Acoustics / Electroacoustics
An echo is a repetition or a partial repetition of a sound due to REFLECTION. REVERBERATION is also reflected sound, but in this case, separate repetitions of the original sound are not distinguishable. For a repetition to be distinct from the original, it must occur at least 50 ms afterwards without being masked by either the original signal or other sounds. In practice, an echo is more likely to be audible after a 100 ms delay. See: PRECEDENCE EFFECT regarding echo suppression.
Perceptual effects of a direct sound combined with a reflected sound (at an angle of incidence of 40o) according to the delay time and relative intensity. Very short delays cause an image shift sideways and tone colouration due to PHASING; longer delays contribute to a spatial impression of reverberation, and stronger delays greater than 50 ms disturb the sound image and are perceived as echoes (after M. Barron, "The subjective effects of first reflections in concert halls - the need for lateral reflections," Journal of Sound & Vibration, 1971, vol. 15, pp. 475-494.)
Sound Example: Echo at Rolley Lake, B.C.Sound Example: Multiple echoes (called 'slap echoes') produced under a parabolic bridge, Stanley Park, Vancouver, B.C. The source sound is a stick hitting a metal can. See: PARABOLIC REFLECTOR.
In tape recording, an artificial echo, called tape echo, may be obtained by combining a sound with a version of itself that has been delayed, usually by the time taken for the tape to move from where it has been recorded to where it is picked up by a playback head. Many other kinds of echo are possible using similar tape FEEDBACK techniques. Digital delay units are also used to create echoes that have no added noise as found in analog DUBBING.
See also: ECHO CHAMBER, ECHOLOCATION, SONAR, TAPE MUSIC. Compare: PHASING, PRINT-THROUGH, SOUND-ON-SOUND, TAPE LOOP.
Sound Example: B.C. ferry horns with 0.25 sec. echo.