Sound waves travel at about 345 meters/second, so that the sound coming directly from a source within a large room will generally reach a listener after a time of anywhere from 0.01 to 0.2 seconds.
Direct sound will decrease by 6 dB for each doubling of propagated distance.
Shortly after the arrival of the direct sound, a series of semi-distinct reflections from various reflecting surfaces (walls and ceiling) will reach the listener. If these early reflections occur within about 50 – 80 milliseconds and are reasonably similar to the direct sound, they are not heard as separate from the direct sound. For rapidly varying sound, such as speech, the limit is around 50 ms while for slowly varying music, the limit is closer to 80 ms.
The source is perceived to be in the direction from which the direct sound arrives provided that (1) successive sounds arrive within about 35 milliseconds, (2) the successive sounds have spectra and time envelopes reasonably similar to the first sound, and (3) the successive sounds are not too much louder than the first. This is referred to as the precedence effect.
The reflections that reach the listener after the early reflections are typically of lower amplitude and very closely spaced in time. These reflections merge into what is called the reverberant sound or late reflections.
If the source emits a continuous sound, the reverberant sound builds up until it reaches an equilibrium level. When the sound stops, the sound level decreases at a more or less constant rate until it reaches inaudibility.
For impulsive sounds (ex. a ballon pop), the reverberant sound begins to decay immediately.
