Page 387 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

14.3.7 Source level
The source level (SL) of sonar or an echo sounder is the sound pressure in the transmitted pulse at the
beam axis 1 m from the transducer. If the transmitting response (S) is known, then the source level is: SL5S110 log P
where
P 5 transmitter power,
S 5 transmitting power response. A widely used formula is:
SL5170:9110 log P1E1DI
where
SL 5 source level in dB re 1 μPa, P 5 transmitter power in watts, E510 log η,
η 5 transducer efficiency,
DI 5 directivity index.
The constant 170.9 incorporates conversion from watts to pascals and can be derived as
follows:
SL510 logð1=4πÞ110 log P1E1DI
The factor 1/4π represents the source level from an omnidirectional source, supplied with 1 W electrical power and with 100% efficiency. When 1 W sound power is distributed over a sphere with radius 1 m and surface 4π m2 the sound intensity will be:
ð1=4πÞ W=m2 The connection between pressure and intensity is:
where
I 5 intensity,
ρ 5 density of water,
p 5 pressure,
c 5 sound velocity (in water).
p5OIρc
Seawater at temperature 10C with salinity 35 (PPT) at the sea surface has the following values:
ρ 5 1027 kg/m3 c 5 1490 m/s
A sound density of 1/4 W/m2 will in this environment correspond to a sound pressure of: p53493106 μPa5170:9 dB re 1 μPa
14.3 Transducers 379