We are all used to what light does around us. For the most part light travels in straight lines. I see you because of the light reflected off you. With our stereoscopic vision, I can estimate how far away you are from me. So, humans are equipped to detect range and direction. That’s incredibly useful in everyday life. We’ve evolved with a good ability to sense of our local environment.
We have two ears. So, to a degree we can judge the direction a sound is coming from in the air. That does get more complicated as the wind blows, with reflections and other noises clutter up our environment. How far away something is presents us with guess work. In the dark, a loud person close to us isn’t too difficult to guess. A quest person, far away is much more difficult to guess.
Sound in water behaves with some of these characteristics but temperature has a significant impact.
Our human experience of sound in water isn’t all that good. That’s down to the interface between air and water. For us water is not our natural environment. Our ears are attuned to sounds in the air.
There’s a device called an expendable bathythermograph (XBT). A nice title that makes this device sound high-tech and whizzy. The truth is that it’s remarkably simple. It’s a thermocouple, for measuring temperature at the end of a long wire that uncoils as it descends in water.
Thrown overboard at sea, the XBT sinks. At the surface a chart is drawn of the temperature profile of that point in the sea. Knowing the temperature profile, it’s possible to calculate what the sounds will do in that seawater. There are variations in the speed of sound in water with temperature.
Key factors to consider with the propagation of sound in water is pressure and temperature. Depth and pressure are simply related. Salinity has an impact too but that’s not the major factor at greater depths. At the deepest point in the sea the temperature is relatively constant.
A student project of mine was to design a sound velocity meter for use in the sea. The idea was to directly measure the speed of sound in water. It was what’s called a “sing‐around velocimeter.[1]”
What all this amounts to is that sound may not travel in a straight line in sea water. So, if a sensor on a surface boat picks-up a sound it may not be so easy to say where it’s coming from without a lot of additional information.
Those searching for the missing submersible in the North Atlantic are aware of the tricks that sound can play in seawater. Let’s hope that the sounds that have been reported as being detected prove to be useful in finding those in peril.
[1] https://pubs.aip.org/asa/jasa/article/85/S1/S112/649512/Measurement-of-the-sound-speed-in-air-by-sing
John Vincent 