initial study has recently been augmented with tests on a range of subjects weighing between these two extremes. Also, since only the first half‐cycle of the acoustic stimulus is thought necessary to elicit the response, the concept of frequency to characterize the sound stimulus is inappropriate. Therefore, thresholds were measured as a function of signal rise time. A parallel effort has involved modeling a biologically relevant stimulus: the acoustic field generated by an attacking predator. [Work supported by ONR.]

The notion that the otolithic organs in the fish ear function as accelerometers has been accepted for over 50 years. By direct excitation, they are capable of sensing the acoustic particle acceleration of an incident acoustic wave. The particle acceleration is a vector field that, for monopole sources, is directed along the source bearing. There are, in effect, many tens of thousands of these accelerometers in each of the six otolithic organs. The spatial orientation of their response axes are ordered but complex. The compliant swim bladder functions as a secondary generator of acceleration at the otoliths, either by direct connection as in otophysans or by acoustic scattering. The swim bladder (or other gas cavities) thus makes it possible for some fish to sense acoustic pressure as well as particle velocity. This mode of excitation of the otolithic organs is referred to as the “indirect” path. The effect of a coupled swim bladder is to extend the hearing range and lower the threshold of hearing. The direct signal is related to a vector and contains information about the source direction. As both directional direct-path and nondirectional indirect signals impinge simultaneously on the otolithic organs, some means of segregating them is required for directionalization to occur. This segregation has been demonstrated by Buwalda (1981). Exactly how source bearing is extracted from the direct signals is unknown as is the mechanism for separating the direct and indirect signals, but theories abound (e.g., Buwalda 1981; Rogers et al 1988; Schuijf 1975). It is generally believed that the indirect (pressure-related) signal serves no role in directionalization other than serving to resolve the 180° ambiguity that occurs because particle acceleration alone only yields the line of bearing to the source (Schuijf and Buwalda 1975). The pressure component is otherwise believed to have no role or to be a negative factor in directionalization. This paper proposes that this viewpoint is not supported by the data and suggests how pressure detection might contribute to directionalization.

This paper describes an investigation of a prototype cylindrical wave‐number calibrator (CWC). The CWC is a water‐filled device designed to generate nonacoustic‐wave‐number pressure fields propagating axially in the water within the calibrator. It can therefore be used to simulate the effects of flow noise on a device placed in the water. Insight into the design of the CWC is obtained from an analysis of an infinitely long, liquid‐filled, cylindrical chamber with an axially‐symmetric, normal‐velocity distribution on the wall of the chamber. Additional insight is provided by a study of the propagation speed in the fluid inside an infinite‐length elastic tube surrounded by another fluid. A finite‐length CWC is then investigated with several computer models of the CWC using the CHIEF acoustic‐radiation program and the ATILA finite element program. These models are integrated into a computer program that uses the electroacoustic transfer matrix for the system to compute electrical drives designed to produce t...

A technique has been developed to demodulate periodic broadband ultrasonic interrogation signals that are returned from multiple scattering sites to simultaneously determine the low-frequency displacement time histories of each individual site. The technique employs a broadband periodic transmit signal. The motions of scattering sites are separately determined from the echoed receive signal by an algorithm involving comb filtering and pulse synthesis. This algorithm permits spatial resolution comparable to pulse-echo techniques and displacement sensitivities comparable to pure-tone techniques. A system based on this technique was used to image transient audio-frequency displacements on the order of 1-10 μm peak (≥ 50 nm/√Hz) that were produced by propagating shear waves in a tissue phantom. The system used concentric transmitting and receiving transducers and a carrier signal centered at 2.5 MHz with an 800 kHz bandwidth. The system was self-noise-limited and capable of detecting motions of strongly reflecting regions on the order of 1 nm/√Hz. System performance is limited by several factors including signal selection, component hardware, and ultrasonic propagation within the media of interest.

