As well as absorption it is essential that the sound be diffused when it strikes a surface, in high end home cinema installations. Ideally we want the acoustic equivalent of a matt surface. Unfortunately most surfaces in a home cinema room, including large areas covered with absorptive acoustic panels, act like acoustic mirrors, with varying shades of darkness. In order to have a matt surface one needs a ‘bumpy’ wall and many things can be used to provide this. Unfortunately the bumps need to be at least an eight, and preferably a quarter of a wavelength in size to be effective.

This results in the requirement for very large objects at low frequencies, 1.25-2.5 m at 34 Hz, and very small objects at higher frequencies, 1.25-2.5 cm at 3.4 kHz. If the objects are too small, that is, less than one eight of a wavelength, they will not diffuse properly. If they are too big, that is, greater than about a half a wavelength, they will behave as acoustic mirrors in their own right and so will not diffuse effectively.

Clearly effective diffusion is a difficult thing to achieve in home cinema installations, in an ad hoc manner. Curved and angled structures found in many home cinema designs can help at mid and high frequencies, and at very high frequencies, greater than about 4 kHz, the natural rough textures of materials such as brick and rough cut stone are effective.

Because of the need to achieve well-defined diffusion characteristics, diffusion structures based on patterns of wells whose depths are formally defined by an appropriate mathematical sequence have been proposed and used. The design of these structures is quite involved and will not be mentioned here. However, home cinema designers would need to take into consideration such structures, if a high end dedicated home cinema installation is the case.

Flanking paths, which are the main limitation to sound isolation in home cinema installations, arise when there are other paths that the sound can travel through, in order to get round the sound isolating structure. Typical paths for flanking in a home cinema installation are the building structure, heating pipes, and most commonly ventilation systems or air leaks.

Home cinema designers can reduce the effect of the building structure by building a ‘floating room,’ which removes the effect of the building structure by floating the room on springs away from it. In practice, ensuring that no part of the building is touching the dedicated home cinema room by any means (plumbing pipes and electrical wiring conduits are popular offenders in this respect) is extremely difficult.

The effect of ventilation systems and air leaks are also a major source of flanking in many home cinema installation cases. In fact in the domestic situation the sound isolation is almost entirely dominated by air leaks and draught paths, and it is the removal of these that allow double glazing salesmen to advertise a dramatic improvement in sound isolation, despite having two 4 mm panes of glass in the double glazing.

So in order to have good sound isolation in home cinema installations, one needs good partitions and an air-tight, draught-free structure. Achieving this in practice, while still allowing the occupants in the home cinema to breathe is a challenge.