Oceans of noise - Whale and Dolphin Conservation Society

2.2. Types of sound source
Sounds can be transient (short duration sounds with obvious start and finish points) such as those
produced by explosive blasts or percussion piling, or continuous (such as drilling). Transient sounds
are usually described by their peak level (the maximum amplitude measured in dB) with an
accompanying indication of how this varies in time. If a transient sound is impulsive (with a duration
of less than 0.5 seconds) it is best described by its energy level. Continuous sounds and long duration
transient sounds are conventionally described by their mean square pressure.
2.3. The propagation of underwater noise
Source path receiver model of sound
When describing the propagation of underwater noise it is useful to apply a simplistic model to this
process. These models are based on the sonar equation and perhaps the simplest of these is the Source
Path Receiver Model. The basic parameters of this model are:
(1) Source: the noise source, e.g. ship, sonar etc. Parameter of interest = source level (SL)
(2) Path or medium: the water column. Parameters of interest include transmission loss (TL), and
ambient noise level (NL)
(3) Receiver: e.g. whale, hydrophone etc. Parameters of interest include signal to noise ratio (SNR),
received sound intensity level (RL) and detection threshold (DT).
A simple model of sound propagation is:
RL = SL −
TL
Where RL is the received level, SL is the source level and TL is the transmission loss.
Transmission Loss (TL)
Transmission loss is the decrease in intensity of a sound as it propagates through a medium, and is the
result of spreading, absorption, scattering, reflection and rarefaction. Transmission loss can also be
estimated by adding the effects of geometrical spreading (TLsp), absorption (TLa) and the transmission
loss anomaly (A). The transmission loss anomaly includes scattering loss and losses due to reflection
and rarefaction at boundary interfaces.
TL = TLspreading
+ TLabsorption
+
For simplicity we'll only deal with spreading (TLsp) and absorption loss (TLa):
TL TL TL + =
g
(1) TLsp - Spreading loss
a
A
Spreading loss is a major component of transmission loss and is range (distance) dependent. Two
forms of spreading loss are common underwater (1) Spherical or Geometrical spreading loss (TLg),
and (2) Cylindrical spreading loss (TLcy).
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