Dr. Robert E. Knowlton

Considering the Coastal Acoustic Carpet

Abstract

A curious natural sound was described in 1943 in The Journal of the Acoustical Society of America –

"When putting out to sea at night in a small boat one is concerned about little things. We imagined a leak or that we were dragging a blackberry bush under the keel, but there was no leak and no blackberry bush. ... As we proceeded on our course the sound grew louder until it was easily heard on deck and the separate ‘crackles’ were clearly noticeable. We imagined that it might come from pebbles on a beach rolled back and forth by surf, but the nearest beach was six miles away and there certainly were no pebbles rolling about beneath us." (Hulburt 1943)

Readers of Soundscape will likely guess the source of this sound. Snapping shrimp, described at least as early as 1818, by Thomas Say, are ubiquitous and some would say conspicuous, yet their reputation far exceeds their recognition. These same finger-sized decapod crustaceans (genus Alpheus and Synalpheus) that can individually create loud “snap” sounds are responsible for the dominant crack-ling sound of shallow tropical and subtropical waters (Johnson 1943, Everest et al. 1948). Yet much of snapping shrimp behavior, ecology and acoustics remains elusive.