Absorption of Modes in Home Cinemas - January 26th, 2009

It has already been discussed that modes in dedicated home cinemas behave differently to diffuse sound and this has various consequences.

1. The modes are not absorbed as strongly as sound which visits all surfaces of a cinema room. This is due to both the reduction in the number of surfaces visited and the change in absorption due to non-random incidence.

2. This reduction in absorption is strongly frequency-dependent and results in less absorption and therefore a longer decay time at the frequencies at which standing waves occur.

3. The decay of sound energy in dedicated home cinema systems is no longer a single exponential decay with a time constant proportional to the average absorption in the room caused by the acoustic panels, etc. Instead there are several decay times. The shortest one tends to be due to the resonant modes in the home cinema room. This results in excess energy at those frequencies with the attendant degradation of the sound in the room.

The energy in a mode decay as a function of time is related to the reverberation, and the absorption in a mode has a big effect on the frequency response. This effect will be analysed separately in a future article.

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Filed under: Technical Articles — admin @ 9:34 pm

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