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- In audio signal processing and acoustics; an echo is a reflection of sound; arriving at the listener some time after the direct sound. When dealing with audible frequencies; the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10 of a second. Thus; since the velocity of sound is approximately 343 m/s at a normal room temperature of about 20°C; a reflecting wall must be more than 16.2 m from the sound source at this temperature for an echo to be heard by a person at the source.