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echo
In audio signal processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound. The word ''echo'' derives from the Greek ἠχώ (''ēchō''),〔(ἠχώ ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus〕 itself from ἦχος (''ēchos''), "sound".〔(ἦχος ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus〕 Echo in the folk story of Greek is a mountain nymph who loved her own voice. Animals that use echoes are cetaceans (dolphins and whales) and bats.
==Acoustic phenomenon==
If so many reflections arrive at a listener that they are unable to distinguish between them, the proper term is reverberation. An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived. Echoes are reflected off walls or hard surfaces like mountains and privacy fences.
When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/15 of a second. Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 25 °C, the reflecting object must be more than 17.2 m from the sound source at this temperature for an echo to be heard by a person at the source.
Sound travels approximately 343 metres/s (1100 ft/s). If a sound produces an echo in 2 seconds, the object producing the echo would be precisely that distance away (the sound takes half the time to get to the object and half the time to return). The distance for an object with a 2-second echo return would be 1 sec × 343 metres/s or 343 metres (1100 ft). In most situations with human hearing, echoes are about one-half second or about half this distance, since sounds grow fainter with distance. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes. The strength of an echo is frequently measured in dB sound pressure level SPL relative to the directly transmitted wave. Echoes may be desirable (as in sonar) or undesirable (as in telephone systems).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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