Abstract
Marine mammals are vulnerable to ship collisions when are near the surface. Here, acoustical laws of reflection and propagation can limit their ability to hear and locate the noise from approaching vessels. Defining the physics of near-surface acoustical propagation as it relates to ship noise and hearing is central to understanding and mitigating ship strikes. Field data from controlled ship passages through vertical hydrophone arrays demonstrate a confluence of acoustical factors that poses detection challenges including (i) downward refraction; (ii) spreading loss; (iii) Lloyd’s mirror effect; (iv) acoustical shadowing; and (v) masking of approaching ship noise by ambient noise and distant ships. A highly directional, dual-frequency parametric sonar has been developed to mitigate these challenges and alert marine mammals of approaching vessels. The system projector is a planar array, comprised of 45 elements, band centered to transmit a high carrier frequency along with a lower sideband signal. The nonlinearity of water is used to demodulate the mixed high-frequency carrier into a lower-frequency waveform audible to both manatees and whales. The bow-mounted arrays project a narrow beam directly ahead of vessels, and fill in acoustical shadows in an effort to alert marine mammals of the approaching danger. [Research supported by DOD Navy Legacy Program.]
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