Potential causes of increasing low frequency ocean noise levels

Abstract

A simple expression for the global average ambient noise is derived in terms of parameters representing the characteristics of the sound sources, the number of such sources and the propagation properties of the deep ocean, and used to explore possible causes of increasing low frequency noise in the deep ocean. A 5 dB increase in noise level over 35 years, reported by [Andrew et al., ARLO 3, 65-70 (2002)], is explained by the increase in the total number of ocean-going ships and their average tonnage, with no evidence of any significant change in source level during that time. The theoretical effect of increasing transparency to low frequency sound, which has been proposed as a possible cause of part of this increasing noise [Hester et al., Geophysical Research Letters 35, L19601 (2008)], is less than 1 dB to date. Predicted changes in the 21st century might be positive or negative depending on the relative contributions from the change in pH (which is expected to increase globally averaged noise) and the change in sea surface temperature (expected to decrease noise). © 2012 Acoustical Society of America.