Room Modes in a Home Cinema - October 26th, 2008
Home cinema installations usually suffer from problems due to low frequency modes. At low frequencies, the standing wave modes of the cinema room are separated in frequency. The frequency response in a home cinema is uneven meaning that some frequencies are emphasised, where modes are strong, and some suppresses, where modes are weak, leading to colouration of the received sound. This is most critical for a dedicated home cinema installation, particularly with the increasingly widespread use of subwoofers, and material with high bass content.
Common solutions include choosing appropriate home cinema room dimensions, speaker locations and listening positions, to flatten the frequency response of the cinema room as much as possible and avoid degenerate modes. Even when the home cinema room dimensions have been carefully chosen, however, the frequency response of the room will still be uneven and acoustic treatment is needed.
Particularly prominent room modes in a home cinema installation are usually treated with bass absorption often referred to as bass traps or bins. It is not possible to treat this problem with diffusion because the sizes of the diffusers become prohibitively large. Porous absorbers are not usually used or specified in the home cinema design stage, as they would have to be extremely thick to provide significant bass absorption. In practice, many people place porous absorption in corners of home cinema rooms, thinking this will absorb the modes. The particle velocity in the corners is very low, so the absorption is ineffective.
For these reasons, dedicated home cinema designs would include resonant absorbers. Home cinema installations require some kind of low frequency modal treatments, and it is the home cinema designer’s responsibility to accurately model and integrate such devices.