Listening to the heartbeat of the ocean
The ocean is a noisy place! Underwater acoustics explores what the watery world sounds like and what impact humans are having on it. So what can listening to this underwater environment tell us about the habitat? Chris Smith spoke to Philippe Blondel from the University of Bath...
Philippe - The world of underwater acoustics is listening to the sounds in the oceans. Even if Jacques Cousteau called it the world of silence is actually very noisy. It’s hissing, crackling, sizzling, and we can hear the sounds from natural processes – the wind, the waves, the rain, or the storms above. We can hear the sounds of the animals around – for example, this whale, but the dolphins we heard about earlier. All these animals make their own noise and then with the noise we create with ships passing by or with our own tools – for example, sonar where we sent pings in the oceans and listen to the echoes like bats do to image the environment and the weather.
Chris - I think the discrimination is active versus passive sound. There are sounds we can make and that’s active acoustics, and passive acoustics is where we’re just eavesdropping on what nature is already providing for us.
Philippe - Exactly. Active acoustics or sonar is like sonar for bats or dolphins, we create our own source of sound like a loudspeaker; send the sound, get the echoes – how long it takes to come back tells us how far the target is and how the echo is changed by the target tells us what kind of target it is.
Chris - Why does a field like yours exist though? What can we actually learn about the underwater world by listening to it?
Philippe - We can learn a lot with these tools. With passive acoustics for example, we can detect sounds even 1500 kilometres away. The oceans are a very big place and we know very little about them. One of the speakers mentioned around 5 per cent. So, we know less about our own planet than other planets like Venus.
Chris - You sent me a sound earlier and you’ve described this as coming from a glacier. I’ll just play that… (sound recording) Tell us what that was.
Philippe - This starts with a hiss which is a background noise of icebergs in the sea and they're melting. So they're breaking into smaller parts. The small bubbles of air inside create small pops like an ice cube in a glass. And then the big noise, the big bang, is part of a glacier falling off which is a natural process. It’s becoming larger now with climate change. These glaciers add a lot of freshwater and we have big blocks of several tens of tons falling into the ocean.
Chris - So does this mean, given that you’ve identified that almost sonic fingerprint for that process happening that by listening for those sounds underwater, you have some idea of how often they're happening and on what sort of scale, and therefore, you have a rate at which glaciers might be melting.
Philippe - Yes, and we can do that without being there in person because the polar environments are very dangerous and challenging. We can put instruments on the seabed for years at a time and gather information – not only from this glacier but all the other glaciers around. At the same time, we listen to the waves, we listen to the human footprints or acoustic footprints. So we have a complete view of what's happening under the oceans and what is hidden to us usually.
Chris - Here’s another clip you sent me…(sound recording) That one sounds quite different and it has irregularity to it. I’d say that’s manmade.
Philippe - Definitely, spot on. This is a ship and we can hear the propeller of a ship going like that (brrrr) and creating this noise. The shipping industry is well aware of its problem so they're trying to reduce their acoustic footprints. But these sounds can travel over very large distances up to hundreds of kilometres away.
Chris - What impact –just very briefly – could they have on animals that are exposed to them because they’ve got to live with this noise going on?
Philippe - It’s likely being next to a motorway. When there's a lot of traffic, you have to shout very loud to be heard by others. Sometimes you cannot sleep at night. If you're relying not on sight but on sound to find your prey, that’s going to affect how easily you can eat and live around these places.