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7.2.2 Transmission Loss (TL)
As the acoustic pulse propagates through the water, it experiences
spreading, which causes the energy of the signal to be dispersed over an
ever increasing area. This diminishes the energy at any specific point as
distance increases.
The acoustic pulse will also experience absorption by the water. The rate at
which the acoustic pulse is absorbed is directly related to the pulse
frequency. The higher the frequency, the higher the absorption rate.
However, typically the higher the frequency, the higher the acoustic resolution
can be achieved. Thus another trade-off must be made, to use the highest
frequency possible, while achieving the desired range capability.
7.2.3 Noise Level (NL)
Noise level is environment specific, which can often be the reason for
acoustic systems experiencing different levels of performance in different
locations or even when operating at different times.
There are numerous sources which contribute to the background noise level
underwater. All of which, make the detection of the return acoustic signal
increasingly more difficult.
From an environmental perspective, marine life such as snapping shrimp can
cause a reasonable level of noise. Also wind and rain can be a factor if
operating close to the water surface.
Man made sources of noise include those from machinery – such as
vessel noise (thrusters and props) and also noise from ROVs and AUVs.
Multipath effects can also add to the background noise. If operating acoustic
equipment in an enclosed area/close to a structure, the acoustic signals tend
to 'bounce around' which can cause sporadic operation of acoustic
equipment.
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