I feel bad for audio and video engineers. Usually, when consumers buy luxury goods, they can't wait to show them off. But when they buy audio/video gear, they want to hide it. While watches have grown bigger and cars have become flashier, A/V gear has been shoved into closets and built into walls and ceilings. Squeezing a subwoofer into a wall has proven a challenge, though. Most subwoofers use large drivers with magnets that are too massive to fit in the 4-inch-deep cavity of a standard wall.

The latest solution to this problem emerged from the mind of Laurie Fincham, chief scientist at THX. It's called THX Balanced Bass-line technology. Fincham tossed the big woofers and substituted a bunch of 4-inchers, each in its own enclosure. He turned the little enclosures sideways so they could fit inside a wall. And he arrayed them in two banks of six, facing each other to cancel out vibration. Fincham took the idea to loudspeaker manufacturer BG Radia, who turned it into an actual product: the BGX-4850 in-wall subwoofer system. I say "subwoofer system" because the BGX-4850 comprises four subwoofer modules, each incorporating a dozen 4-inch woofers. A 2,200-watt digital amplifier powers the system.

Although 4-inch drivers might seem inadequate to deliver powerful bass, combining 48 of them gives you about 20% more radiating area than two 18-inch woofers. The smaller drivers can move (and stop) faster than big drivers, so they have the potential to deliver tighter, better-defined bass. However, each of the 4-inch drivers can move only about ? inch forward or back, while a typical 18-inch woofer might more three or four times that far. Thus, from a purely acoustical standpoint, the BG Radia doesn't produce as much bass as a pair of 18s. BG makes up for this using an equalization circuit built into the amplifier.

Each subwoofer module comes with two paintable grilles: one 7 inches wide, the other 15 inches wide. Your installer can cut a hole in the drywall, install the subwoofer module, and cover it with the 15-inch grille. For a more discreet installation using the narrower 7-inch grille, new drywall can be laid over the installed subwoofer, leaving only the slot into which the driver's vent is exposed.

SETUP

I was reluctant to tear holes in my walls to accommodate the BGX-4850's four sub modules. Fortunately, because each of the modules is self-contained, I didn't have to. The woofers don't vent into the wall, as most in-wall speakers do, so they perform essentially the same if they're placed against a wall as they do inside it. (In fact, BG will soon be offering on-wall and freestanding versions of its subwoofer system.)

The BGA-2104 amplifier uses Neutrik Speak-On connectors, a type common to professional audio products. BG includes a pair of Speak-On plugs that your installer can connect to your speaker cables, two cables per plug. The amp has two RCA inputs and one XLR jack; just connect one of these to your receiver or surround sound processor.

The amp has an alphanumeric display on the front. By punching and turning the volume knob, you can adjust things like crossover frequency, phase, and, of course, volume. It doesn't let you adjust the volume of each sub separately, however, and it doesn't let you equalize them individually - any equalizer you connect will affect the system globally.

Research shows that four subwoofers deliver more consistent bass response throughout a room than a single subwoofer can. However, getting four subs to work in concert can be complicated. In my home theater, I have a bass "sweet spot" - a subwoofer position that I've found delivers nearly flat bass response when I'm sitting in the middle of my couch. With four subs, though, that didn't work. I ended up spending a great deal of time experimenting with different placements of the woofer modules, using a measurement microphone and a laptop running TrueRTA real-time spectrum-analysis software to evaluate each positioning scheme.

In my room, placing all four subs up front, near the TV, gave me the best performance. In this position, the bass measured almost dead flat in every seat on my couch. It was also the only position I found in which I got truly powerful deep bass from the system - with every other positioning scheme I tried, I couldn't get the low-end grunt I expect from movies like King Kong. If I were installing BGX-4850s, I'd mount the four subs where I'd be likely to get the greatest deep bass output - in the four corners of the room, say - and then use a subwoofer equalizer (such as the Audyssey MultEQ feature, built into many receivers) to smooth out any bumps in the frequency response. This would probably be the most practical way to get powerful, consistently smooth bass from this system.