echo, reflection of a sound wave back to its source in sufficient strength and with a sufficient time lag to be separately distinguished. If a sound wave returns within  1⁄10 sec, the human ear is incapable of distinguishing it from the orginal one. Thus, since the velocity of sound is c.344 m (1,130 ft) per sec at a normal room temperature of about 20°C (68°C), a reflecting wall must be more than 16.2 m (56 1⁄2 ft) from the sound source at this temperature for an echo to be heard by a person at the source. In this case the sound requires  1⁄20 sec to reach the reflecting surface and the same time to return. Bats navigate by listening for the echo of their high-frequency cry. Sonar and depth sounders work by analyzing electronically the echo time lag of sound waves, generally between 10 and 50 kilohertz, produced by underwater transducers. Radar sets broadcast radio waves, usually between 100 and 10,000 megahertz, pick up the portion reflected back by objects, and electronically determine the distance and direction of the objects. A sound echo that is reflected again and again from different surfaces, as by parallel walls in a tunnel, is called reverberation. When a surface reflects sound it partially absorbs and partially reflects the energy. As the process is repeated the sound becomes weaker and weaker and eventually ceases.

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