Physical Mechanisms Underlying the Acoustic Signatures of Breaking Waves

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Authors
  1. Dallas, C.A.
  2. Tollefsen, C.D.S.
Corporate Authors
Defence Research and Development Canada, Atlantic Research Centre, Halifax NS (CAN)
Abstract
The characteristic sound of breaking waves is an integral component of the ambient noise along the coastline. The underwater sound generation from breaking waves has been thoroughly studied, but very little work has been published on the airborne sound caused by breaking waves. Acoustic recordings from a field study were analyzed in an attempt to determine the sound generation mechanisms. A microphone was deployed at Osborne Head, Nova Scotia from June to August 2011 on a grassy cliff above a beach with abundant breaking wave activity. Photographs, weather and ocean wave data were collected to assist in interpreting the audio recordings. Individual breaking wave events were located in the data and their one-third-octave band spectra were analyzed. The levels in the 40 to 200 Hz bands increased by 10 to 25 dB as the wave breaking occurred, with the largest increases observed for frequencies less than 100 Hz. The power spectra of the breaking waves revealed a harmonic structure for frequencies below 200 Hz. The band-level increase and the harmonic structure in the power spectrum was used to postulate that the breaking wave mechanism which give rise to its characteristic sound is the large collapsing air volume present for wave heights greater than 1 m. An approximate resonance frequency was calculated by modelling the wave as a pipe closed at one end, and the resonance frequency coincided with typical maxima found in the power spectra.

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Keywords
airborne sound;breaking waves;acoustics;wave noise
Report Number
DRDC-RDDC-2016-R150 — Scientific Report
Date of publication
01 Aug 2016
Number of Pages
28
DSTKIM No
CA043291
CANDIS No
804594
Format(s):
Electronic Document(PDF)

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