Echoes and reverberation

Sound bounces off solids and liquids, following the same laws of reflection as light: it bounces off at the same angle it arrived at, all in one flat plane.

An echo is sound reflected back to you from a far-off hard surface: a mountain, a cliff, the end of a long corridor.

Why you need distance for an echo. Your brain hears two sounds separately only if they arrive at least 0.1 s apart. In 0.1 s, sound (340 m s⁻¹) travels 34 m. But it has to go to the wall and back, so the wall must be at least 34 ÷ 2 = 17 m away.

Echo sums: always halve. You clap and hear the echo 0.5 s later. The sound travelled 340 × 0.5 = 170 m, to the wall and back, so the wall is 85 m away. Then check: does the answer make sense? An echo from 850 m inside a school corridor would not.

Echoes are strongest from hard, smooth surfaces. Soft things like curtains absorb sound; rough surfaces scatter it.

Reverberation. In a big empty hall, sound bounces many times, and the reflections arrive less than 0.05 s apart, so the sound seems to linger after the source stops. That's why singing in a bare bathroom or empty room sounds fuller. In a concert hall, too much reverberation makes speech muddy, so halls use sound-absorbing panels, cushioned chairs and curtains.

An Indian marvel. In the Whispering Gallery of the Gol Gumbaz in Bijapur, Karnataka, a soft whisper against the wall is heard several times right across the huge dome.