The P220 is an 8 inch passive radiator with aluminium dome. Extra mass can easily be added to tune the  resonance  frequency. A simple screw mount provides fixing of  stainless steel discs in the desired quantity. The values of Vas and Cms allow for extensive tuning and various box sizes. The  stated  resonance  frequency of the passive radiator is taken at  3mm excursion, to give a realistic value. It can be calculatd by the following equations:

Fres= sqrt[1/(Cms corr.) x Mms x 4 pi^2)]  with  Cms corr.= 1/[(1/Cms) + 1/((Vb/Vas) x Cms)]