What are echoes and reverberations?

Understanding why mountains sometimes talk back.

We have all heard echoes

When you’re in the middle of a mountain and you scream, you can hear that sound. While it could be the mountain talking back (that would be fun), the sound you hear is an echo.

But why can you hear them?

It’s weird that you can hear sound coming back again. When you hear repeated sounds, what you are actually hearing are reverberations.

What exactly are reverberations?

Reverberation is the persistence of sound after it has been produced. Sound doesn’t disappear instantly. The time it takes to disappear is called the reverberation time, and it can take something from milliseconds to seconds.

They create background noise

If you’re hearing a radio show and you’re constantly hearing background noises at the same time, usually there’s a lot of reverberation in the room.

This means the average listener won't listen to the show correctly, which is a common speech intelligibility problem.

Reverberation time depends on the surfaces sound waves encounter.

We’ve talked about sound behavior before.

If sound waves are absorbed, reverberation time will diminish quickly. If they’re diffused, the reverberation time won’t diminish, but the waves won’t repeat themselves in the same place. This is why a combination of diffusers and absorbers can help you create good room acoustics.

Echoes are a by-product of reverberations

When the reverberation time is long enough, an echo can occur.
An echo is a separate repetition of the original sound, after a few seconds of the production of sound.
The occurrence of echoes depends on the distance between the sound source, the reflecting surface, and the listener.

Flutter echoes are a problem in architecture.

Flutter echoes are echoes that are trapped between two reflecting surfaces. This happens, for example, in hallways, where you can have two flat walls that make sound bounce repeatedly between them.

Next: How do you control noise?