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Echo  

Murray Campbell and Mary Térey-Smith

The repetition of sound after a short time interval. In addition to the applications discussed below the term is used for a signal-processing device (also known as a delay) that produces a slightly delayed playback of sounds either by a tape loop or by digital delay; see Electric guitar §2 .

See also Organ stop .

Natural echoes arise from the reflection of a sound wave by a solid surface, such as a wall or cliff. For the echo to be perceived as distinct from the original sound, the extra path length travelled by the reflected sound wave must have a minimum value of around 17 metres, corresponding to a minimum time interval of 50 milliseconds between direct and reflected sounds.

The reverberant sound field in a concert hall is created by multiple reflections of sound waves. In a well-designed hall, the direct sound reaching a member of the audience is followed by a series of reflections within a time interval of around 35 milliseconds. These ‘early delayed arrivals’ are not heard as separate echoes; because of the ‘precedence effect’ they are perceived as a reinforcement of the direct sound. Subsequent reflections blend smoothly into the reverberation. A concave surface, focussing sound waves into a particular part of the hall, can give rise to an audible echo; a ‘flutter echo’ can arise from successive reflections between parallel walls....