Acoustical Characteristics of Noise Produced by Snapping Shrimp

Some marine animals produce sounds which, under certain conditions, completely dominate the ambient noise in the sea. The snapping shrimp (not to be confused with the edible shrimp) are the most widely distributed of these animals; they are frequently less than 3 cm in length and produces the sound by snapping of the claw. Shrimp noise is likely to be found around the world in tropical and subtropical waters less than 55 meters deep wherever rock, coral, or other material on the bottom provides interstices in which shrimp thrive. This paper describing the acoustic output of the snapping shrimp (Crangon and Synalpheus) is based upon measurements made off the Southwestern and Southeastern coasts of the United States, the Hawaiian Islands, and several islands in the Southwest Pacific. Over a shrimp bed, the noise spectrum is found to be roughly independent of frequency from 2 to 24 kc (the upper limit of measurement), whereas the ambient noise normally present in the deep sea decreases with frequency. There is a broad peak in the shrimp spectrum somewhere between 2 and 15 kc. Over a shrimp bed the noise at 20 kc is about 25 db above the ambient noise which accompanies a sea state of 2 (waves less than a meter high, not including swell). For low sea states, shrimp noise is appreciable a mile or more from the boundary of the bed. The maximum diurnal variation of shrimp noise is from 3 to 6 db, the noise being greater at night. Single snaps from isolated specimens have been subjected to a Fourier integral analysis which indicates a spectrum comparable to that measured over shrimp beds where the underwater sound consists of a multitude of snaps. A typical peak sound pressure at the distance of a meter from a single shrimp is of the order of 200 dynes/cm2.