by Alan Lofft, (bio), Axiom Audio
Catching the Waves — All of Them
When you think of some enormous waves in Hawaii rolling in toward the shore (and a surfer dude on his board about to ride one), there's an attractive visual analogy with the way long wavelengths of deep bass behave in any room.
Imagine standing on a cliff above the beach and looking out at the succession of waves. The crests of the waves and the troughs between them represent what happens when a single subwoofer generates the long wavelengths of deep bass — pressure waves that radiate out non-directionally throughout the room (think of your subwoofer as a "pressure source"). Those waves meet the sides of the room, and depending on the dimensions of the room and where you happen to be sitting, they reflect back and forth length-wise and cross-wise, creating big peaks of deep bass energy and valleys or "nulls" of no energy.
(At this point, as nice as the image is, we'll abandon the Hawaiian wave analogy, though it should be said that if you put a big concrete wall in the path of the incoming rollers, the wave energy would be reflected back towards the incoming waves, creating multiple wave behavior in both directions. Anyone who has spent time on boats may have noticed, and felt, the effect of intersecting waves.)
If your chair or couch unfortunately happens to be located in one of the valleys or troughs of the reflected waves, you're not going to hear much deep bass. But if you get up from your chair and walk a few feet back or to the left or right, chances are you'll hit one of the peaks (we also refer to those as a room's "resonant modes") and the bass will be spectacular, perhaps too much of a good thing.
Standing Waves
Those peaks and dips in bass energy are called standing waves because the peaks and nulls don't change unless you change the physical dimensions of the room and the frequency of the bass tones. Besides, even if you did alter the room's dimensions, you'd be left with a whole new set of peaks and troughs to deal with.
So the problem becomes one of defeating or lessening the intensity of the big peaks and somehow smoothing out the level of the nulls — in effect reducing the irregularities in perceived deep bass in multiple locations, so that everyone in the room can enjoy deep, powerful and smooth bass without "boom" (unless there's an authentic boom on the movie soundtrack or in the musical selection!).
With some care in placement of a single subwoofer and the listening location, one listener can experience fairly smooth and deep bass in a rectangular room. Unfortunately, other listeners or viewers seated elsewhere in the same room will hear different bass response, which may have noticeable irregularities. Trying to reduce some of the largest peaks — too much bass at one or two frequencies — is possible with equalization (EQ) for one listener and one location. But attempting to use multiple measuring locations and applying EQ simply doesn't work. Adaptation to our own room's idiosyncrasies also helps us to ignore gross irregularities in deep bass heard when we stroll around a room with only one subwoofer operating; once seated, we tend to "tune out" or ignore some of those irregularities — not all, but some.
Experiments with Multiple Subwoofers
Considerable experimentation over the last decade by leading researchers Dr. Floyd E. Toole, Todd Welti, Sean Olive, Allan Devantier and others into the behavior of deep bass in rectangular, square and odd-shaped rooms yields really interesting and practical solutions to the fairly dramatic seat-to-seat variations in bass response experienced in most rooms.
The conclusions are fairly straightforward (and mildly annoying for single-subwoofer owners) and apply mainly to rectangular rooms:
- Using one subwoofer, only one listener can truly enjoy smooth and extended deep bass response in a single seating location; listeners in other locations will hear different and irregular deep bass.
- Multiple subwoofers, optimally placed in rectangular rooms, will greatly reduce the seat-to-seat variations normally experienced using a single subwoofer, resulting in listeners in different seats hearing much smoother and more consistent deep bass.
- Even numbers of additional subwoofers work best.
- Two subwoofers or four subs deliver the greatest benefits in smoothing out irregular bass for multiple listening seats (four subwoofers are more effective than two). The really good news, according to Todd Welti, an associate of Dr. Toole's, whose white paper, "Subwoofers: Optimum Number and Locations," explored in detail multiple subwoofer performance: "The conclusion I came to was that two subwoofers give you about 90% of the performance that is possible, and that four take you about as far as you can reasonably expect to go."
- There seem to be no advantages to using more than four subwoofers in most rectangular rooms.
The most effective arrangement of two subwoofers in a rectangular room includes one at the front wall in the middle of the shorter dimension, and one sub at the back of the room in the same relative position. Alternatively, one sub can be placed along each sidewall (the longer dimension) at roughly the middle with the second sub against the opposite sidewall in the same relative location.
Four subwoofers were found to be most effective when two subs are placed at the middle location of the short wall at front and back and two subs at the middle location of each sidewall, opposite each other.
Placing one subwoofer in each of the room's four corners was found to be similarly effective. The third beneficial arrangement of four subs located two across the front wall (shorter dimension) and two across the back wall, opposite the two subs at the front wall.
The 25% Solution
One alternative setup that worked extremely well but is crazily impractical for most living setups is what I've dubbed "The 25% Solution." As Todd Welti told Wes Phillips, an AV columnist for On Home Theater.com: "You shrink the whole room by 25% and put the subwoofers at the corners of that virtual room. You get incredible performance, but that's just not practical in most rooms. But if you use two or four subwoofers in the corners or the wall midpoints, you can get pretty good performance."
There you have it – scientific justification for your spouse as to why you absolutely need a second (or fourth) subwoofer this year. Watch our homepage next month for the video on placing two or four subwoofers in your room.
