Upper cut-off frequency = c * At / (2 * p * V)

Where:

V = Volume of cavity

At = area of port, i.e. horn throat area

c = Speed of sound

In two-way horn systems where the front of the driver loads a mid-horn, and the back loads the bass horn, it is of great importance that the dimensions of this cavity be calculated correctly. This is to ensure that there is a mechanical crossover between the two horns. However, in a back-loaded-only system such as this, it is really not that critical. The cavity's only mission here is to create a roll-off from a frequency where wavelength = an odd multiply of the horn's length, to avoid annulling when the out-of-phase waves from the back of the driver meets the in-phase waves from the originating at the front of the driver. We want to load ca 3 octaves into the horn (40Hz-320Hz). Theory then prescribes a relatively small cavity (ca 1,5 litres, space taken up by driver included) which also ensures good coupling of the cone's movements to the horn.