How do echoes work?

Question

I am not a sound engineer, but this site looks like there may be someone who can answer a question I have today. On a nostalgia site about Freeport, Texas--where I was a child sixty years ago--someone mentioned that she remembers just going outside and shouting to hear an echo. But that no longer works. I remember being able to do that as a child, too. Can someone say a little about how echoes work. Would I be more likely to be able to hear an echo in a rural setting than in an urban one?

Answer 1

Sound travels at approximately 340m/s. This means that if you make a sound and it reflects off a surface 170 meters away, you will hear the bouncing sound a second later (half a second to get there and half a second back). If the sound was loud enough and there isn't much background noise you should hear repetitions every one second fading away (assuming you have a large flat surface behind you).

Effectively you have two effects combined. The delay caused by the distance and the attenuation in volume, caused by the distance again. A third effect is less but still noticeable - the diffusion of the sound.

We distinguish echoes (delay) from reverberation which is also a reflection effect, by the amount of direct reflections (one or two usually for echo, many for reverb) and their distance (in time) from the direct sound. As a rule of thumb, if you can't distinguish the start of each repetition its a reverb. If you can, it's an echo.

You should be able to hear echoes in both environments if there is a sufficiently large surface to reflect the sound but in an urban environment you are expected to have more noise so you may not be able to distinguish the echoes as easily as in a rural setting which is averagely more quiet.

Answer 2

Nice explanation Schizo, I would only add that there are other factors that affect how easily sound can travel through the air.

The weather for one, the humidity, wind etc. Also, frequency of the wave also determines the distance it can travel(so composition of the sound; harmonic content), plus certain sounds can be heard to echo better than others(e.g. fast attack & decay is easier to hear after diffusion).

But the biggest factor is obviously the reflection surface. Maybe the area has changed over time; moss grown on surface etc. This would absorb and dissipate the sound, so little reflection would result.

It's an interesting question, I'd like to see how the area has changed.