The Cabinet & the Subwoofer

I’m in the interesting position of telling you how to do something with a subwoofer that you shouldn’t do. No, it’s not illegal. But in the ears of those for whom sound quality is more important than ergonomics, décor, and/or domestic tranquility, it’s definitely heretical.

So let me say at the outset that subwoofers are finicky beasts, and a change as small as a matter of inches in the placement of a sub in your room can make a difference between bass that sounds boomy (or wimpy), bass that sounds good, and bass that sounds great. If you have the room in your room—and the willingness—to put a sub (or, better, multiple subs) where the bass sounds best, then there’s absolutely no reason to do otherwise.

However, if you have other considerations that are at least as important as (or more important than) sound quality, you may have the need or desire to hide your subwoofer. Your first consideration should be to use a good in-wall sub, yet there’s a variety of reasons and situations that could rule out this solution. Before you fall into despair over the intractable “can’t live with them, can’t live without them” nature of subwoofers, though, there’s the somewhat contentious option of stashing that poor, unloved sub in the depths of your A/V cabinet. Yes, it’s difficult—if not downright impossible—to hide a sub in a piece of furniture and make it sound as phenomenal as it would if it were ideally positioned in your listening room. On the other hand, if hiding the sub in a cabinet is your least-worst option, there are things you can do to mitigate or even eliminate some of the drawbacks.

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Resonance Is Futile
A former custom-install designer and dealer, Steve Colburn is no stranger to the conflicts that arise between high-end audio performance and equally high-end home décor. Nowadays, he’s in charge of product development and training at Triad Speakers, a company that has a whopping amount of expertise in high-performance architectural speakers and subwoofers as well as ways to maintain that performance when concealing subs—or LCRs, for that matter—in furniture. When asked for his advice, Colburn told me, “There are three major factors that must be addressed for a successful cabinet subwoofer install: rattles, resonances, and airflow.”

When it comes to rattles and resonances, Colburn began, “Installing a powerful subwoofer into a cabinet turns that cabinet into a bucking bronco that must be tamed. Anything loose or not dampened or constrained will vibrate and rattle. So it doesn’t make sense, for example, to try to turn a cabinet that will house silverware into a subwoofer. First, look at a cabinet’s function and determine if it is compatible with housing a subwoofer at all.” (I guess the cabinet holding my extensive collection of wind chimes and gongs isn’t a good choice.)

Colburn suggested that when you evaluate a cabinet, take a close look at the physical construction. “Areas subject to rattles and resonances are thin and flimsy back panels (sometimes as thin as an eighth of an inch!), internal dividing panels, drawers, doors, and shelves.” Having a cabinet built from scratch, of course, is ideal because you can have the cabinetmaker use thick wood—5/8 to ¾ inch is good, but thicker is better—and choose cabinet hardware “with vibration control in mind. There is door and shelf hardware that captures snugly rather than using just spring tension or gravity,” Colburn noted. “The cabinet itself can be glued and screwed together.” Unfortunately, not all of us have the wads of cash needed for a custom cabinet and are forced to work with off-the-shelf (ahem), preconfigured furniture. So what the heck can be done about the rattles and resonances that are bound to lurk in less-than-stellar cabinets?

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I put that question to SnapAV’s vice president of content, Eric Harper. He suggested using adhesive felt or some other sort of soft damping material where doors and shelves touch the cabinet. (Triad’s Colburn agreed and additionally recommended using pins and sockets to solidly capture and hold the doors when closed.) Moving on to the rest of the cabinet, Harper pointed out that “wood panels and other nonmoving parts are hard to fix, if at all.” He proposed using a “dense sound barrier adhered to the back” of panels and doors because it “adds density to the lightweight wood to keep it from moving as much.”

Nick Colleran, at Acoustics First Corporation, concurred and emphasized that “there should be enough mass to the cabinet to prevent unwanted resonance.” As an example of what to use, he suggested the company’s Blockaid mass-loaded vinyl. It can be cut to size and adhered to surfaces inside the cabinet that need dampening.

