Scuba Diving: Sound and Hearing

We take sound for granted while moving through the blanket of air we call atmosphere. Divers, however, quickly realize that sound can be a source of confusion and consternation under water because the direction it’s coming from is so difficult to determine.

Why?

If you’ve ever banged on your tank to attract your buddy’s attention, only to watch him turn around and around in vain to locate the source, you’ve experienced yet another example of the failure of our land-adapted organs to serve us under water.

The Speed of Sound: Air

While the speed of sound varies depending upon air temperature, it is often given as 344 meters per second. Although we’re aware of the presence of sound, we can’t see it moving. We have to imagine it: the ripples that form when a pebble falls into a still body of water, for example. The ripples move outward in a regular pattern until they strike another object, or lose all of their energy.

The human ear is designed to detect these sound waves rippling through the air unseen. More importantly, for this analysis, as sound waves move through the air they actually push air particles into small clusters, which then alternately spread out like the compressed coils of a Slinky moving through the entire spring.

The Speed of Sound: Water

Air is lightweight for its volume or size. Water, however, is approximately 800 times denser than air. This far greater density allows sound vibrations to move more quickly through water than through air because water particles are packed much closer together than air particles. As a result, vibrations can move from one particle to another more quickly: four times faster, as a matter of fact, or 3,000 mph.

So What’s the Problem?

If sound moves so well through water, why isn’t that a good thing? Put simply: it’s too much of a good thing. Sound moves so quickly through water that our land-mammal brains cannot discern its direction. The brain normally determines a sound’s direction by calculating the minuscule time lapse between the vibrations striking the ear closest to the sound and then the other.

That’s fine for land. But because sound travels so fast in water, the brain is unable to carry out the normal calculations due to the time lapse being 400 percent shorter than in the air. Eyes, lungs, feet and now ears: Once again we confront the shortcomings of a land animal venturing into the watery kingdom.