The Cabinet & the Subwoofer
SoundandVision.com | By Darryl Wilkinson | May 30, 2014
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.
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?
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.
Colburn 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.
Sean Hotchkiss, technical director at Grand Home Automation (in Hudsonville, Michigan), told me that although the company prefers to install architectural subwoofers in their clients’ walls whenever possible, they regularly hide subs in cabinets, too. Rather than attempt to isolate an in-room subwoofer, they often mount in-wall subs into the cabinet’s structure. One way to do this, he mentioned, is to use Triad’s InCabinet Sub Adapter. According to Triad, the InCabinet Sub Adapter is a special bracket (for use with the company’s InWall Series subs) that’s designed to fit “between the subwoofer and the cabinet base and is primarily used when down-firing the subwoofer through the cabinet. The MDF adapter mates with the speaker box and provides needed isolation to prevent buzzing and unwanted noise.” If down-firing the sub through the cabinet’s bottom isn’t an option, Hotchkiss explained that you can mount an in-wall sub into a side of the cabinet in much the same way you’d install the sub in a wall.
Near the end of our conversation, Hotchkiss stressed that airflow is important for another reason—one that people often forget or don’t realize. Powered subwoofers, he told me, like any other component in your system, generate heat. Heat, of course, can change the nature of how a component performs. Perhaps more important, it can also dramatically reduce its life span. So you have to take care to provide enough airflow around the integrated electronics to keep the heat from building up inside the cabinet.
Location, Location, Location
Dangerously armed with a little knowledge and a lot of overconfidence, I was ready to fire up the chain saw, strap a caulking gun to my belt, and basically re-engineer any nearby piece of furniture in my house. Fortunately, the kitchen cabinets and the piano were safe because BDI had shipped two cabinets my way for the express purpose of sneaking subwoofers inside. BDI’s Novia 8426 cabinet, as a matter of fact, was specifically designed to house a sub in a large hidden cavity located on the left side behind a shallow media storage area. The wide, quad-door Corridor 8179 wasn’t specifically designed to conceal a subwoofer, but we chose it because the internal cabinet sections are large enough to house a relatively hefty sub. By chance, I had two subs available as well: a $350 Atlantic Technology SB-900 (8-inch front-firing driver, ported, 125-watt internal amplifier) and a $999 GoldenEar ForceField 5 (12-inch front-firing active driver plus a 12.75 x 14.5-inch down-firing passive radiator, 1,500-watt switching amplifier).
As I mentioned, a subwoofer’s placement in the room has an enormous effect on its performance. Unfortunately, whereas you can work on rattles, resonances, and airflow in a cabinet, there’s usually little you can do about its location. Curious as to how much difference it would make, I listened to each of the subs in three locations: 1) at each sub’s individual optimum placement in the room; 2) inside each cabinet; and, in order to determine how much of an effect the cabinet itself had on the sub’s performance, 3) sitting next to each cabinet. I primarily used music tracks with hard-hitting bass and lots of dynamics, such as “Lemme Try Your Bass (Interlude)” from SMV’s Thunder, the Jennifer Warnes classic “Way Down Deep” (The Hunter), and selections from Pink Floyd’s The Dark Side of the Moon, among many others. In addition to music, I fed a sweep tone through the subs to pinpoint and isolate the rattle and resonance locations and attempt to fix them.
It might surprise you to find out that the hidden-in-the-cabinet location wasn’t always the worst performing. In some cases, the subwoofer sounded better inside the furniture than sitting next to it—although I could never get an in-cabinet sub to sound as good as the same sub in its optimum room placement. One example came with the Atlantic Technology SB-900 sub in the BDI Corridor 8179 cabinet. The SB-900 was compact enough to test in several different orientations, including firing forward through the slotted door, firing backward toward the wall (with the cabinet’s back panel removed), and firing down through the large vent holes in the bottom of the cabinet with the sub propped up on squares of Acoustics First’s Vib-X Vibration Isolation Pads.
Firing back toward the wall was unbearable because, in addition to sounding boomy, the sub rattled the crap out of the wall. Situating the SB-900 so that it fired forward into the room produced reasonably respectable bass, certainly nowhere near as boomy as what the backward orientation generated. After I listened with the cabinet door open and then closed, I noticed that the door itself was restricting a small amount of the sub’s energy and dulling its character ever so slightly. Interestingly, the best bass performance came with the SB-900 firing down through the pre-cut holes in the bottom of the cabinet. (Since the cabinet sits on legs that raise it about 4 inches off the floor, I didn’t have to worry about venting toe kicks.) With the sub in this position, the bass output became much tighter and a bit deeper. It wasn’t the absolute best that the SB-900 could sound, but it was more than acceptable—much better than the sloppy, tubby bass from the sub sitting next to the cabinet, and for sure a hell of a lot better than it would sound in other locations that people might be tempted to use.
