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.
Sweet Echo, sweetest nymph, that liv'st unseen
Within thy aery shell
By slow Meander's margent green.
And in the violet-embroidered vale,
Where the love-lorn nightingale
Nightly to thee her sad song mourneth well;
Canst thou not tell me of a gentle pair
That likes thy Narcissus are?
Oh, if thou have
Hid them in some flowery cave,
Tell me but where,
Sweet queen of parly, daughter of the sphere,
So may'st thou be translated to the skies,
And give resounding grace to all heaven's harmonies."