Page 380 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

372 CHAPTER 14 Underwater Acoustics
where
I 5 intensity,
p 5 pressure,
ρ 5 water density, and
c 5 speed of sound in water.
14.2.3 Decibel
The decibel is widely used in acoustic calculations. It provides a convenient way of handling large numbers and large changes in variables. It also permits quantities to be multiplied together simply by adding their decibel equivalents. The decibel notation of intensity I is:
10 log I=Io
where Io is a reference intensity.
The decibel notation of the corresponding pressure is:
10 logðp2 =ρcÞ=ðρ2o =ρcÞ 5 20 log p2 =po
where po is the reference pressure corresponding to Io.
Normally, po is taken to 1 μPa, and Io will then be the intensity of a plane wave with pressure
1 μPa.
As shown in the example, the decibel number is the same for pressure and intensity. It is there- fore common practice to speak of sound level rather than pressure and intensity. The reference level is in both cases a plane wave with pressure 1 μPa.
14.2.4 Transmission loss
14.2.4.1 Geometrical spreading
When sound is radiated from a source and propagated in the water, it will be spread in different directions. The wave front covers a larger and larger area. For this reason the sound intensity decreases (with increasing distance from the source). When the distance from the source has become much larger than the source dimensions, the source can be regarded as a point source, and the wave front takes the form as a part of an expanding sphere. The area increases with the square of the distance (Figure 14.2) from the source, making the sound intensity decrease with the square of the distance.
EXAMPLE
A pressure p 5 100 μPa
In decibels: 20 log 100/1 5 40 dB re 1 μPa
The intensity will also be 40 dB re “the intensity of a plane wave with pressure 1 μPa.”