Sound Pressure Levels
Underwater sound pressure levels
Set out below is a table of sound pressure levels in order-of-magnitude steps. Sound sources typical of some of the levels are shown for comparison.
(dB re 1µPa)
|1,000,000||240||Maximum linear source level. Cavitation begins to occur at the face of transmitters.
Seismic air gun (1m from source)
|100,000||220||Typical active sonar transmission level
Beluga whale call (1m)
|10,000||200||Large ship broadband (1m)|
|100||160||Large ship broadband (100m)|
|10||140||Fin whale call (100m)|
|0.1||100||Ambient noise, sea state 4|
|0.001||60||Ambient noise, sea state 0 (flat calm)|
Comparison of underwater and airborne sound
To make a comparison of these underwater sound pressure levels with the values quoted for sounds in air an important point must be addressed. The sound pressure levels in air are usually referenced to the threshold level for human hearing which is now standardised at 20µPa. Thus if an rms sound pressure level of 100 Pa is measured in water the result is quoted as 160dB re 1µPa, whereas the same level in air is quoted as 134dB. The conversion factor of 26dB (20log(20/1)) is due to higher reference level, but mention of the different reference level is often omitted.
The comparison of Intensity values corresponding to the air and water measurement are also subject to a large adjustment before meaningful comparisons can be made.
The relation between pressure and intensity is often quoted as P2 proportional to I, but this is only true if the acoustic impedance is constant. Acoustic impedance is a measure of how the pressure in the medium rises when a sound of a given intensity is applied. If the impedance is low, say 415 Pa.s/m as in air, there is a small rise in pressure. If the acoustic impedance is high, say 1,540,000 Pa.s/m as in water, the rise in pressure is approx 3700 times the rise in air pressure for the same intensity wave.
So to compare a sound pressure level in air and water, take the sound pressure level of a loud rock band of say 120dB re 20µPa and find the sound pressure that would be observed in water for a source emitting at the same intensity. There is an additional 26dB because the measurement is from a reference level of 1µPa instead of 20µPa, and there is 36dB additional rise in pressure due to the higher impedance of water. So the value measured will be 180dB re 1uPa. A noise level of 180dB is the roughly the noise level heard from a large container ship about 10m away.
Comparing the noise a conventional submarine whose sound level, at worst, would be little more than the background ocean noise, to a rock band indicates that someone may have misinterpreted the sound measurement scales.