The Subwoofer DIY Page
Sealed Systems: Box Calculations
last updated: 13 April 2011
To determine the box size for a sealed enclosure system, you will need to know the following Theile/Small parameters for the driver:

Vas = Equivalent air compliance (litres)
Qts = total Q of the driver at Fs
Fs = resonance frequency of the driver (Hz)

Choose a final Qtc (total Q of system at resonant frequency) for your design. Recommended values for Qtc are from 0.6 to 1.0. Transient response degrades with higher Qtc values, but the power handling of the system increases. A Qtc of 0.7 will usually give pretty good results, but you can use a higher figure if the subwoofer has a low resonant frequency (<20 Hz) or if it's being designed for car use.

Then,

Qr = Qtc/Qts
Vr = Qr^2-1
Vb = Vas/Vr
Fb = Qr*Fs
F3 = Fb*((1/Qtc^2-2+((1/Qtc^2-2)^2+4)^0.5)/2)^0.5


If Qtc>(1/2)^.5
then dBpeak = 20*log(Qtc^2/(Qtc^2-0.25)^0.5)
Else dBpeak = 0

where,

Vb = net box volume (litres)
Fb = box resonant frequency (Hz)
F3 = -3dB frequency (Hz)
dBpeak = maximum peak or dip in system response