Geoscience Reference
In-Depth Information
Babanin
et al.
(
2001
)(seealso
Babanin
et al.
(
2007b
) and
Section 8.3
for additional dis-
cussions). In the current section, detailed description of the Lake George field experiment
will also be given. This complex experiment (
Young
et al.
,
2005
) is relevant for the acous-
tic methods of the present section and will also be referred to throughout the topic with
respect to other topics.
The ambient sound level in the ocean at a given frequency may vary by 20 dB, increasing
with the wind speed (e.g.
Knudsen
et al.
,
1948
;
Wenz
,
1962
;
Kerman
,
1988
,
1992
;
Ding &
Farmer
,
1994
). Wind and wave effects are most marked in the 0
10 kHz band. The
general mechanisms of sound creation in this band are understood, although their inter-
relationships are not. Wind pumps energy into the wave spectrum, causing wave growth
which can lead to breaking. The whitecapping from a breaker creates bubbles near the
surface, and bubbles emit sound. However, it is known that the wind dependence is indirect.
In most situations, it is the hydrodynamic evolution of the waves that determines whether
breaking occurs (e.g.
Babanin
et al.
,
2001
,
2009a
,
2010a
). The wave-breaking bubbles
can either generate the sound themselves as described below, or can transform pressure
fluctuations in the air into acoustic noise in the water (
Didenkulov
,
1992
).
Subdividing the 0
.
1kHz
−
.
1 kHz-10 kHz band allows more detailed explanations. In general, it
is above 0
5 kHz that the wind-dependent component of the sound spectrum dominates
(
Wenz
,
1962
). Furthermore,
Bass & Hey
(
1997
) and
Babanin
et al.
(
2001
) showed that the
sound spectrograms due to breakers become evident above 0
.
5 kHz. Theoretical work (e.g.
Meyer
,
1989
) suggests that the bubble-formation process dominates the acoustic spectrum
at frequencies greater than 0
.
.
5 kHz. From the basics of bubble acoustics briefly described
below, 0
5-10 kHz corresponds to the natural emissions of millimetre-sized bubbles at
near-surface depths. Frequencies around 0
.
5 kHz are likely to be produced by bub-
ble clouds, not individual bubbles (e.g.
Prosperetti
,
1988
;
Lu
et al.
,
1990
;
Tkalich & Chan
,
2002
).
It has been well known since the time of
Rayleigh
(
1917
) that individual bubbles oscil-
late volumetrically with a natural frequency that depends on their size (see
Leighton
,
1994
,
for a review), suggesting an obvious application to instruments analysing bubbly flows.
The simple-harmonic solution to the Rayleigh-Plesset equation describing bubble-acoustic
oscillations shows that a single bubble's natural frequency is inversely related to bubble
size, according to
.
1-0
.
3
γ
0
P
0
ρ
w
1
R
0
,
ω
0
=
(3.28)
(
Minnaert
,
1933
) where
γ
0
is the ratio of the specific
heats of the gas,
P
0
is the absolute liquid pressure and
R
0
is the equivalent spherical
radius of the bubble. If the number of bubbles is assumed infinite, continuum approxima-
tions based on
(3.28)
permit overall acoustic properties of a bubbly cloud to be calculated
(e.g.
Commander & Prosperetti
,
1989
;
Duraiswami
et al.
,
1998
). The acoustic properties
of bubbles have been the basis of several oceanographic instruments (e.g.
Phelps
et al.
,
ω
0
is the sound radian frequency,
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