This is my attempt to illustrate how we can detect whale calls on an ocean bottom seismometer. But this is not typically how people measure sound underwater! Usually, they use an instrument called a hydrophone. Here’s an example of a Reson TC4033:
Pretty, isn’t it? (A little aside: I chose this one because I used it to collect data for my master’s project and I also worked at Reson for two years. Oh memories…).
Hydrophones are basically underwater microphones. They contain a special crystal called a piezoelectric transducer (my friend Alexis made a sweet blog post describing these: magic crystal blog post) Remember what a transducer is? It converts between different forms of energy, in this case acoustic to electric.
Good quality hydrophones are calibrated so that if you measure a certain voltage (at a certain frequency), you can say exactly how loud the sound wave was when it hit the transducer. There are all kinds of reasons why measuring the loudness (amplitude) of a sound wave is useful when you’re underwater. For example if you measure a really loud sound, it might be closer, or a really quiet sound, it might be further away. It’s really much more complicated, because sound travels in all sorts of weird and wonderful ways underwater, but you can think of it like that for now.
But guess what? Measuring the loudness of a fin whale call (or any whale for that matter) can tell us lots of useful things. Like how far can they communicate with each other? How will ship noise disturb their communication? How far away are they from that hydrophone you’ve got dangling off the side of your boat?
The problem with trying to do this on seismometers is that they are not calibrated to measure the loudness of sound – they just measure how much the ground moves. So how do we deal with that? Find out in another (future) post! Ooh, it’s cliffhanger!