OTHER TECH

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.