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Figure 3.11 Acoustic time series showing grouping of sound pulses at breaking events. Note that
wave frequency is 0 . 75 Hz so that, with 100% of waves breaking, crests are about 1 . 33 s apart. Figure
is reproduced from Manasseh et al. ( 2006 ) © American Meteorological Society. Reprinted with
permission
analysis. While this preliminary result supports the contention that R 0 can be a proxy for
local breaking severity, much work is required to determine the true relationship between
bubble size and breaking severity under a wider and more realistic range of breaking
conditions.
Explicit calibration of the bubble size in terms of breaking strength, that is quantitative
parameterisations of the wave energy loss across the spectrum in terms of the bubble size,
or spectrum of the bubble sizes produced in the course of wave breaking, is still to be
accomplished. Qualitatively, however, the new method signifies a tested technical means
for studying spectral wave energy dissipation by non-intrusive remote-sensing passive-
acoustic devices.
An itemised outline of the measurement and analysis procedure was formulated in
Manasseh et al. ( 2006 ) as follows:
(1) A submerged hydrophone monitors sound continually;
(2) A prior statistical classification-accuracy analysis has determined a sound-pressure threshold
optimally discriminating breaking from non-breaking events. When the instantaneous sound
pressure exceeds this predetermined level, a very brief pulse of sound is captured, assumed to
be due to a single, freshly formed bubble;
(3) The pulse frequency is rapidly measured, and translated into the bubble's radius;
(4) Running statistics on the rate of detection of bubbles and the mean bubble size during breaking
events are collected;
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