Bass Traps in Home Cinemas - October 23rd, 2008
Dedicated home cinema designs offer bass traps in order to absorb the low frequencies in the room. Bass absorption is usually done with resonant absorbers. Resonant absorbers are mass spring systems with damping to provide absorption at the resonant frequency of the system. The mass might come in the form of a membrane made of plywood or mass-loaded vinyl. Alternatively, the vibrating air in the neck of a hole might form the mass, as is the case for a Helmholtz resonant absorber. The spring usually comes from an air cavity. Damping is most often provided by sound being forced through a porous resistive material; mineral wool, fibreglass or acoustic foam.
Bass trapping devices like resonant absorbers, usually only provide a narrow band absorption in the home cinema room. In a home cinema installation, we need to cover a wide bandwidth, so a series of bass traps are required, each tuned to a different frequency range. Dedicated home cinemas often use multiple bass traps, to ensure a smooth response to the whole seating area. That aspect is taken care in the design stage, where efficient and elegant ways are used, in order to integrate bass traps in the cinema room. A smooth low frequency response is then achieved and creates an unrivalled home cinema experience.
The home cinema design stage is probably the most important part of the whole process. One problem with modal control in using bass traps in a home cinema installation, is estimating how much resonant absorption to use. Although the theories allow the absorption coefficient for Helmholtz absorbers to be estimated, the meaning of absorption coefficient at low frequencies is problematical. At low frequency the sound field in a home cinema room is not diffusive, and consequently the effect that the bass trap has is not calculable through simple statistical laws.