Getting good bass in custom home cinema installations has traditionally been a hit-or-miss affair. Remedies for unacceptable situations typically included spending more money on subwoofers with a “better” driver and a bigger power amplifier. Very occasionally, some form of acoustic treatment has been employed, but most such acoustic panels were of little value at very low frequencies.

In stereo it was common to think single-mindedly of a sweet spot, and to arrange for everything to be optimum for a single listener. At low frequencies, a DSP equaliser can be used to reduce the audible excess of objectionable room resonances, thus delivering respectable bass to a single listener. However, the existence of the standing waves between and among the room boundaries ensures that other cinema seats experience different bass.

Delivering similarly good bass to everybody occupying the home cinema seating area means that the room resonances must be physically manipulated in a manner that reduces the point-to-point variations in sound pressure. Conventional acoustics attacks the problem with absorption, damping the resonances by draining energy from the offending modes, resulting in lowered pressure maxima and elevated minima. Low frequency absorption in home cinema installations is always a good idea, but it can be difficult. Traditional low-frequency absorbers were bulky devices, some of which are hostile to even progressive concepts of interior decor. They still exist, but there are some different bass-trap devices that are more elegant. The options fall into several categories, and the effectiveness of each depends on the home cinema designer’s knowledge, on where in the room to place the acoustical material or devices.

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.