Far, hard and facing you
Why Do Canyons Echo So Well?
Canyon walls are distant, hard and steep — the three things an echo needs.
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Step 1 of 5
Distance gives you the delay.
You need over a tenth of a second to hear the echo separately, which means a wall at least 17 metres away. Canyon walls are far further.
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Step 2 of 5
Bare rock is an excellent reflector.
A forest is quiet because it is eating the sound Hard dense surfaces bounce almost all the sound back. Trees, soil and snow absorb it, which is why forests are so quiet.
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Step 3 of 5
Steep walls aim it back at you.
Sound reflects like light: the angle out matches the angle in. A near-vertical wall sends your shout straight back down the canyon.
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Step 4 of 5
Two walls give you repeats.
Sound can bounce from one side to the other several times, each trip returning a fainter copy. That is the fading repeat you hear.
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Step 5 of 5
Each bounce loses energy.
Some sound is absorbed at every reflection and some spreads away. The echo fades to nothing after a handful of trips.
Some ancient amphitheatres were shaped deliberately so a whisper on stage reaches the back row clearly.
The short version
You need a wall at least 17 metres away to hear the echo separately. Bare rock reflects nearly all of it where a forest would eat it, and two steep walls trade the shout back and forth until it fades out.
Try it yourself
Shout across a valley with rock on one side and trees on the other. Only the rock answers.