March 2012

Passive Radiator Subwoofer Systems
Article By Jeff Poth

Difficulty Level

  First, a few links you may enjoy:

Dayton SA1000 Subwoofer Amplifier A truly fine subwoofer amplifier.

Creative Sound Trio12 Reworking surplus cabinets to create a new subwoofer.

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!

Passive Radiators
Passive radiators are a version of a tuned, or resonant, enclosure. Passive radiator systems and vented (bass reflex) systems both use the behavior of masses of air, the enclosed air in the box acting like a spring joined to a mass (the passive, or the air in the vent tubes). In the case of the passive radiator it's quite easy to picture, since we see the spring-like suspension components in the spider and surround, and can push it and set it in visible motion, like any other resonant mechanical system. With air vents the action is similar but typically distributed over a smaller surface area of air. Vented systems are perfectly fine when used properly, but they're very impractical in today's high Xmax, low Vas subwoofer drivers. The reason is that the spring interacting with the mass of a vent is a low volume of air and thus less compliant, shifting tuning upwards for a given port length, vs. a larger box. Said another way, a 3" diameter 5" long vent might tune to one frequency in a two cubic foot box, but that frequency would be higher in a 1 cubic foot box. This is why resonant systems must be “tuned", to make sure the vent is contributing the right amounts of the right frequencies. Here is a nice link to help explain.

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.

Sound
While there's a lot of art in higher frequencies, subwoofers are more about brute force. How loud, how low, how much power. As such, I try not to wax too poetic about the sound. They go louder, lower, and more dynamic than previous subs I've had, including the quad of Trios in the smaller sealed cabinets. Pretty simple eh? There is however a limit to what extra output can provide. Most subwoofer responses are limited by the modal behavior of a room, and there are several solutions to this. I'm fortunate in that my room is joined to most of the rest of the house, so there are irregular semi-walls and whatnot distributing the dimensions of the space over a much broader range than your typical rectangular listening room, but in my space there is still some lingering modality. Many subs attempt to correct this with EQ, but there is a limit to what can be accomplished this way. The peaks and dips are very high amplitude, which brings excursion and power handling outputs into play if one were to try to EQ them. The alternative is a much more compelling solution- use more subs! My two location system combined with my room's lack of symmetry works pretty well, but there's more smoothness in bass response to be had from more sub locations -- a system promoted by Geddes (remember my waveguides? His design) and others. That's it for next time!