Bandpass enclosures were the magic box from the late '80s through the '90s, but at the turn of the century they became all but obsolete. They were a finicky bunch and had to be designed with great precision. This required a good dose of mathematics and an understanding of how bandpass enclosures respond acoustically to a vehicle environment. When done correctly, the output gain of the sub could be substantial, often offering three to five decibels of gain over the same woofer in a sealed enclosure, and sound quality that was fairly uncompromising. In retrospect, bandpass enclosures were often significantly larger than a sealed box, and therefore less desirable. But the single largest contributing factor to their abandonment came with technological advancements in speaker design that allowed newer subs to work in ever-smaller enclosures with greater efficiency.
This month we get a chance to look at Swiss Audio's SBP212 bandpass enclosure - a somewhat retro design built around two 12" subwoofers. It's a nice-looking piece that reminds me of units I built in my install days. Like most, though, it's sizeable, measuring 28 11/44" wide, with a height and maximum depth of 14 31/44" and 16 11/44" respectively.
The SBP212 enclosure is pretty typical as bandpasses go. Its single reflex design utilizes three chambers amid the two subs. The outer two chambers are ported, each holstering a pair of 3" polished aluminum "Venturi" ports on the front with large radius, noise-reducing flanges that terminate their ends. The chamber in the center of the enclosure is sealed and common between the two subwoofers. A clear acrylic panel bolted to the front provides a good look at this chamber and the face of the two subs inside - a cool thing to show off to your friends. B-grade, 15mm (0.6") MDF frames the enclosure, while a dense, black automotive carpet dresses up the exterior and sets-off the aluminum ports.
The subs used in the SBP212 don't look like anything from Swiss Audio's regular lineup, and could likely have been engineered specifically for this enclosure. Peering through the unit's acrylic window, you can see that these drivers use poly-mica cones, big, fat foam surrounds, and dust caps embossed with the Swiss Audio logo. What you don't see from this angle is their generic stamped steel frames and moderate-size motors - the latter consisting of a single 18mm (31/44") magnet and a vented pole piece to increase thermal/power capacity.
Power enters the rear of the enclosure through two terminal cups that accept up to 10-gauge wire. Each of these subs has a 4-ohm impedance rating, so the SBP212 will be best used with an amplifier that makes power at a 2-ohm mono load. Because they share the same sealed chamber, you won't want to wire these subs on separate channels of an amp. The slightest power deviation between the two could reap havoc on the subs, damaging, even shattering the cones from the pressures inside the bandpass.
With 400 watts of power handling, the SBP212 system should yield good output. Let's hope the designers at Swiss were able to balance a little sound quality into the mix, too.