My experience with the GoldenEar ForceField 5—one of the most impressive under-$1,000 subwoofers I’ve heard recently—was a bit different. The 18-inch depth of the sub prevented me from upending it and firing the active 12-inch driver down through the vent holes of the BDI Corridor 8179 cabinet. But I did test it facing into the room as well as facing toward the wall, with the passive radiator facing down in both instances. Once again, the wall-facing position battered the back wall mercilessly. The ForceField 5 performed better firing forward, but it lacked the dynamics and near-bowel-movement-inducing deep-bass output this sub creates in its ideal room placement. Placing the sub in the cabinet raised the radiator a good 6 inches from the floor boundary and changed its character significantly. Placing the sub next to the cabinet brought back some of the depth, but the overall sound was weak and failed to energize the room.
That’s not what happened with the BDI Novia 8426, however. Since that cabinet is designed to cover the subwoofer, I was able to place the GoldenEar ForceField 5 directly on the floor, with the active driver firing through the slots in the side of the cabinet. The bass regained some of its depth and fullness, but it still couldn’t rival what I knew the sub was capable of in the right spot. Atlantic’s SB-900 sounded pretty much the same inside the Novia 8426 or next to it on the outside; yet, as with the ForceField 5, the SB-900 still suffered from being “out of place.” One advantage of the Novia 8426’s design, however, is that I was able to set the SB-900 on top of one of Auralex Acoustics’ original SubDude models. The interesting combination of raising the SB-900 up off the floor approximately 2 inches along with having the acoustic isolation provided by the SubDude served to noticeably improve the nature of the sub’s sound. The SB-900 still sounded a bit tubby, but it gained control and texture.
Meeting Bass to Bass
What is there to conclude from all this moving and shaking of subwoofers and cabinets? The first rule of thumb for hiding a sub in furniture is that there is no first rule of thumb. There’s really no way to determine ahead of time the subtler aspects of how good a particular sub will sound in a certain cabinet at a set location in a given room. On the other hand, if you’re OK with sacrificing a little performance in return for making a sub disappear, and you’re willing to spend a little time experimenting plus a little cash chasing rattles and dampening resonances, it’s most definitely possible to get respectable performance out of a sub hidden in a piece of furniture. And if you choose the furniture wisely (see “Construction So Good, It Hertz”), you’ll minimize the effort required to adapt it for this role. As the Rolling Stones might have said it, “You can’t always get the bass you want. But if you try sometimes, you just might find, you get the bass that you need.”
I approached BDI at the start of this project for two reasons. First, the company makes an extensive line of well-built A/V furniture that’s perfectly suited for this sort of task. Second, BDI makes a cabinet that’s purposely engineered to hide a subwoofer and soundbar. I could have picked up some flimsy piece of crap at the local discount store to deal with, and perhaps had more of a playing field for torture-testing some of the vibration isolation devices and padding materials mentioned in this story. As it turns out, when it comes to installing subs and speakers in A/V furniture, BDI’s cabinets were awesome—too awesome, if you like to muck around with dampening materials.
It’s worth explaining why to help you assess any cabinet you’re planning to acquire for this purpose. First, BDI’s credenza-style Corridor 8179 ($2,149) and subwoofer-hiding Novia 8426 ($1,749) aren’t ready-to-assemble pieces of furniture that ship flat. Both BDI cabinets ship built. (You do have to install the shelves, glass tops, and a few other items—but nothing major.) It’s an event when the delivery truck shows up with a 79.25 x 28.25 x 20.25-inch cabinet that’s contained in an even larger box, with an overall package weight of 237 pounds. That’s impressive by itself, but it’s not all that makes these cabinets special.
For example, both cabinets use shelf pins that screw into metal grommets inserted in the side panels. While that keeps the pins from rattling, each pin has two rubber O-rings on the half of the pin that fits snugly into cutouts on the bottom of the cabinet’s half-inch-thick shelves. Disappointingly, where I was looking forward to using plenty of sound-deadening material and acoustic caulk, the task of shelf resonance control had already been taken care of, and in a much simpler manner. Unlike a lot of cabinets that have the back panel either stapled or screwed on, both the Corridor 8179 and the Novia 8426 have a 0.19-inch-thick back panel that rests in grooves in the top and bottom of the cabinet. Rather than fuss with rattle control in the section where I put the subwoofer, it was a simple matter to lift the back panel out of the grooves and remove it altogether. This also ensured there’d be plenty of airflow for cooling purposes. To add insult to injury, large ventilation holes were already cut into the bottom panels of the leg-supported Corridor 8179. (So much for using the chain saw.)
It wasn’t a fault of either cabinet, but one thing that gave me a good deal of trouble—until I discovered the cause—was the fact that when cables and cords vibrate against the cabinet’s sides, back, or shelves, or even against the sub itself, they can induce rattles that sound as if they’re coming from somewhere else inside the cabinet. Lightweight components, such as low-end Blu-ray players, also like to sympathetically vibrate when the right frequency is hit. Since this is an airborne phenomenon rather than a vibration transferred directly through a shelf, you’ll need to use some sort of dampening material directly on the component’s chassis—or move the component somewhere else, if possible.—DW