The ears of fishes are remarkable sensors for the small acoustic disturbances associated with underwater sound. For example, each ear of the Atlantic cod (Gadus morhua) has three dense bony bodies (otoliths) surrounded by fluid and tissue, and detects sounds at frequencies from 30 to 500 Hz. Atlantic cod have also been shown to localize sounds. However, how their ears perform these functions is not fully understood. Steady streaming, or time-independent, flows near a 350% scale model Atlantic cod otolith immersed in a viscous fluid were studied to determine if these fluid flows contain acoustically relevant information that could be detected by the ear's sensory hair cells. The otolith was oscillated sinusoidally at various orientations at frequencies of 8-24 Hz, corresponding to an actual frequency range of 280-830 Hz. Phase-locked particle pathline visualizations of the resulting flows give velocity, vorticity, and rate of strain fields over a single plane of this mainly two-dimensional flow. Although the streaming flows contain acoustically relevant information, the displacements due to these flows are likely too small to explain Atlantic cod hearing abilities near threshold. The results, however, may suggest a possible mechanism for detection of ultrasound in some fish species.

Applications and research utilizing supercavitation for high-speed underwater flight has motivated study of the phenomenon. In this work, a small scale laboratory experiment for studying supercavitating projectiles has been designed, built, and tested. Similar existing experimental work has been documented in literature but using large, elaborate facilities, or has been presented with ambiguous conclusions from test results. The projectiles were 63.5 mm in length and traveled at speeds on the order of 145 m/s. Measurement techniques are discussed and used to record projectile speed, supercavity dimensions, and target impact location. Experimental observations are compared with a six degrees-of-freedom dynamics simulation based on theoretical models presented in literature for predicting supercavity shape and hydrodynamic forces on the supercavitating projectile during flight. Experimental observations are discussed qualitatively, along with quantitative statistics of the measurements made. Successful operation of the experiment has been demonstrated and verified by agreement with theoretical models.

Proliferation of supercavitating torpedoes has motivated research on countermeasures against them as well as on the fluid phenomenon which makes them possible. The goal of this research was to investigate an envisaged countermeasure, an acoustic field capable of slowing or diverting the weapon by disrupting the cavitation envelope. The research focused on the interactions between high pressure amplitude sound waves and a supercavity produced by a small free-flying projectile. The flight dynamics and cavity geometry measurements were compared to control experiments and theoretical considerations were made for evaluating the effects. Corrugations on the cavity/water interface caused by the pressure signal have been observed and characterized. Results also show that the accuracy of a supercavitating projectile can be adversely affected by the sound signal. This research concludes with results that indicate that it is acoustic cavitation in the medium surrounding the supercavity, caused by the high pressure amplitude sound, that is responsible for the reduced accuracy. A hypothesis has been presented addressing the means by which the acoustic cavitation could cause this effect.

During ASIAEX, surface directional wave spectra were recorded for nine consecutive days. Data are analyzed in this paper to show the characteristics of the surface wave field in the East China Sea. Three surface spectrum models, Pierson‐Moskowitz, JONSWAP, and Toba, are evaluated and compared with the ASIAEX RMS surface wave height data. The data‐model comparisons show that the JONSWAP‐Mitsuyasu model is the best fit to the experimental results. Model performance can be improved by a combined model, JONSWAP‐Mitsuyasu for U/cp 0.8. The preliminary results suggest that locally measured wave age can be used to find the most appropriate surface model for acoustic prediction purposes.

This paper addresses the scattering of acoustic plane waves from submerged objects that are partially covered with a compliant coating. Beam incidence of a planar wave on cylinders of finite and infinite length are considered. The infinite cylinder is analyzed using a two‐dimensional collocation technique, while a three‐dimensional boundary element technique is used to study the finite cylinder. The coated and uncoated surfaces are modeled by assuming that the specific local impedance is constant along arcs of the cylindrical surfaces. It is shown that discontinuities in the impedance factor between adjacent regions leads to large pressure and normal velocity gradients on the surface in the vicinity of the discontinuity, but such effects have little influence on the scattered far‐field pressure.

