The Noise of Melting Icebergs

Abstract

Icebergs seem to have escaped serious attention as sources of underwater sound. We have measured the noise of icebergs by using sonobuoys dropped by an aircraft at distances between 200 and 10 000 yd. Two isolated icebergs 130–150 ft high were measured at a location northeast of Newfoundland. Noise apparently originated by the bergs was found to have a spectrum flat to about 8 kHz, with spectrum levels of −37 and −42 dB re 1 dyn/1 cm2 at 200 yd from the two bergs. In the laboratory, we have observed the noise of ice melting in a small pressure vessel containing a hydrophone. Two kinds of noise were heard. One was an impulsive noise apparently due to thermal cracking that disappeared under an applied pressure. The other was a sizzling frying sound that became louder as the pressure increased but occurred only with cloudy milky ice and not with clear ice free of air. We surmise that this noise is caused by the implosion of tiny air bubbles trapped in the ice as their walls become broached during the melting process. Alternatively, the noise of real icebergs may be due to tiny explosions of trapped air bubbles having high internal pressures. Iceberg noise should therefore depend on size, depth, air content, rate of melting, and origin of the berg.