Ambient Recording ASF-G XLR Podręcznik - Strona 9

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7. Underwater Acoustics

The acoustic underwater differs in many ways from the airborne sound we are used

to. The sound velocity underwater is varying between 1450 and 1550 m/sec

depending on temperature, salinity and depth. This results in 4 to 4.5 times bigger

wave lengths as well as in a special sound phenomenon called the SOFAR, also known

as Underwater Sound Channel. Horizontally sound can travel over huge distances

whereas vertical propagation is strongly attenuated. Here are some useful advices:

 Sound Channels (USC, SSC,...) can be avoided by knowing their local depth

 To avoid the noise from braking waves on the boat carcass, take long enough

cables to go down deep enough or prepare to record in greater distance to the

vessel.

 Take time for detailed micro-phasing when recording in pools or artificial

environments as bigger wave lengths underwater lead to greater distances

between sound maxima and minima.

 Never forget how far low frequencies can spread underwater. Therefore, avoid

touristic areas for documentary shots.

 Water owns a reflection factor close to 1. Thus airborne sound virtually can't

couple directly into water. This phenomenon can be used. If barriers reach

close to the water surface (reefs, wave-breakers, e.g.) you can use them as

acoustic barriers, independent of the current depth. This way you may find

acoustic cover in a pool with 1m depth behind an 80cm wooden block, even

though the wavelength underwater wouldn't suggest this.

ASF-G