Adding a second sub can give you more even bass over a larger area. THX recommends placing the subs across from each other, in the middle of each wall. Using four subs can give you more even bass over a much larger area than one or two subs can. It's best to place a sub in the middle of each of the four walls. But if a door or other obstruction prevents this, you can place them in the corners.

Maybe you've heard of somebody adding a second subwooer to his home theater for whatever reason. You might have even heard of somebody with these subs. But FOUR?

Before I get to the technical arguments for four subs, consider the practical side. With prices continuing to drop, manufacturing quality on the rise, and high-performance subs getting ever smaller, you can now get multiple subwoofers for what you used to pay for one first-rate model. So why would you want to add more subs? The answer lies in the way we now use our audio systems.

As we've become addicted to giant HDTV screens and surround sound for films and music, our houses have become the movie palaces of old. The days of listening to monolithic stereo speakers from a single perfect position are gone. As a good host, we now have to be concerned about the sound quality at a number of seating locations for family and friends who drop by to see the latest flick. Multiple subwoofers, properly placed, can help address the sound-quality issue by producing uniform deep bass across a large listening area.

In my house, we have a traditional Sunday-night gathering in our theater. (Being in the A/V industry has its benefits, since I have to test the latest equipment somewhere.) When I added a second sub to the theater, my family was very impressed and remarked on the great sound. I neglected to tell them that they were the only beneficiaries of the upgrade, since the sound had always been that good at my seat.

About a month after that, I was at my friend's house, and he couldn't wait to show me his latest addition: a second subwoofer! He placed the shiny new $500 compact sub right next to the original one (a $99 special) and proceeded to crank them both up and smile. As you can imagine, it sounded like the latest incarnation from the local car-audio emporium. After my hearing came back, I unplugged the old sub and repositioned the new one. Unfortunately, I wasn't able to set him up with dual subwoofers, but the new unit sounded remarkably better.

In a typical home theater, many factors work against perfect sound, starting with the room's acoustics. Obviously, my friend missed a few items, like subwoofer placement, seating locations, calibration, matching equipment (two mismatched subs tend to sound worse than the lesser of the two), and an external equalizer. I'll start by addressing placement and work into the rest of the issues.

Room modes — uneven frequency response caused by the combining and canceling of reflected sound waves — have always been a factor in the placement of both subs and seating. Modes create nulls at certain points where a frequency might not be heard at all and other areas where the sound waves combine to create peaks that increase the volume of some frequencies.

Let's make the problem real by using your home theater. Place your sub tightly in a corner of the room. Now, turn on your receiver's test signal for the subwoofer output. (You could also use a test disk with a continuous subwoofer tone or loud music with repeated low bass beats.) Turn up the volume to where it's almost too loud. Now walk into the room's center (that is, halfway between the front and back as well as an equal distance from either side). As you approach this spot, you might hear the volume decrease. The center is typically a null point for some frequencies and a peak for others. (Depending on the shape of your room, the quietest spot might not be at the exact center.)

Now walk up to any wall and put your head near it. The sound will be louder, since all the room modes hit their peaks near the wall. Next, go back to the center to give yourself a reference at the quieter location, and then walk throughout the room. The perceived volume of the sub should change as you go from place to place. These modes are just one of the problems with acoustics in small rooms.

Some great research has recently been published on how subwoofers interact with small-room acoustics. Harman International helped to lead this charge with Todd Welti's Subwoofers: Optimum Number and Locations (available at harman.com/about_harman/ technology_leadership.aspx). Based on the research of well-known speaker researcher Dr. Floyd Toole, this white paper has been largely responsible for the increased interest in multiple subs.

The paper assumes you have a typical rectangular room and are trying to locate one, two, and finally, four subwoofers. (Avoid using a square room. Modes have a big effect on bass distribution in spaces of less than 30 x 30 feet, and that problem is only exacerbated if your home theater is square.)

To accurately position a single sub, you have to eliminate any self-respect you might have and crawl around on the floor. Move your chair, couch, or beanbag, and place your sub in the key listening position. Then go around on your hands and knees, listening for the location with the best sound. That doesn't necessarily mean the loudest spot — you're looking for clear and even bass. (Use a DVD or CD that really gives your low-frequency driver a workout.)

There's no hard-and-fast rule for subwoofer placement, so it's best to start in one of the room's corners and then move in and out and several feet up and down the wall from where you're thinking of putting the sub. (For more setup tips, see "Subwoofer Setup Secrets.") As you look for the optimal spot, you'll find some locations where the bass is boomy and too loud, and others where it almost disappears. Once you've found the best location, move the sub there and return to your listening position.

Making slight changes to your listening spot can also improve the sound. Avoid sitting in the exact center of the room or right against a wall. If your couch is near a wall, bring it out a little. And if your chair is in the center of the room, move it back or forward or a bit to the side.

If you're concerned about sound quality at several listening positions as opposed to just one, you should definitely consider using multiple subwoofers. Adding a second sub helps to even out the room response over a larger area than just a single seat. The most common method of adding a second sub is to place it opposite your first one (that is, across from the first sub on the opposite wall).

But this traditional formula has recently been modified based on Dr. Toole's research. He recommends placing the subs in the middle of the front and back walls. (THX recommends placing them in the middle of the left and right walls.) Toole's method requires some equalization to flatten the bass response so all the seats in the primary listening area hear solid, even bass. My own experiences have shown that you can get excellent results by using a high-quality twelfth-octave equalizer and two good subs. To get an even more consistent bass response over a large seating area, move up to four subs. (Why not three? Because symmetry is key to making this work. Every sub needs to have a counterpart placed in the opposite position.)

Four subs have been used as recommended solutions in demo rooms by THX, the Home Acoustics Alliance (HAA), and others. At CEDIA Expo 2006, HAA's Gerald Lemay set up two identical rooms, one with a single sub in an uncalibrated space and another with four subs with EQ in a calibrated space. The goal was to show what a properly calibrated home theater sounds like compared to the typical drop-it-in-place setup. The idea of buying three more subs might be daunting, but the results were impressive.

As with two subs, the recommended placement for four subs is to put one in the middle of each wall. If a door or some other obstruction prevents you from placing a sub right in the middle of a wall, the four corners of the room are acceptable as a secondary location.

As with two subs, you'll need to use good-quality EQ to get the best results with four. Unfortunately, calibrating subwoofer EQ by ear isn't really an option. If you've invested in the gear for a two- or four-sub system, consider contacting someone with the proper equipment to tune your setup. The Home Acoustics Alliance (homeacoustics.net) has listings of local certified technicians who can provide the necessary expertise, or contact an HAA-certified CEDIA installer (cedia.net).

Sean Greer is the owner of and senior design engineer at Experience AV Home Theater (ExperienceAV.com) in Montrose, Colorado.

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