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Summer 2010

Creative Sound Trio12
Reworking surplus cabinets to create a new subwoofer.
Article By Jeff Poth

Difficulty Level

 

  As a dedicated audio scrounger, I diligently check hifi forae, craigslist, surplus sites like deepsurplus.com, apexjr.com, sales pages at madisound.com and parts-express.comand garage sales; looking for anything hi-fi that tickles my fancy. Much to the pain of my poor wallet, I am successful reasonably often. In this case the pain is being borne by my garage space. This is the story of a find of subwoofer cabinets and what happened to them. Many other people are repurposing cabinets for various applications, one popular one is a relatively inexpensive two-way project called "Econowave".

Browsing at the sales section of diyaudio.com I came across a mention of freebie Miller & Kreisel (M&K) cabinets at deep surplus deepsurplus.com. There were 100 pallets of mostly subwoofers, but also multi-driver surround and center channels. I work 20 minutes north of there, and made a number of trips with my trusty 1995 Subaru, filling half my garage with cabinets. These were mostly faux Cherry (vinyl veneer) subwoofers mostly, but a smattering of the smaller offerings, and a number of their larger, dual driver push-pull cabinets. Some were picked up on behalf of other DIY'ers, some will be worked into subs for friends at cost, and others will find other service.

When using a cabinet you must first determine if it is a good match for your desired application. You'll want to model bass response in any of the available box programs (midrange and treble applications are quite a bit more involved). I usually use WinISD to model bass response for sealed or vented bass enclosures. You'll need to know cabinet volume, cutouts, and other applicability concerns. For sealed subs like these it's pretty much just the cutout and cabinet size. Mine were mated to the Creative Sound Trio 12 creativesound.ca, which is appropriate to the box volume (sealed). I measured the cutouts and much to my chagrin, they were 12 inches overall in diameter with an 11 inch through hole. Too small for the vast majority of 12 inch woofers; which are usually more like 12.25 inches and have some variance on the through hole. What to do? The first thought was a router jig. This would have required the use of some centering mechanism, and would have needed to be very precise indeed. I went to Plan B.

As you can see, a drum sander powered by my trusty drill made short work of expanding the holes. One must take care not to create divots, and to keep moving at a steady pace to keep the hole true. It took about 20 passes at medium/high speed, rotating clockwise around the recess while the bit was spinning clockwise (forward cycle). I also used a screwdriver to buy myself a little more clearance at the bottom of the hole, as the drum sander leaves a little bit of a lip. For some drivers, including the Trio 12, the inner hole needed no expansion, for others, I used the same sander to make what clearance was needed. I also took the edge off the inside lip of the through-hole, so as to make a small amount more airflow clearance out of the back of the frame. I did only a small amount here, as the baffle wasn't terribly thick (1") and the value of this operation in subwoofers is not very large. The exception is with very thick baffles. Ideally, you want a minimum of obstruction of airflow coming off the back of any driver (or the front, for that matter).

Once I had opened the hole up, it was time to clean up the work. The baffle cutouts had been coated black by M&K, as a skinny halo of MDF around your driver frame tends to be rather unattractive. To replicate this without making a mess on the vinyl veneer, I used a black chisel-tip sharpie. Veneer colored stain is also available in applicator pens, which is the better solution if the hole is somewhat out of round after a rough iteration of "Plan B". Keep some paper towels and rubbing alcohol handy for slips, and if you're working with real veneer then by all means MASK IT. Blue painters tape is the order of the day. You'll also want to seal most edges like this with some wood glue. Normal yellow wood glue (Titebond II is my favorite) and a couple damp paper towels, and you should be able to run a little bit around the edge (I use my finger) to seal it. This keeps the MDF (typical) or ply from absorbing moisture at the cut edge and toughens the material somewhat. You'll want to be diligent about wiping any off the cabinet face, and mask it when in doubt.

Once the wood glue dries (give it about an hour), you then want to gasket the joint to the driver. Soft weather stripping works well here. You can use however much, in whatever thickness, you like, but I like to go relatively light with it. Some people go to great lengths to ensure no air escapes their cabs, but so long as any gaps are small enough not to whistle, you're usually okay. You don't want a 100% airtight box anyway, as temperature changes will induce a static pressure or vacuum within the box unless there's an equalization path. In this case I gasketed at the edge of the cutout, but you can apply to the driver frame if you prefer. The Trio 12 doesn't even really need a gasket, so long as you have a nice level mounting surface- the rubber trim ring can act as either a front or a rear gasket.

