Introduction to Acoustics

5.2.5 Ambient Noise
There are essentially two types of ocean acoustic noise:
manmade and natural. Generally, shipping is the most
important source of manmade noise, though noise from
offshore oil rigs is becoming increasingly prevalent. See
also Table 5.2 in the Marine Mammal section for specific
examples of manmade noise. Typically, natural noise
dominates at low frequencies (below 10 Hz) and high
frequencies (above a few hundred Hz). Shipping fills in
the region between ten and a few hundred Hz and this
component is increasing over time [5.25, 26]. A summary
of the spectrum of noise is shown in Fig. 5.15.The
higher-frequency noise is usually parameterized according
to the sea state (also Beaufort number) and/or wind.
Table 5.1 summarizes the description of the sea state.
The sound-speed profile affects the vertical and angular
distribution of noise in the deep ocean. When
there is a critical depth (Sect. 5.1.2), sound from sur-
Table 5.1 Descriptor of the ocean sea surface [5.24]
Underwater Acoustics 5.2 Physical Mechanisms 161
face sources travels long distances without interacting
with the ocean bottom, but a receiver below this critical
depth should sense less surface noise because propagation
involves interaction with lossy boundaries, the
surface and/or bottom. This is illustrated in Fig. 5.16a,b
which shows a deep-water environment with measured
ambient noise. Figure 5.16c is an example of vertical
directionality of noise which also follows the propagation
physics discussed above. The shallower depth is at
the axis of the deep sound channel while the other is at
the critical depth. The pattern is narrower at the critical
depth where the sound paths tend to be horizontal since
the rays are turning around at the lower boundary of the
deep sound channel.
In a range-independent ocean, Snell’s law predicts
a horizontal noise notch at depths where the speed of
sound is less than the near-surface sound speed. Returning
to (5.2) and reading off the sound speeds from
Fig. 5.16 at the surface (c =1530 m/s) and say, 300 m
Sea criteria Beau- Wind speed 12 h wind Fully arisen sea Seafort
range mean Wave Wave Fetch b,c state
scale knots knots height a,b height a,b Duration b,c naut. miles scale
(m/s) (m/s) ft (m) ft (m) h (km)
Mirror–like 0