Near-Surface Observations of Wind and Rain-Generated Sound Using the SCANR: An Autonomous Acoustic Recorder

Abstract

Recent oceanic ambient noise observations demonstrate a clear linear relation between sound pressure and both wind speed and rainfall rate. Wave whitecapping increases the sound level but the relation is still tenuous. Assessment of the true correlation of the ambient sound with its sources, a necessity for the understanding of the generating mechanisms, requires precise measurement of the surface phenomena made at the immediate location of the sound observations. Most noise records have lacked such proximate environmental measurements. Furthermore, most ambient noise has been recorded at depths of 250?4000 m and the data were low-pass filtered, both effects minimizing the acoustic response from small-scale and (or) short-term surface phenomena. Near-surface observations of ambient sound associated with rapidly changing wind and rainfall events were made using the Self Contained Ambient Noise Recorder (SCANR) system which provides autonomous digital tape records of ambient sound at selected frequencies. The SCANR was placed at 4 m depth from a pier in Narragansett Bay, Rhode Island. Wind and rainfall were monitored simultaneously with the ambient noise recordings at narrow bands cantered at 15 and 25 kHz and a broad band. A correlation of 0.97 of the 15 kHz band with wind speed was obtained for a 24 h period with winds ranging from 0.2?15 m s?1. At higher sustained winds (12?15 m s?1), a pronounced decrease occurred in sound pressure which appears due to increased absorption of the sound generated at the surface by the whitecap-produced bubble layer. Use of direct sound pressure output, in lieu of logarithmic sound levels, best showed the immediate acoustic response to changes of the wind field and rainfall rates associated with passing squalls. Rain-produced sound attained 35 dB increase within 2?3 min during a passing line squall which was tracked with an MIT weather radar at Cambridge, Massachusetts. The SCANR system has proved useful for observing the near-surface ambient sound field at both broad and narrow bands up to 30 kHz. It is easily deployed and retrieved while nearby observations are made of wind speed, whitecapping intensity, and rainfall rate.