Peter H. Rogers

Underwater Sound as a Biological Stimulus

Of all the sensory stimuli discussed in this volume, only sound allows longrange transmission of information underwater. This is a consequence of the extraordinarily low attenuation of sound in water and the ability of sound speed gradients in the ocean to channel sound so that it can propagate without interaction with the surface or bottom.

Role of the Fish Ear in Sound Processing

Despite the considerable interest in fish hearing in recent years (see Schuijf and Hawkins 1976; Tavolga, Popper, and Fay 1981), the contributions of the fish ear or brain to hearing and sound processing remain relatively unknown. Our knowledge about the fish ear is more limited as compared to our knowledge of the ears of amphibians, reptiles, birds, or mammals. Extensive studies of representatives of these classes have demonstrated that their inner ears are involved in the analysis of sounds, rather than simply as a system for transduction of mechanical to electrical energy.

Parvulescu Revisited: Small Tank Acoustics for Bioacousticians

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 animals natural environment. The significance of these differences is magnified by the nature of the fishs 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.

Reverberation vertical coherence and sea-bottom geoacoustic inversion in shallow water

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.

Multipole Mechanisms for Directional Hearing in Fish

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

Ultrasonic displacement sensor for the seismic detection of buried land mines

A system is under development that uses seismic surface waves to detect and image buried landmines. The system, which has been previously reported in the literature, requires a sensor that does not contact the soil surface. Thus, the seismic signal can be evaluated directly above a candidate mine location. The system can then utilize small amplitude and non-propagating components of the seismic wave field to form an image. Currently, a radar-based sensor is being used in this system. A less expensive alternative to this is an ultrasonic sensor that works on similar principles to the radar but exploits a much slower acoustic wave speed to achieve comparable performance at an operating frequency 5 to 6 decades below the radar frequency. The prototype ultrasonic sensor interrogates the soil with a 50 kHz acoustic signal. This signal is reflected from the soil surface and phase modulated by the surface motion. The displacement can be extracted from this modulation using either analog or digital electronics. The analog scheme appears to offer both the lowest cost and the best performance in initial testing. The sensor has been tested using damp compacted sand as a soil surrogate and has demonstrated a spatial resolution and signal-to-noise ratio comparable to those that have been achieved with the radar sensor. In addition to being low-cost, the ultrasonic sensor also offers the potential advantage of penetrating different forms of ground cover than those that are permeable to the radar signal. This is because density and stiffness contrasts mediate ultrasonic reflections whereas electromagnetic reflection is governed by dielectric contrast.

Anomalous sound propagation in shallow water due to internal wave solitons

At-sea experimental data and numerical simulation results are given to show that, acoustic normal-mode coupling induced by internal solitons could be an important loss mechanism for shallow water sound propagation.<<ETX>>

Use of the swim bladder and lateral line in near-field sound source localization by fish

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%, seven of the eight 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 deg 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 deg ambiguity of source direction from the particle motion vector remains unresolved.

Seabottom Acoustic Parameters from Inversion of Yellow Sea Experimental Data

The direct measurement of seabed parameters over a wide frequency range for a large sea area is often very difficult and always costly and time consuming. Inversion techniques for obtaining seabed acoustic parameters are often an attractive alternative. In this paper, seabottom acoustic parameters obtained from inversion of experimental data from the Yellow Sea are reported. The experiments were conducted over many years by using explosive signals at three different flat-bottom sites with water depths of 28.5 m, 36.5 m and 76.2 m. Several different inversion techniques, all based on normal mode models, were employed. Depending on the site, geoacoustic models of varying complexity were required. Using these techniques, the sound speed ratio of seabottom to water for the three different Yellow Sea sites is found to be about 1.056 – 1.072. The seabottom attenuation in the Yellow Sea for all of the sites exhibits a strong nonlinear frequency dependence in the frequency range of 80–1500 Hz. The acoustic attenuation coefficient is found to be proportional to frequency to the 1.6 – 1.9 power. This result differs from the widely used Hamilton’s seabed geoacoustic model, which has a linear frequency dependence.

Probing signal design for seismic landmine detection

This paper addresses the design of time-domain signals for use as seismic excitations in a system that images buried landmines. The goal of the design is the selection of a signal that provides sufficient contrast for the post-processed landmine image in the shortest possible measurement time. Although the goal is relatively straightforward and the problem appears similar to one of system identification for a linear time invariant (LTI) system, practical implementation of many commonly accepted approaches to the system-identification problem has proven difficult. The reason for this is that the system under consideration exhibits observable nonlinearity over the entire range of drive levels that are of interest. The problem is therefore constrained by the requirement that nonlinear effects be tolerable rather than imperceptible (i.e. that the nonlinearity be sufficiently weak that the system can be reasonably characterized as linear). Several candidate signal types that have been shown to offer good noise immunity for the LTI system identification problem were considered. These included circular chirps, binary-sequence-based (BSB) signals, and numerically optimized randomly seeded multisines. Based on purely experimental figures of merit, circular chirps with flat amplitude and linearly swept frequency offered the best performance among the signals that were tested.