It can take a while to get used to hearing sound underwater. You are constantly receiving visual input through an entire dive, so your brain learns to compensate for refraction and other visual properties of water. However, audible input is not constant, so each time a boat drives by it catches you by surprise. What makes sound different underwater?
The largest difference is speed. At the surface, sound travels at about 340 m/s. In sea water that increases to about 1500 m/s, over four times faster! You constantly use the speed of sound to distinguish the audible world around you; your brain is unable to cope with this drastic increase, and thus noises sound like they are coming from all over.
With the relatively slow speed of sound in air, your brain uses the distance between your ears to pinpoint the sound's origin (through a process called triangulation). For example, let's say you hear a sound immediately to your left. The sound will reach your left ear before your right. If your ears are a distance of 8 inches = 0.2032 meters apart, we can determine the time difference as t = 0.2032 m / (340 m/s) = 0.000597 s. Small, yes, but large enough for your brain to detect. If the sound reaches your ears at the same time, then the sound is coming from directly in front, above, or behind you (ignoring sound waves bouncing off walls). Visual input fills in the missing blanks to pinpoint the sound's origin.
Now let's repeat the same calculation underwater. The time differential from a sound immediately to your left is t = 0.2032 m / (1500 m / s) = 0.000135 s; below the threshold your brain can distinguish. Combine this with no visual input directly in front of you, and your brain just assumes that all sound is coming from directly on top of you. Humans don't like loud noises (such as those from boat engines) from directly above, so this tends to trigger fear reflexes.
Sound bounces off walls and objects, further aiding your brain in the triangulation process. The ocean is a much wider expanse in which sound travels freely, further altering your underwater perception.
This time we discussed how the location of sound underwater is perceived. Next time we'll look at how the actual sound is altered.