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Background
I’m contemplating a built-in DIY subwoofer in our living room. I’ve been using couple of “throw-down” subs for the past several years, a couple of 12” Shiva drivers dropped into some old ’70s tower speakers I had taking up space in the closet.
Even though the boxes are pretty big, they’re only about 2.0 cu ft. each, which is on the small side for the Shivas. Response peaks out at about 40 Hz, and some serious EQ boost is required to get them down to 25 Hz. They’re powered by an Adcom GFA-555II amp, which delivers 325 watts per channel into the 4-ohm load.
Here are both unequalized and equalized graphs with the subs in their current location.
Baseline Response
Equalized Response
Believe it or not, even with the modest power and huge EQ, these subs have been adequate for the 9200 cu. ft. family room in the house we moved into a few years ago (that includes all other areas of the house open to the room). At the maximum volume I listen to, the Adcom’s clip lights will flicker at about the time the drivers reach their limits in the depth charge scene of U-571.
Here are the subs in their present location. Normally they are hidden behind a chair.
The Proposal
Basically, I want to get these things off our living room floor. This family room is smaller than our last one, not to mention much more “classy,” so these big boxes on the floor behind the chair are just out of place.
I had my eye on a built-in cabinet/book case (right behind where the subs are sitting) for a “stealth” sub installation soon after we moved in, but figured it would be too problematic and dismissed it as an option. Here’s a picture of the built-in with the subs moved out of the way.
I’ve also toyed with the idea of an IB, as there is attic space available above the built-in (where you see the AC vent in the picture). However, it would cost a minimum of several hundred to a thousand dollars, money we just don’t have right now. (Okay, I do have the money, but I’m lusting after a fretless bass at the moment, and some new equalizers for the HT system. It’s my perennial problem, too few dollars chasing too many hobbies and interests! :laugh: ) Not to mention, the work may involve moving the AC duct and other stuff I’m not sure I want to deal with.
That brings me back to the built-in. I originally dismissed it for a number of reasons. Primarily, I feared vibration would be horrendous, both from the cabinet itself and everything on the shelving above it. Then there was the problem of outfitting two 12” speakers in a long, “low boy” um, “enclosure.” Mounting the drivers side by side, the only apparent viable option (each door opening is 17"x17"), would create a time alignment issue at the listening position, not to mention the prospect of drilling holes everywhere to properly secure a baffle board over or behind the cabinet openings was unappealing. Properly sealing off the other door openings, while leaving the doors themselves intact (for obvious aesthetic reasons) looked daunting as well. On top of all this, there is a gas jet valve for the fireplace inside the cabinet.
I think I’ve come up with solutions for most of these issues. For the first three, I think I can address them all in one fell swoop by utilizing an IB-type manifold, mounting the two drivers opposing each other. That should take care of the vibration issue, and fit nicely into the hole where the last door (nearest the corner) would be removed. Unlike a baffle board, it shouldn’t be hard to secure a manifold inside the cabinet without drilling holes in the front of it. I though I would try to utilize the removed door’s original hinges for a fold-out grille frame. Naturally, this will all be hidden from view by the chair that normally sits catty-cornered in front of the built-in.
About the only thing I haven’t figured out yet is what to do about the gas jet. Anyone have any ideas? It’s a square shaft that a handle fits onto. If not, there’s a fireplace store in the neighborhood; I might drop by and see if they have some kind of extension.
Potential performance
Okay now for the good stuff, crunching the numbers. The built-in cabinet has 17 cu. ft. internal volume. Giving up maybe 1.5 cu. ft. for the manifold, we have about 15.5 cu. ft. total to work with, which means each speaker will see a little less than 8 cubic feet (or 225 liters). That’s nearly 2 cu. ft. (or 48 liters) more than the largest size Adire recommended for the Shiva in their white paper.
I’m no subwoofer-building expert, but I figure I’ll have no problem hitting 20 Hz or lower with this thing with no help from the equalizer. That should mean I'll be able to get output comparable to the small-box set-up without maxing out the amp and drivers like I am now – right? (Based on the fact that EQ boost will no longer be needed to get the added extension.)
Yes, I know it would probably be better for each driver to have its own enclosure, but that’s just not practical here. Or even possible…
So – whatd’ya think? I’m open to any suggestions or tips on possible “gotchas.”
Keep in mind that this is a low-budget proposition, using the drivers and amps I already have on hand. I also have most of the lumber on hand, primarily a good-sized sheet of 3/4” furniture-grade plywood that I’ll build the manifold out of. A primary consideration is that I have to be able to restore the cabinet back the way it was in the event that we ever move again. So any permanent, visible damage to the front of the cabinet is out of the question.
Aside from better extension, what I can I expect from this set up, compared to what I’ve had before with the smallish boxes? What effect will the larger enclosures have on detail, dynamics, etc.? My expectations are pretty low-key; if it performs at least as good as the “throw-down” subs, I’ll be happy.
Regards,
Wayne
