The Subwoofer DIY Page
Ported Systems - Box Calculations
last updated: 13 April 2011
The Subwoofer DIY Page

To determine the optimum box size and tuning for a ported 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 its resonant frequency
Fs = resonance frequency of the driver (Hz)
Dv = internal diameter of port (cm)

then,

Vb = 20*Qts^3.3*Vas
Fb = (Vas/Vb)^0.31*Fs
F3 = (Vas/Vb)^0.44*Fs
dBpeak = 20*LOG(Qts*(Vas/Vb)^0.3/0.4)

where,

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

Note that this is only one of many possible alignments for a ported system with a given driver. Many types of alignments exist, each with its own particular advantages and disadvantages. This particular alignment will give you a maximally flat response within the system's passband, but it won't necessarily produce the best results with every driver, particularly those with very low Qts values (EBS alignments may work better with low-Qts drivers). You can use the "comparisons" section of the PORTED.XLS file provided on this page to derive a ported alignment that best suits your needs.