Echo

Echo is a sound heard when it is reflected back to its source after striking some object. When we shout or clap our hands, we produce sound waves that travel through the air in all directions. We first hear the sound when the waves reach our ears by the most direct path (see Sound). If the waves also hit a large object, such as the side of a building, they bounce back and may reach our ears a second time. The second sound is an echo. It is probably named for the nymph Echo in Greek mythology.

Sometimes we do not hear echoes. We may not hear an echo if the original sound is too weak, if the object absorbs rather than reflects the sound, or if the reflecting object is too small. We may not be able to tell the difference between the direct sound and its echo if the reflecting object is less than about 30 feet (9 meters) away, because they arrive at nearly the same time.

At other times, we may hear more than one echo from just one original sound. Such repeated echoing usually occurs in valleys and canyons where there are many sound-reflecting surfaces. The sound waves bounce from wall to wall and may often produce several echoes. Such repeated echoes are called reverberation.

Echoes can help us find out how far we are from echo-producing objects. Sound waves travel about 1 mile (1.6 kilometers) in five seconds. It takes 10 seconds for sound to reach an object a mile away and return. If a person who stands and shouts at the edge of a canyon hears the echo five seconds later, the canyon is about one-half mile (0.8 kilometer) wide.

Sound waves traveling through water also produce echoes. A device called sonar uses underwater echoes to measure depth and to locate underwater objects. It is used in navigation to locate underwater channels, and can even detect schools of fish.

Contributor: Thomas A. Griffy, Ph.D., Prof. of Physics, Univ. of Texas, Austin.

Echo

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