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 hull, 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 80 cm wooden block, even though the wavelength underwater wouldn’t suggest this.
