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 (spherical spreading).
Shortly after the arrival of the direct sound, a series of semi-distinct early reflections from various reflecting surfaces (walls and ceiling) will reach the listener.
The early reflections should arrive within about 50 milliseconds (ms) for rapidly varying sounds, such as speech, or up to 80 ms for slowly varying music, in order that they not be heard as separate from the direct sound.
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.
