A low sea-state study of the quieting effect of monomolecular films on the underlying ambient-noise field

Abstract

A series of tests, conducted at low sea states, show that the reduction of surface-related ambient noise beneath monomolecular (slick-forming) films begins between 1 and 2 kHz and extends to at least 70 kHz. Although the amount of noise reduction varies between experiments, attenuations of up to 8 dB are common. Most of the experiments are conducted in the absence of whitecapping, where for wind speeds greater than 2 m/s, the ambient noise spectra beneath the films generally resemble those of nonfilmed, lower sea-state conditions. Laboratory experiments conducted with these same films spread upon a reservoir of seawater show that the regular bubble entrainment associated with vertically falling drops can be dramatically suppressed, and the air entrainment accompanying plunging liquid jets is characterized by a conspicuous increase in the number of smaller bubbles. Preliminary studies providing simultaneous in situ acoustic and video monitoring of the ocean surface from a meter beneath it suggest that, in the absence of whitecapping, the ambient-noise reduction beneath the slick results from a dramatic decrease of microbreaking events within it. The acoustic signatures of these microbreaking events are distinguished by individual oscillating bubbles.