The overall goal of this project is to develop and demonstrate a system for non-invasive in vivo measurement of the complex elastic moduli (stiffnesses and loss factors) of cetacean head tissues. This system is ultimately intended to provide a portable diagnostic capability for use in stranded animal assessments. The primary technical objective is to remotely generate and detect mid-frequency elastic waves within the body of a living cetacean and to use the measured propagation parameters of these waves to obtain the complex elastic moduli by inversion. A further technical objective is to extract moduli in this manner for intracranial tissues. This objective carries considerably more technical risk since both the wave-generating ultrasound and the probe ultrasound will be attenuated, distorted and scattered by the passage through the skull. The final objective is to develop a prototype portable version of the technology and use it to perform examinations of stranded animals. Data collected with this system is envisioned to serve two purposes: 1) provide basic knowledge of in-vivo elastic properties, which is non-existent for marine mammals, and 2) provide a potential basis for non-invasive diagnostics of tissue pathologies, both naturally occurring or otherwise induced.

Naturally occurring internal solitary wave trains (solitons) have often been observed in the coastal zone, but no reported measurements of such solitary waves include low‐frequency long‐range sound propagation data. In this paper, the possibility that internal waves are responsible for the anomalous frequency response of shallow‐water sound propagation observed in the summer is investigated. The observed transmission loss is strongly time dependent, anisotropic and sometimes exhibits an abnormally large attenuation over some frequency range. The parabolic equation (PE) model is used to numerically simulate the effect of internal wave packets on low‐frequency sound propagation in shallow water when there is a strong thermocline. It is found that acoustic transmission loss is sensitive to the signal frequency and is a ‘‘resonancelike’’ function of the soliton wavelength and packet length. The strong interaction between acoustic waves and internal waves, together with the known characteristics of internal waves in the coastal zone, provides a plausible explanation for the observed anomalous sound propagation in the summer. By decomposing the acoustic field obtained from the PE code into normal modes, it is shown that the abnormally large transmission attenuation is caused by ‘‘acoustic mode‐coupling’’ loss due to the interaction with the internal waves. It is also shown that the ‘‘resonancelike’’ behavior of transmission loss predicted by the PE analysis is consistent with mode coupling theory. As an inverse problem, low‐frequency acoustic measurements could be a potential tool for remote‐sensing of internal wave activity in the coastal zone.

If fish are to behave appropriately with respect to objects and events in their environment they must process an acoustic scene that is often complex (Fay and Popper 2000). A presumptively important part of such behavior is the ability to determine properly the direction from which a sound emanates. Although the question regarding mechanisms for sound-source localization in fishes has been of interest since Karl von Frisch and Sven Dijkgraaf (1935) performed behavioral studies in European minnows (Phoxinus laevis), the mechanisms remain poorly understood, with relatively few biologically plausible models. A localization mechanism that exploits the amplitude, time, or phase difference between the ears as employed by terrestrial vertebrates is not available to fish because the ears are very close together, the speed of sound in water is more than three times faster than in air, and the close impedance match between the fish’s body and water precludes usable diffracted paths (van Bergeijk 1964, 1967; and see Sand and Bleckmann, Chapter 6). Another major difficulty that any model must address is a resolution of the so called “180 ambiguity” that arises because the axis of particle motion associated with a passing sound points both toward and away from the sound source (for review of sound localization by fish see Fay 2005; Sand and Bleckmann, Chapter 6). Current models of directional hearing in fish with mechanisms to resolve the 180 ambiguity include the “phase model” proposed by Schuijf and colleagues (e.g., Chapman and Hawkins 1973; Schuijf 1975; Schuijf and Buwalda 1975) that compares the phase of the pressure and particle motion components of sound or the phase of the direct-path particle motion and the particle motion of sound reflected from surfaces or objects; an “orbital” model by Schellart and de Munck (1987; de Munck and Schellart 1987) in which sound pressure and particle motion together cause the otolith orbits to rotate either clockwise or counterclockwise depending on whether the source is to the left or right; a computational model by Rogers et al. (1988) that also uses both pressure and particle motion; and, a more algorithmic approach pointed out by Kalmijn (1997) by which a fish could make its way to a sound source by