514cab.submaterial250.jpgColburn suggested an inexpensive method of reducing vibration in shelves: Line them with “rubbery kitchen shelf liner material.” Then he went a step further. If possible, you should “shore up existing cabinets by screwing their flimsy back panels into the wall.” (That might not be such a good idea if you’re renting an apartment.)

Acoustics First’s Colleran told me that trying to incorporate a subwoofer into a furniture cabinet “presents a situation similar to soffit-mounting speakers in professional studios and broadcast facilities.” (The rules of acoustics evidently treat everyone equally, regardless of budget or application.) “The speakers,” he said, “should be decoupled from the structure with vibration mounts.” He recommended setting the sub (or speakers) on top of the company’s Vib-X Vibration Isolation Pads. “Vib-X pads are based on the materials used to quiet submarines (which otherwise could be made quiet by the enemy). If it works for those subs, it should work for your sub-woofer.” (Of course, now that he’s told me, he has to kill me.)

SnapAV’s Harper was of the same opinion (with the sub isolation tip, not the killing-me part). “The feet on most subs are pretty rigid,” he added, “so mechanical vibrations will transfer through them to the cabinet. If the feet unscrew, look to swap them for a more forgiving/spongy foot.” Fortunately, I didn’t have to look too hard for an example of a “forgiving/spongy foot.” I’d recently received a sample of SV Sound’s SoundPath Subwoofer Isolation System, which consists of four or six “optimized durometer elastomer” isolation feet (along with screws of varying thread sizes and lengths) that can be used to replace a sub’s stock feet.

Rather than retrofit a sub with new isolation feet, Auralex Acoustics takes a slightly different approach, as explained to me by the company’s technical marketing director, Robb Wenner. Auralex makes two patented subwoofer isolation platforms, the compact SubDude-II and the larger SubDude-HT, both of which consist of a velour-covered inert structural layer that “floats” on a cushion of Auralex’s Platfoam acoustic isolation material. Placing the subwoofer on top of either SubDude model decouples the sub from whatever the SubDude is sitting on, whether it’s a stage, the floor in your room, or the bottom of a cabinet.

Airflow: It’s a Blast!
Taming the rattles and resonances of a wood-and-particleboard bucking bronco isn’t all there is to putting a subwoofer in a piece of furniture. As Triad’s Colburn mentioned at the outset, there’s also the issue of airflow. Here’s how he described it: “If subs are to generate the large bass waves they are supposed to and not generate any additional nasty chuffing and puffing noises, their airflow must not be restricted. Ideally, whatever grille or opening [that] vents the subwoofer’s output into the room should have the same number of square inches of clear opening as the driver’s surface area.” In other words, don’t vent a 12-inch circular driver through a 4-inch square hole.

Cabinet style and construction will determine where it’s best to vent the sub into the room. With a cabinet on legs, for instance, it often works well to cut an appropriately sized hole (or multiple vent holes) into the bottom of the cabinet and use a down-firing sub. Cabinets with toe-kick panels require grilles or vents to be installed in the kick panels, and Colburn was careful to point out that the grilles need to be substantial enough that they won’t rattle and make the problem worse. You can probably get away with fudging the total vent opening down by 10 to 20 percent, according to Colburn. But if you can’t figure out a way to create enough vent space through the bottom, you’ll need to find a way to vent the sub through a larger opening in the cabinet where it won’t be visible.

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COMMENTS
Manbat's picture

VAF of Australia produce such a sub called the Platform:
http://www.vaf.com.au/picture-gallery.asp?dialog=true&file_id=34&folder_...

and
http://www.vaf.com.au/detail.asp?audio=Subwoofer&grunt=s200r26747

They can custom design the size of the unit. Cheers!

JayhawkLaw's picture

I'm no engineer, but am I'm I way off base in thinking that a lot of the problems associated with in-cabinet subs be at least alleviated by using a sealed sub like your recommended SVS SB-13 Ultra?

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