Radiator Subwoofer Systems
First, a few links you may enjoy:
A big sub amp driving big subs. Life with four of the trios was very nice indeed. Unfortunately, it was not to last, two of the subs went to a friend. This left two sealed 12"ers…. I missed the big dogs. I wound up receiving a pair of passive radiators that needed re-foaming.
And so I knew I had to make a pair of subs with the trios and
the passives. Being sensitive to underdamped alignments and the boomy character
they impart, I decided to tune these nice and low. 500 grams of added mass and
3.4 cubic feet was about right. This gave me about 6dB more at 20 Hz and 10dB
more at 14 Hz, both in sensitivity and in max output. Quite a boost!
With a passive radiator, where it is a mass of a solid object
instead of air, the density can be much higher, and we can tune it with mass
rather than length of vent. There are some other differences. In larger cabs,
where the vents don't need to be as long for a given tuning, vents can have
some advantages. One big advantage is cost. Passive radiators are significantly
more expensive than tubes of air. Some of the nonlinearities (read: distortion)
are different between the two types. But in a small cabinet, say 3.4 cubic feet
tuned to 14 with the Trio12, a vent would need to be 4 feet long if it were 4"
diameter- which is still dramatically smaller than the surface area of the 15"
passive. And 4" diameter vent is significantly limiting to a 12" as capable
as the Trio12. Below is an impedance sweep showing the tuning action.
In any case, I re-foamed the passives, unfortunately losing
the pictures in the process (it was a hoot, keeping it aligned was lots of fun)
and began considering how to get my preferred cabinet together. As it turned
out, the optimum volume for a build with these was about 3.5 cubic feet, so I
looked for things that could be repurposed to subwoofer cabinets. Being serious
subs, there were not many contenders, but I didn't want to spend a lot of
money on the project seeing as I'd come by the passives for shipping cost and
already had the amp and Trios. As it turned out, fortune favored me here, a
neighbor was getting rid of some very heavy duty dressers- MDF and 2.25" thick
on the top panel and no less than 0.75" on any other panel, with most being 1" or more. Heavy beasties, but
they'd serve nicely, and fit my space a lot
better than a pair of big sonotubes. I removed all extraneous hardware and
laminated some Baltic birch plywood for driver-mounting panels (normal cabinet
thickness is .75", but for a megasub I wanted 1.5"), then glued it in place.
Because I was starting with a shell from the dresser, I
retrofit strips of plywood into a bracing scheme. They were shimmed into place
to ensure that they were tightly fit, and crossed with other strips to keep the
bracing rigid. This was a slow, additive process with lots of clamps and glue
and shims, but the result was a very VERY sturdy enclosure. The back was
provided with anchoring points for a removable rear panel, as shown.
The great thing about the low-tuned enclosure is that the passive takes over where the driver is at its worst, limiting excursion needed from the Trio12. Vented systems “take over" where they're tuned, with the vent or passive radiator motion causing the output, and the driver doesn't have to move (much) to create SPL. This system is tuned extremely low, about 14 Hz. The Trio doesn't need to try to do the extreme stuff. A challenge in typical vented systems is that below tuning, the vents act as an acoustic short circuit- SPL drops off dramatically, and cone excursion uncontrolled. So the driver flops around without contributing significant output. Passives suffer from this less than vented systems since there's a barrier between the cabinet volume and the outside air. Because it's tuned so low, there is a very small bandwidth over which any unloading can take place. For reference, most subwoofer systems are tuned between 20 and 30 Hz and the higher they're tuned, the more unloaded bandwidth there can be.