Ship strike acoustics: It is all just shadows and mirrors

Abstract

Whales are vulnerable to collisions when near the surface and in shallow water. Here the physics of near-surface sound propagation may play a crucial role in their survival. Ships sufficient in size to mortally injury whales generate acoustic spectra dominated by very low frequencies. Since the ocean’s pressure-release surface severely attenuates frequencies that are generated at distances less than a wavelength from the surface, whales may not detect low-frequency sounds generated by approaching ships. The Lloyd Mirror Effect predicts sound pressure levels at the surface approximate zero and empirical acoustic measurements support the prediction, while quantifying another important phenomenon associated with large ships, Acoustical Shadowing. The confluence of these acoustic propagation effects pose significant ecological consequences for marine mammals at the surface. While aerial and shipboard monitoring networks help navigate ships away from whales sighted on the surface, these programs are not reliable during conditions of poor visibility and darkness. Further understanding the acoustics of whale/ship collisions could augment protection efforts and result in more consistent and effective management strategies. To mitigate collision risks at times when surveillance programs are ineffective, ships could be projecting directional acoustic signals specifically designed to defeat the Lloyd Mirror Effect and Acoustical Shadowing.