Acoustically ‘Small’ and ‘Large’ Dedicated Home Cinemas - November 19th, 2008
The concept of critical frequency allows us to define the difference between dedicated home cinemas which are ’large’ and ’small’ in acoustical terms. In an acoustically large home cinema room the critical frequency is below the lowest frequency of the sound that will be generated in the room, whereas in an acoustically small home cinema the critical frequency will occur within the frequency of the sounds being produced in it.
Most home cinema systems are designed and installed in acoustically small rooms; an example of an acoustically large room would be a concert hall, or even a large recording studio.
There are two different approaches in regards calculating the critical frequency of a dedicated home cinema, prior installation. The first is to assume that modal behaviour in the home cinema room dominates once the mean free path is equal to one and a half wavelengths. The expression (A) below is useful for making rapid assessments of the like hood of achieving a particular critical frequency in a given home cinema installation.
However, the real critical frequency may well be higher because a home cinema room can have significant modal behaviour at high frequencies if the absorption is low. Because of this the accepted definition of critical frequency is based on the modal bandwidth, hence equation (A). The main consequence of modal behaviour is the frequency and spatial variation caused by it. However, if a given frequency excites more than one mode in the home cinema room, both the spatial and frequency variation will be reduced.
The critical frequency is defined as when the modal overlap equals three, so at least three modes are excited by a given frequency, and is given by expression (B) below. Expression (B) clearly shows that larger home cinema installations will have their critical frequencies generally lower than smaller ones. That is why big cinema rooms are acoustically ‘large’ as well.