Particle motion and sound propagation near boundaries

Abstract

It has been well-established that the inner ears of fishes act as particle motion detectors (Popper and Hawkins, 2018) and that fishes use particle motion to determine the direction of a sound source (Hawkins and Popper, 2018). Many soniferous estuarine fishes live in very shallow water at depths less than 3 m, where sound propagation is complicated by reflection and transmission of waves by and through the substrate and the surface. In these very shallow environments, maxima and minima of sound pressure often occur at different locations than maxima and minima of particle motion, making it difficult to relate hydrophone measurements of sound pressure to the effects of sound on the fishes that perceive particle motion. A recent study (Sprague et. al, 2016) used hydrophones to measure sounds of vessels in nearby channels to determine sound exposure levels at very shallow oyster toadfish Opsanus tau nesting sites. In this study we model the propagation of vessel sounds from deeper channels into nearby very shallow regions to determine the relationship between the pressure and particle motion both in the water and in the substrate in these environments and to gain insight into sound propagation in similar situations. Our results indicate that the sound waves in the very shallow regions contain significant particle motion components parallel to the seafloor (horizontal) and a reduced particle motion component perpendicular to the seafloor (vertical).

Hawkins, A. D., and Popper, A. N. (2018). “Directional hearing and sound source localization by fishes,” J. Acoust. Soc. Am. 144, 3329–3350.

Popper, A. N., and Hawkins, A. D. (2018). “The importance of particle motion to fishes and invertebrates,” J. Acoust. Soc. Am. 143, 470–488.

Sprague, M. W., Krahforst, C. S., and Luczkovich, J. J. (2016). “Noise propagation from vessel channels into nearby fish nesting sites in very shallow water,” Proceedings of Meetings on Acoustics 27, 010012.