The formula says 17.2 metres, because the sound must travel to the wall and back, so with that speed, the delay needs to be over 0.1 seconds for our ears to separate the echo. But in my city, yaar, I hear echoes between concrete towers at far shorter distances in the quiet of a 4 a.m. shift. My worry about the noise, and my aging ears, have tuned me to listen for what the textbooks say you shouldn't.
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