Reverberation
Reverberation is the sound that can still be heard in a room after the source has stopped.But let's see how this works step by step.Step #1A sound is produced by the source (S), travels to the receiver (R) with decreasing intensity with the distance. The sound pressure level loses 6dB per doubling of the distance (true only if there is no reflection on the walls, or in so-called "free field"). This is the direct sound.Step #2After the direct sound has arrived to the receiver, some reflected sound will also arrive.The source is not sending in only one direction but in all directions (even if one directions is stronger because of directivity of the source). These waves reaching the receiver after bouncing (once or more times) on the walls (or other objects in the room) are called early reflections. They play an important role for spacial recognition and intelligibility depending on there intensity.At each bounce, the sound is partially absrobed and partially reflected, with more or less diffusion. Step #3As time goes, more and more reflected sound reaches the receiver with various delay and intensity. The more bounces, the more absorption, until it dies.Thise are the late reflections or simply the reverberation.This also plays a role for spacial information and a detrimental role for intelligibility.The time it takes for a sound to disappear after the source has stopped is called Reverberation Time. It's usualy defined as the time it takes to the reverberated sound to decrease by 60dB.The word "echo" is sometimes used to describe "resonance" in a room. It can be correct but it's often improper. Here is why.An echo is defined as a reflected sound that comes after the original sound with a "certain delay". If that delay is long enough, then the reflection is perceived as a separate event. In the examples below you will notice that when the delay is bigger then 40ms, then we can clearly identify two events. If the delay is less then 40ms, the sound appears to be only one but "sounding a bit funny".
Voice with echo of increasing delay, from 0ms to 100ms, in steps on 10ms.
Hand clap with an echo delayed by 0ms, 10ms, 20ms, 30ms, 40ms, 50ms, 60ms, 70ms, 80ms, 90ms and 100ms.
You have to count the claps to know which one is witch...
In the examples above there is only one reflection. If there are more reflections, the same "40ms rule" apply. Listen to the following example but note that in examples with delays less then 40ms, there are actually some additional reflections coming later then 40ms (it's a multiple echo).