Though Jacques-Yves Cousteau once dubbed the ocean "the silent world," we now know that is not exactly true. The ocean is full of sound. Our ears just aren't specialized enough to hear all the different frequencies.

Ocean animals make noise all the time. They make noise while swimming, when they are frightened, to find food, to send out warnings, to check out their surroundings and to talk to each other.

Dolphins and some whales use a process called echolocation. First, they send out a series of clicks and whistles and then listen for echoes as the sounds bounce off objects, such as other fish, boats, the ocean floor or reefs, in their path. From the direction and strength of the echo, these animals can develop a mental image of their environment. They can "see" the size of objects in their path and how far away the objects are.

The sonar we use to study the ocean floor works like echolocation. By sending out signals and retrieving the echoes, we can develop pictures of all the features on the ocean floor. We can also find objects on the bottom, like shipwrecks or mines, and in the water column, like submarines or large schools of fish.

Scientists found one part of the ocean that conducts sound a bit differently from the rest. It is called the SOFAR channel, which stands for SOnic Fixing And Ranging Channel. Low-frequency sounds can travel for hundreds of miles in this channel. Any shallower or deeper in the water column the sound will fade out much faster. It has been suggested that whales use this channel for communication over great distances.

Information provided by: http://www.onr.navy.mil