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.For example, as shown in Figure 5.1, at 500 Hz, which is in the middle of the hearing range for most fish, sound suffers only 1 dB of attenuation in 100 kilometers of propagation in seawater and 1 dB in 10,000 kilometers of propagation in freshwater. In comparison, at 500 Hz, electromagnetic radiation attenuates 1 dB in just I m, and blue-green light attenuates 1 dB in less than 3 m. The attenuation of sound in water is also several orders of magnitude lower than its attenuation in air, which is itself rather low.The relationships presented in the following sections are fundamental to sound fields both near and far from the source. Although the fundamental theory presented in this chapter is applicable to all ranges and frequencies, the emphasis is on true "sound" (i.e., compressional waves), rather than "hydrodynamic" (i.e., essentially incompressible) flow, which may be the dominant stimulus at close range and low frequency (see Chap. 4 for a treatment emphasizing the latter stimulus). Because of the characteristics of underwater sound propagation, fish may simultaneously receive many signals from many distances. Understanding the acoustical stimulus to the fish requires the application of these fundamental relationships. The Nature of SoundSound is a longitudinal mechanical wave that propagates in a compressible medium. By wave, we simply mean a disturbance that propagates; by longitudinal,

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A scanning underwater, ultrasonic system has been developed to measure the in vivo vibrational amplitude response of fish hearing organs to low-frequency sound. The fish is insonified with a 20-MHz source while stimulated in a single-frequency sound field. Spectral analysis of the echoes due to the acoustic impedance mismatches at the auditory organs provides the desired amplitude information. A computer-controlled scanning system allows nearly simultaneous measurements of the response of the various organs. The system is capable of measuring displacements of 12Å with a spatial resolution of 0.28 mm.

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In an attempt to determine whether or not a place-type mechanism for frequency discrimination exists in the ears of bony fishes, an experimental investigation of three species was conducted. Goldfish, Oscars, and Kissing Gouramis were exposed to intense single-frequency sound fields for 2 h and then sacrificed. During the exposure, the fish were constrained inside a waveguide at a point of maximal acoustic pressure and minimal particle velocity. The saccular maculae were examined under a scanning electron microscope to determine the location of hair cell damage as a function of frequency. Preliminary results indicate that the location of the damage may not depend on frequency. [Work supported in part by ONR and NIH.]