Bowers & Wilkins DB1 subwoofer B&W Room Acoustics Compensation
Sidebar 3: B&W Room Acoustics Compensation
Testing subwoofers is not as lonely a business as I had thought. One Stereophile reader, Darren Gum, wrote to criticize my use in reviews of extra hardware to achieve more transportable subwoofer setups; eg, the virtual spectrum analyzer in my Velodyne DD-18 subwoofer (reviewed in June 2004), and Bryston's 10B-SUB outboard electronic crossover ($2395; reviewed in May 1994 and November 2005), which I used with subwoofers that didn't have high-pass feeds for the satellite speakers. Gum worried that most audiophiles wouldn't have access to the extra gear, and that different results might have occurred had I used only my ears.
His argument made sense. Most readers set up their subwoofers by ear, unless the dealer comes by to "tune" the system. Using my ears to set a subwoofer's polarity, match its output to that of the Quad ESL-989s, and set its low-pass filter's frequency would either work as well as my extra gear or not. I decided to try it both ways: I first set up the B&W DB1 using only my ears, then set it up again using my measuring gear.
Before setting up the DB1, I measured the Quads' room response using the signal generator and spectrum analyzer built into the Velodyne DD-18 subwoofer that sits at the center of my room's front wall. Clearly, using Velodyne's electronics was not part of the B&W's setup instructions, but had proven helpful in setting up other subs.
After setting the DD-18's volume control to "0" so that its 18" drive-unit would remain silent while I took measurements of the B&W, I rested the Velodyne calibration mike on the back of my listening chair, at my seated ear height of 37" above the floor and triggered a repeated 20200Hz sweep tone that I fed into the Auxiliary input of my Bryston BP-26 preamplifier. The Velodyne's sweep signal revealed that the Quads' output was relatively flat down to 80Hz, then fell 10dB by 35Hz (fig.1). I then measured the DB1's uncorrected in-room response, which revealed strong output from 20 to 200Hz, with prominent room modes evident at 45 and 65Hz (fig.2).
Following the online instructions, I set up B&W's Room Acoustics Compensation program by plugging the DB1's soundcard into a USB port on my Windows laptop; connecting one cable to the soundcard's minijack connectors to send its test signals to the sub, and another to receive the feed from the calibration mike; and plugging a third, long USB-to-RS232 cable from a second USB port on my laptop into the RS232 connector on the DB1's service panel.
That done, I ran B&W's SubApp Windows program. When I first clicked the on-screen Run button, the DB1 played some rapidly stepped test tones, then stopped and indicated that the volume level was too low for it to take a measurement. I checked and found a loose connection in a cable plugged into the soundcard output. I fixed it, then successfully reran the compensation routine, which involved the App taking eight different measurements, without the Quads connected (fig.3).
To set up by ear, I turned the Quads back on and matched the DB1's volume, phase, and polarity settings, ran the Room Acoustics Compensation, and listened for the tightest bass. I left the DB1's low-pass filter at its default setting of 80Hz, and selected a low-pass filter slope of 24dB/octave. I saved these settings as Preset 1. To my surprise, the Velodyne's spectrum analyzer showed a surprisingly flat in-room response (fig.4). The sound was spectacular, especially playing organist James Busby's recording of Herbert Howells' Master Tallis's Testament, from Pipes Rhode Island (CD, Riago 101, no longer available). I was delighted to hear extraordinarily tight, clear, sustained organ-pedal notes reproduced with good pitch definition and solidity, and with enough power to shake loose objects in my listening room.
To set up by gear, I returned the DB1 to its factory default settingsno room compensationby selecting a new Preset to clear out the previous Room Acoustics Compensation settings. I ran SubApp a second time, and used my laptop's keyboard to adjust the virtual gain sliders in SubApp to match the outputs of the DB1 and the Quads while watching the resulting room-response curve on the TV monitor. I then adjusted the DB1's low-pass filter frequency to achieve the flattest line on the sweep signal display. The display showed that a low-pass frequency of 70Hznot the 80Hz I'd used beforeand a low-pass filter slope of 24dB/octave gave the flattest line representing the room response. Finally, I found that setting the DB1's phase to 90° gave the flattest response on the TV monitor, not the default 0° setting I'd used before (fig.5). I loaded the new settings into Preset 2, then again played Master Tallis's Testament. I heard the same clean, deep bass, and this time it was even more focused and solid.Larry Greenhill