Properties of Acoustic Panels in Home Cinemas - November 23rd, 2008
Porous absorbers, such as carpets, curtains and other soft materials found in home cinemas, work due to frictional losses caused by the interaction of the velocity component of the sound wave with the surface of the absorbing material.
The velocity component arose because the air molecules have to move between the compression and rarefaction states. A given pressure variation will require a greater pressure gradient, and hence higher peak velocities, as the wavelength gets smaller with rising frequency. Because the pressure gradient of a sound wave increases with frequency, the friction due to interaction with an acoustic wall treatment will also increase with frequency, and therefore the absorption of acoustic wall panels also rises with frequency.
Clearly, the larger the surface covered with acoustic panels in a home cinema installation, the higher the friction and therefore the absorption. This means that porous acoustic panels which consist of a large number of fibres per unit volume, such as high-density rockwool or fibreglass, will tend to have a high-level absorption. This also explains why curtains in home cinema rooms which are draped to a fraction of their cloth area absorb more strongly than ones which are flat.
Because acoustic panels in home cinema installations interact with the velocity component of the sound wave, they are affected by the space between them and the wall and their thickness. This is due to the fact that at the surface of the home cinema’s wall, velocity component is zero, whereas at a quarter of a wavelength away from the wall, the velocity component will be at a maximum. An acoustic wall panel will absorb more strongly at frequencies whose quarter wavelength is less than either the spacing of the panel from the wall, or the thickness of the acoustic panel, if it is bonded directly to the home cinema’s wall.
Although in principle there could be a variation in the absorption coefficient as the frequency increases above the quarter wavelength point, due to the inherent variation of the velocity component as a function of wavelength at a fixed distance from a surface, in practice this does not occur unless the cinema’s acoustic panels are quite thin.