Board thickness 18 mm. Bass reflex fit front (clear bass ) or back ( strong bass) ,because of reflection off wall. Bass reflex is not tube, just hole. Bass loudspeaker do not fit at strictly at center. Dimensions frankly change but volume must remain identical.

BASS	BASS
16-19CM	29-32CM
19-22CM	32-35CM
22-25CM	35-39CM
25-29CM	44-49CM