Multiple Partitions in Home Cinema System Designs - November 22nd, 2008
There are only two ways to achieve sound isolation of a dedicated home cinema room, using either stiffness or mass. Stiffness proves to be effective at low frequencies due to the fact that the sound wave must push against the stiffness of the cinema’s partition. This is known as the stiffness controlled isolation region.
As the frequency rises the partition needs to move less distance to re-radiate a given level of sound, and so it gets less effective, until at the resonant frequency of the partition its level of attenuation is at its lowest value. This is due to the fact that at resonance, the home cinema’s partition can be moved easily by the incident sound wave and so re-radiates the sound effectively. As the frequency rises above the partition’s resonant frequency, the mass-controlled region of isolation is entered. In this region, it is the fact that the sound must accelerate a heavy mass that provides the isolation. Because more force is required to move the home cinema’s partition at higher frequencies, the attenuation rises as the frequency rises.
At even higher frequencies there are resonances in which both the thickness of the dedicated home cinema’s partition, and the way sound propagates within it, interact with the incident sound to form coincident resonances that reduce the attenuation of the partition. Damping can be used to reduce the effect of these resonances. Most practical partitions in home cinema installations operate in the mass-controlled region of the isolation curve with coincident resonances limiting the performance at higher frequencies.
Plaster board walls, which are probably used in all dedicated home cinema installations, appear to have a significant coincidence resonance. The performance of a single partition increases by 3 dB every time its mass is doubled, but the coincidence resonances limit the ultimate performance of single partitions. Single partitions in high end home cinema systems are not recommended, because the cost and size get unreasonable for large attenuations.
The solution is to have two or more partitions which are independent of each other. If the two partitions are truly independent then the total attenuation, or effective sound isolation, is the product of the attenuations of individual partitions, which is the dB attenuation is the sum of the dB attenuations of the individual home cinema’s partitions. In practice, the partitions of the home cinema room are not independent, although the isolation is improved dramatically, but not as much as would be predicted by simply summing the dB attenuations. Coincidence resonances also reduce the effectiveness of a partition, and it is important that home cinema designers ensure that the two partitions have different resonances. This is most easily assured by having them made with either a different thickness, or a different material.
As an amusing example, assume double glazing made with similar thicknesses of glass. Because of the effect of the coincident resonances, the doubled glazed unit with 4mm glass is actually worse than a single pane of 4mm glass!
In dedicated home cinemas, often absorbing material is placed in the cavity between the two partitions to reduce the effect of coincidence resonances, but it is important that home cinema designers ensure that the absorbing materials do not make contact with both partitions or else flanking may occur.