A BASS MEASUREMENT ODYSSEY

Aclosed-off symmetrical, rectangular room with a flat ceiling may be the ideal space to optimize subwoofer performance, but homes with open floor plan spaces have become increasingly popular. The room used for my home theater setup, for instance, is 16 x 21 feet (with an oddly sloped ceiling) with one corner opening up to the front entryway and the right sidewall opening wide to a kitchen and small breakfast area. So even though the main space is 16 x 21 feet, the overall acoustical space the system must deal with is much larger and oddly shaped.

Optimizing such a room requires extensive experimentation, and I wouldn’t attempt it without the use of the sort of measurement tools mentioned earlier. For the exercise here I used the Omnimic system.

All of the room measurement results shown were taken at single mic positions at each listening seat in turn, but the Audyssey calibrations were performed across a 64-inch-wide area heavily weighted toward the center seat. The playback system included Monitor Audio Silver 10 tower speakers plus a pair of SVS SB-3000 subwoofers crossed over to the mains at 90 Hz. My setup also includes center and surround speakers, but to keep things manageable here I’ll only discuss the results for the left and right channels together with the subs. For this feature, SVS kindly loaned us the SB-3000S, which I chose because they’ve been well reviewed and are small and light enough (around 50 pounds) to schlep around a room to different positions.

One subwoofer issue caused me no end of problems until I realized what was happening. Since the SVS subs are set by default to their Auto turn-on positions, they took a second or two to detect bass in the source before turning on. But the Omnimic test tones are rapid sweeps that sometimes didn’t trigger one or both of the subs. The result was much hair-pulling before I realized why I was getting odd results. If you have these or similar subs and are using a measurement program with similar sweep tones, be sure they are set to the always-on position before running the tests.

For all of the measured results, the two subs were set up to operate together in mono and level-matched to produce the same output. When measuring a room or applying room correction using an A/ V receiver or surround preamp that has two subwoofer outputs, you should use just one of them together with a splitter to connect separate cables to each subwoofer (unless your subs have a bridging output to run the signal from sub 1 to sub 2— many don’t). Why not use both sub outputs if your AVR provides them? Because the room correction processing might try to equalize each subwoofer separately, which would defeat the purpose of using two dissimilarly positioned subs to smooth out room modes at more than one seat.

I first positioned the two subs directly next to the left

and right speakers—a common audiophile setup. Fig.1 shows the results. Yikes! The dip at around 50 Hz falls right in the prime bass region, and while the response recovers in the deep bass, no room correction can fix that dip. Reversing the phase of both subs relative to the mains didn’t help, and the results were much the same when I tried moving the subs closer together to the left and right of the center speaker.

I then began the odyssey of trying out different positions for the two subwoofers. My goal was to produce respectable performance at three different listening positions: one at the classic center seat, and the other two at 32 inches to the left and right of center, respectively. I finally settled on a setup with one of the subs slightly left of center at the wall behind my couch and the other just to the right of center at the front wall. Moving the listening couch 18 inches closer to the speakers than in my original setup also helped.

While every measurement situation will be different, patience usually pays off. The curves shown here tell the tale, each of them shown with and without Audyssey room correction. In all of the curves, the left channel (+subs) is in red and the right channel (+subs) is in blue.

( The curves were all 1/ th octave 6 smoothed.) Note that Audyssey also flattened a rise in the 100-200 Hz region. This rise is charac

Above: The layout of TJN'S open floor plan theater where the subwoofer tests and measurements were conducted.

teristic of my room, though such bass deviations are common to many domestic spaces (varying only in frequency and degree). A small 45 degrees phase tweak I made to the rear woofer is also included in the results. For the Audyssey curves, I increased the subwoofer level by 5 db, though for music listening I backed off on this by 1-2 db. A small rise in the bass is generally preferred by most listeners, though some bassheads overdo it!—

Figure 1: Measurements taken at original center seating position with subs next to left/right towers.

Figure 4A: 32 inches right of center with subwoofer placement optimized (Audyssey active).

Figure 3A: 32 inches left of center with subwoofer placement optimized (Audyssey active).

Figure 2A: Center seating position two with subwoofer placement optimized (Audyssey active).

Figure 2: Center seating position two with subwoofer placement optimized (no Audyssey).

Figure 4: 32 inches right of center with subwoofer placement optimized (no Audyssey).

Figure 3: 32 inches left of center with subwoofer placement optimized (no Audyssey).