Anyone who has scuba dived, or even ducked underwater when snorkelling, knows that the underwater world is not silent.
For a scuba divers there is the crackling of exhaust bubbles, the air sucking through the regulator, the ringing and popping in the ears from changing pressures and the clicking of the sealife.
Maybe an anchor chain banging on the seafloor, coral munching fish, clicking of crustaceous, someone’s equipment banging on their tank or a boat motor somewhere overhead. The underwater world is a noisy place. But what does this mean for a scuba diver?
Why Does Sound Travel Faster in Water Than Air ?
Sound travels in waves through a medium. Those waves strike the ear drum which is then, through a series of small bones, converted into the sounds that are understood in the human brain.
The speed of sound through a medium is determined by the compressibility, and the density of that medium.
Density: the amount of material that takes up a specific volume
Compressibility: for a given pressure, how much of the medium can be compressed.
If the medium tends towards being dense, and compressible, then the sound will travel slower through that medium.
Compare the Speed of Sound Through Air and Water
When comparing the medium of air and water, these two factors of density and compressibility have to be considered.
Air: air is not very dense. That is, a large amount can be put in a specific volume; however it is very compressible. Just think of the air being compressed into a scuba tank to demonstrate the low density and high compressibility of air.
Water: water on the other hand is very dense, much more so than air. But it is not very compressible.
As water is around 800 times more dense than air, it could be expected that sound would travel faster through air than water. However, it is the fact that water can’t be compressed that makes water a much better conductor of sound than air.
As a result of all this, sound travels about four times faster through water than through air.
Scuba Diving and the Speed of Sound in Water
One of the phenomenon of being underwater on scuba gear is that it is difficult to clearly detect where a sound is coming from.
The sound of a boat motor underwater will nearly always make novice scuba students to look directly to the surface above them, searching for the passing boat. (Even though this shouldn’t happen, as the scuba dive warning flag should force all boats away from the dive site.) But the boat may be some distance away from the scuba divers, but it can sound like it is close and even passing overhead.
This lack of direction comes from the fact that sound is travelling faster through the water than air.
When in the air, the sounds reaches the ears at different times, usually reaching one ear very slightly ahead of the other ear. It is this slight delay that the brain uses to calculate the direction where the sound is coming from.
However, underwater that difference is substantially reduced. The delay between each ear hearing the sound is only one fourth of that in air, so the brain is not able to do the calculation of where the sound is emanating.
Communicating Underwater for Scuba Divers
Fortunately, this difference in the speed of sound underwater does not present a major problem for scuba divers communicating underwater. The ability of sound travelling through the water can be an advantage in some situations; such as banging a scuba dive knife on a tank to catch the attention of a scuba dive buddy. Or a diver hearing the warning beeps of their dive computer or watch.
Other ways of scuba divers communicating underwater are through sign language or the use of written words on a slate.
The speed of sound in water does not have a great impact for scuba diving. However, being aware of the physics of sound in water is useful knowledge for a scuba diver.
Bruce Iliff - Hello from an Australian freelancer! I’ve been scratching articles together in Australia in a range of subjects for a number of ...
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