Origins of directionality in snapping shrimp sounds and its potential applications

Abstract

Snapping shrimp (genera Alpheus, Synalpheus & Penaeus) sounds are known to dominate high frequency ambient noise in warm shallow waters with a peak-to-peak source levels of up to 190 dB re 1uPa @ 1m. It has been previously shown that these loud sounds are primarily due to the collapse of cavitation bubbles resulting from the ejection of a highspeed jet of water generated by the high-speed closure of the snapping shrimp claw. As the cavitation bubble is small, it is expected to behave as an omni-directional source. However, since snapping shrimp live on or very close to the sea bed, the source and the bottom reflection of the source form a quasi-dipole. The interference between the direct arrival from the source and the closely-spaced bottom-reflected arrival creates directionality. The observed directionality of the snapping shrimp sound is a function of the bottom parameters, because of their influence on the reflected component of the quasi-dipole. Since the shrimp are distributed over a wide portion of the seabed, the directionality resulting from many different arrival angles may be used for geoacoustic inversion at a single receiving location, yielding estimates of superficial bottom parameters such as sound speed from a single receiver station deployment and without introducing deterministic sources. The Acoustic Research Laboratory (ARL) at the Tropical Marine Science Institute of Singapore has developed a compact, high-bandwidth, 3-dimensional acoustic array that can localise these sources in time and space. Data from experimental deployments of this system provides evidence for the directionality of snapping shrimp sound.