THE JBL WOOFS
This speaker system was made from a pair of 15-inch JBL (D140) bass guitar speakers. They are twin dedicated woofers, one cabinet each per side (stereo).
The cabinet was made with 3/8 and 5/8 inch particle board glued together (1") for the sides & back. The back also has 2x4 braces. The sides are also covered with 3/8" panel board on 3 sides. The inside walls are covered with fiberglass.
I tested the speakers with a signal generator and used those specs with theil/small parameters to calculate the port size. The original plan was to make the port size "changeable" by allowing different sized covers (with holes) to be bolted onto it... or bolting on a solid piece to make the cabinet fully sealed. The bolts for that are visible. But the cabinet sounded fine as is, so it's been used in this manner.
Each of these 2 cabinets (pair) have been used in bi-amped mode or tri-amped... and sometimes as the woofer section in a single amped system. They were sometimes set up in the rear, directly to the left & right of the listener. Their natural roll-off meant that they could be used without a lot of filtering, which generated a more natural sound (less phase anomalies). JBL's are very efficient speakers, due to their unique coil structure (flat wound), and among the finest drivers ever made.
The bass from these is very quick & natural. It's deep too (when it has to be)... and not the muddy, boomy, or resonant(hi-Q) nonsense so many commercial subwoofers have. I built these almost 40 years ago!