Source Levels

The source level of a fin whale call is a measure of how loud the call is when it’s made. Having an idea of what the source level is can help us to understand things like:

– how far the animal can be detected using acoustic methods
– how communication between fin whales might be affected by anthropogenic noise in the ocean
– how many individual animals are present in a particular area based on measured energy levels.

Measuring fin whale source levels passively is not a straightforward problem. In theory, we should be able to measure the received level, and if we know where the call was made, we can estimate transmission losses along the acoustic travel path. Adding the transmission loss to the received level should give the source level.  Sounds easy, right?  Well, here are just a few challenges that come up:

– Uncertainty in the position of the whale results in incorrect estimates of transmission loss
– Even if we try to only look at direct path arrivals, there’s a chance that there has been surface or bottom interactions which could drastically change the signal level
– If either the whale of the receiver is near the surface, then there is a possibility of interference between direct and first-bounce arrivals (constructive and destructive interference)
– OBS’s are not hydrophones – we need to find a transfer function to convert from measured velocity fluctuations to acoustic sound level.