It comes back
Shout towards a cliff, a tall rock wall. A moment later, your own voice comes back.
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Rule
A hard surface reflects sound. A return 0.1 s or more late is heard as an echo. The wall is half the round trip away.
- Reflection — sound bouncing off a surface. It follows the same law as light.
- Echo — reflected sound that you hear as a second sound.
A sound seems to last about 0.1 s in your ear. A return sooner than 0.1 s blurs into the first sound. Out and back is the round trip: speed × time. The wall is half of that distance.
| Echo after | Out and back | Wall is |
|---|---|---|
| 0.1 s | 340 × 0.1 = 34 m | 17 m |
| 0.3 s | 340 × 0.3 = 102 m | 51 m |
| 0.6 s | 340 × 0.6 = 204 m | 102 m |
Hard, smooth walls give strong echoes. Curtains take in sound, so their echo is weak. In a big hall, many reflections make sound last longer. That longer-lasting sound is reverberation.