Reverberant Sound in Home Cinema Rooms - November 18th, 2008
Sound is reflected many times before arriving at the listener from all directions. There are so many possible reflection paths, each individual reflection is very close in time to its neighbours, thus there is a dense set of reflections arriving at the listener. This part of the sound in the cinema room is called reverberation and is desirable as it adds richness to, and supports, musical sounds.
Reverberation in home cinema rooms also helps integrate all the sounds from an instrument so that a listener hears a sound which incorporates all the instruments’ sounds, including the directional parts. In fact we find home cinema installations in spaces which have very little reverberation, uncomfortable and generally unsuitable for listening to music in.
The time taken for reverberation to occur is a function of the size of the home cinema room and will be shorter for smaller spaces, due to the shorter time between reflections and the losses incurred on each impact with a surface. In fact the time gap between the direct sound and the reverberation in dedicated home cinemas is an important cue to the size of the room that the soundtrack is being played in. Because some of the sound is absorbed at each reflection on the cinema’s acoustic panels, it dies away eventually. The time that it takes for the sound to die away is called the reverberation time and is dependent on both the size of the home cinema room, and the amount of sound absorbed at each reflection.
In fact there are three aspects of the reverberant field that the home cinema room effects:
1. The increase of the cinema’s reverberant field level :
This is the initial portion of the reverberant field and is affected by the home cinema’s room size, which affects the time between reflections and therefore the time it takes the reverberant field to build up. The amount of absorption in the home cinema room also affects the time that it takes the sound to get to its steady-state level. In other words, the rate at which sound builds up in a dedicated home cinema, depends on the time between reflections and the absorption. That simply means that reverberant sound level will take more time to reach a louder level than a smaller home cinema room.
2. The steady state level of the home cinema’s reverberation field :
If a steady tone is played in a home cinema system, after a period of time the reverberant sound will reach a constant level because at that point the sound power input balances the power lost by absorption because of the acoustic wall panels. This means that the steady-state level will be higher in home cinema rooms that have a small amount of absorptive acoustic panels, compared to cinema rooms that have a lot of absorptive treatments.
3. The decay of the cinema’s reverberant field level :
When the tone in the home cinema stops, the reverberant sound level will not reduce immediately but will instead decay at a rate determined by the amount of sound energy that is absorbed at each reflection with the acoustic panels and other surfaces. Thus in home cinema installations with a small amount of absorption the reverberant field will take longer to decay.