The repeated sound produced when sound waves bounce off another object is called an echo.
An echo happens when sound reflects from a hard surface—such as a mountain, building, canyon wall, or empty-room wall—and returns to the listener after the original sound. The delay depends on the distance traveled by the sound and the reflecting surface.
Human hearing generally perceives a separate echo when the reflected sound arrives at least about one-tenth of a second after the direct sound. At roughly 343 meters per second, that usually requires a reflecting surface about 17 meters away when the source and listener are together. A shorter, blended reflection is usually described as reverberation rather than a distinct echo.
The word comes from the Greek ēchō, meaning “sound.” In Greek mythology, Echo was a mountain nymph cursed to repeat the words of others. Echoes also have practical uses: bats and dolphins use echolocation, while sonar measures distance by timing reflected sound waves.