These particular cabinets came pre-mounted with a quad of "T-Nuts", which are designed to create a strong mounting point. This is particularly useful with MDF which is a soft material, and will not hold up to repeated screw insertion and removal. With ply it's less critical to use a fastener enhancement, but still recommended. There are several options. One can use "T-Nuts", "Hurricane Nuts", and "Screw Inserts". Different people have their favorites, but all work nicely with some care. It's highly recommended to line these, just like other cut edges, with glue, and mount the fasteners while it's still wet. You'll want to make sure you keep the threads clean so you don't have a fastener loaded with dried glue. You can also use staples or other methods to make sure they stay tight. These types of fasteners can be a challenge, so go slow, triple-think, and triple-check. There's nothing worse than a driver that's stuck in its mounting because you had a fastener come loose inside the cab. Another trick is to pre-drill the hole, put in some superglue, and immediately screw in then remove the size of screw you plan to use (Thanks to Dr. Earl Geddes for this trick). You must work quickly with this method, but it's fairly effective, easy, and cheap. The superglue needs a little while to cure before use. This method plasticizes the wood, effectively making it into a socket.

Once this is determined, it's a simple matter of connecting the wire to the driver, then screwing into place. If you're using a plate amplifier, you'll need to make cutouts and install the plate amplifier; otherwise, you may need to install binding posts. Shown is a machined back panel. Forstner bits are used to lower the overall level of the material to allow the mounting of these binding posts.

After mounting the back panel and securing the speaker cable to the braces with zip ties (you don't want it rattling around, there are many acceptable ways to affix it), I stuffed the back section of the cab and added a little batting around the braces to absorb standing waves. Not much of an issue at subwoofer frequencies but darn it, I did it anyway!

These cabs had grills available, though mismatched to the grille pegs. I hacked them off, applied self-adhesive fasteners (Velcro for some, the interlocking peg style for others), and viola!

The driver, in-box, performs as modeled, and is very very VERY capable with a huge amount of output. The unit-to-unit consistency is superb as well. Below is an impedance sweep from the Dayton WT3.

 

More on the Trio 12
The trio12 was not only chosen because of the box size. This model has the XBL^2 motor, which is a split gap magnetic topology that allows for a significant increase in linear excursion. This means the driver is able to move further in and out (excursion) while remaining linear (low distortion). The driver is rated for Xmax of 20mm which means that it can have a stroke of over an inch while remaining quite clean. The XBL^2 motor arrangement has been around for some years and was developed by Dan Wiggins and Adire Audio, which is now defunct. I've previously mentioned it all at this link.

Additionally, this driver has shorting rings, a well-designed steel frame with ventilation under the spider, a large pole piece vent, and excellent build quality. One noteworthy piece was that there were 2 sets of tinsel leads. This is something I've never seen before in a single voice coil driver. These are woven into the spider for durability, and by using a second (unconnected) set, rocking modes from the asymmetrical influence of the leads is reduced (lower distortion). Very cool stuff. I've been around the block a few times and these were some of the loveliest drivers I've seen.

You may remember their big brother, the Subduction 15" woofer from my VSAC 2008 coverage.

The Trio 12 has a slight decrease in materials throughout, changing the cast aluminum frames and copper shorting rings of the Subduction series for stamped steel and brass, and the cone is changed from a honeycomb assembly to thick coated paper. These are very good design decisions for driving the cost down; the Trio is definitely well-optimized to be a bang-for-the-buck leader (and then some). The quality of the materials used remains extremely high.

 

That's All I've Got
I hope this is useful to some of the scroungers like me. A little effort goes a long way, and you can get a project together without doing a full-on woodworking project. The quad of these sub boxes I'm using now rattles the room well before approaching their limits. They could easily double their output in a larger room with more amplification behind them, but it's just not necessary for me. The Trio12 is a subwoofer driver to be envied, and is as good as or better than the most expensive drivers of yesteryear, using modern production and technology to maximize performance. It just happens to be very inexpensive as well at $139 (inexpensive for a high-excursion full-featured 12" subwoofer that is). Stay tuned for a more advanced subwoofer project, utilizing system design to maximize clean output.

 

 

 

 

 

Gryphon Audio

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     
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