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Echolocation

A dolphin does not only use its sonar sounds for navigation, but also to locate certain objects (or other things, like fish). This is of great importance for the dolphin. Since the dolphin cannot look straight ahead because its eyes are on the side of the head, it sends out a sonar sound and the sound waves will be reflected if they hit anything. The way that the sound vibrations come back to the dolphin will tell the dolphin where (in this case) the fish is located.
This way of locating things is called echolocation. The echo (the reflected sound) makes sure the dolphin has an exact location of the object. Because of the nearly constant sonar, the dolphin lives in a complete 'sound-world', and this is especially the case in the dark.

Dolphins produce separate, short click-sounds, with a maximum length of 0.2 milliseconds (this is 0.0002 seconds) each. One second fits 5000 of these sounds. These sounds have a frequency of 150000 Hertz (vibrations per second).
Research has been done on this subject with the dolphins. Firstly, one of the dolphins was 'blindfolded'. Whenever it got near a place where there was food, it would make these sounds at a higher rate. These echosounds are sent out with direction and in this way it can distinguish a piece of plastic (with the same size as the feed) from a piece of fish. In dolphinariums they swim in complete darkness - without losing speed - without hitting anything.
There is a difference between dolphins and other animals in how they use echolocation. The other most important animal with this ability is the bat. The bat has two very large earshells, while the dolphin only has two holes.