3.5 cubic feet of volume tuned to 45hz
Hard to find TS parameters for the 421A
resonant frequency (Fs) is stated to be 40hz
performance not sound is similar to k-140, EV15L
so they dont need a larger box than 3 to 3.5
unlike some 15's which like larger boxes around 5 to 6 cubic.
anything larger will just reduce power handling.
Altecs were offered as upgrade speakers in Ampeg 2x15
and also standard in the 2x15 horn loaded V4B cabinet.