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Let's play an exciting game of "Find the Noisemaker". While your eyes are closed the Noisemaker will quietly move to a different spot in the room and make a noise such as a burp. Without opening your eyes each should point to my location. Open your eyes and see what you should be pointing at the Noisemaker. Accurately doing that using just your ears is richly rewarding. You located the position in the room without looking. How did you do that? Was it a miracle?
The part of your ear that sticks out from your head (the Auricle or Pinna) acts as a locater. Its shape catches and concentrates sound and then sends it into the hole going into your head. When a sound is produced the listener will turn their Pinnas until the sound is at its loudest and equal for both. The loudness is at its peak when the pinnas are both facing the sound capturing it and reflecting it to that small opening. The bigger the Pinna the greater the concentration. A person with Pinnas equal to an elephant's will be easily decipher your nasty comments.
Sound travels about 770 miles per hour in air or 344 meters/second. In addition to the Pinna reaction your computer (brain) is fast enough to be able to detect the difference in arrival times of a sound hitting one ear and then the other if one ear is facing the source. When the arrival time for both ears is the same the observer must be facing directly toward or away from the source of the sound. This difference in arrival times is another way a person can tell where sound is coming from.
When a diver is underwater the pinnas do not function as they do in air. Since they are almost the same density as the wate, it's as if they were surgically removed from your head leaving you with just 2 holes going into the Middle Ear. The ears may also be covered by a hood. Because the speed of sound in water is about 4 times faster (it is about 3400 miles per hour, ~1500 meters/second) the brain is no longer able to tell the difference in arrival times for each ear. The result is the loss of being able to tell where sounds originate underwater. You can clearly hear me banging my tank with my knife to alert you to some important event but you will not know which way to look to find out why I am sending you the message.
Sound does travel further underwaterl. Banging 2 rocks together below the surface can be heard for miles underwater. Contrast that to sound in the air. You could clearly hear the Deep-Six underwater loud speaker for about 3 miles in the open ocean. You will be able to hear the whine of a boat's propeller, but will be unable to tell its origin or direction. Don't surface without a sausage.
Before leaving the topic, Dive Training Magazine (11-2010) published some graphs related to the path and speed of sound underwater:
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