Triple Chamber Bandpass sub uses dual 18" drivers in "Push-push" configuration. Motors in center chamber.
Drawing shows center (red) and outer (blue) chamber details separately. Bracing comprising center chamber "port tubes" only shown.
[Oodles more bracing therein. ]
Tech note here: http://audioheritage.org/vbulletin/s...ead.php?t=6289
JBL Lit here: http://www.jblpro.com/pub/obsolete/4688.pdf