Setting Up Subwoofers in a Two-Channel Audio System

A two-channel audio system can often benefit from using subwoofers to augment the main speakers’ low bass. However, subs in two-channel music-only systems play a role somewhat different from the role they play in home-theater systems, whose owners are generally more concerned with the quantity than the quality of bass output. This is why many subwoofer manuals instruct you to put the sub in a corner, where it will be louder.

In a two-channel system, the principal goal is to match the subwoofers with the main speakers so that they blend seamlessly. You sometimes get better results with two subwoofers; room modes are more easily smoothed out with multiple sources of low bass. However, that can double the cost. You may actually produce better sound if you forgo a single large, expensive subwoofer in favor of two smaller subs. Though that may run contrary to the advice you’ll see in most other publications, remember: we’re setting up a music system. Two smaller subs may radiate bass more evenly into a room than a single big one.

With most subwoofers, it’s best to use your preamp’s line-level output. Run an interconnect from the preamp output jack (it’s beneficial to have two sets of outputs, a common feature of today’s preamps) to each subwoofer’s input jack. If you use the line-level output, you may have the choice of using an unbalanced (RCA) or a balanced (XLR) interconnect. One of the problems you may encounter in adding subwoofers to a system is ground loops, which produce hum that’s especially annoying when heard through subwoofers. Balanced XLR connections tend to have fewer ground-loop problems, since neither conductor is at ground level. Nor do you need the most expensive interconnect you can find; after all, the sub’s crossover will filter out all frequencies above the bass. Some companies sell interconnects optimized for subwoofers, which usually aren’t very expensive.

Matching the levels of the main speakers and subwoofers is utterly essential. The instructions provided with most subs say something like, "Find a recording with deep bass and adjust the subwoofer level to match the main speakers’ level." That’s not very precise. An easier, faster, more accurate way to set a sub’s output level is to use a real-time analyzer (RTA) to display, in graphical form, the entire frequency response you hear at your listening position. That way, you can see when changes in the subwoofer level match the output level of your main speakers. The RTA can be a program you install on your computer, with an accompanying test microphone and mike preamp, or a separate, self-contained component such as the Phonic PAA-6. A computer-based RTA system would comprise a laptop or netbook computer, RTA software (e.g., TrueRTA from True Audio), a calibrated microphone (e.g., the Behringer ECM8000), and a mike preamp with USB output (e.g., the MXL Mic Mate).

Start with these settings on your subwoofer’s control panel:

The following process walks you through the process of setting up two subwoofers in a two-channel audio system, using an RTA and related accessories.

1. Assemble your test equipment: RTA gear as described above, pink-noise CD (if the program lacks its own noise generator), and a tape measure.
   
   
2. Following the instructions that come with the RTA software, measure the frequency response of your main speakers. Before you connect the subwoofers, play pink noise (a signal with equal energy in all octaves) through the main speaker, and use the RTA to view the mains’ frequency response at your listening position. You should be able to see where the speakers "roll off" -- that is, where there’s a significant drop in their bass output -- in your listening room. Look at the bass rolloff on the frequency-response graph and identify the "6dB down" frequency; i.e., where the bass range is 6 decibels below most of the other frequencies. Use that -6dB point as your starting frequency for crossing over the audio signal from the main speakers to the subwoofer.
   
   
3. Position the subwoofers close to the inner or outer side panels of your main speakers. (If the mains have side-firing woofers, don’t block those.) If you put the subs farther from the mains, it will be harder to achieve a good acoustic blend. Measure the distance from the woofer cone of the main speaker to your listening position. Adjust each sub until its cone is the same distance from the listening position as the woofer cone of the corresponding main speaker.
   
   
4. Follow the instructions that came with the subwoofer to connect it to your preamp’s line-out jacks. Use balanced interconnects if you have that option, as they tend to cause fewer ground loops (hum). Plug the subs into AC wall sockets. Subwoofers often have rather powerful amplifiers, so use a heavy-gauge cable. Set the controls or switches on both subwoofer amplifiers as described in the table above. (Note: The settings listed are available on JL Audio subwoofers; not all subs made by other companies will have all of these controls and switches. Use the controls and switches that appear on your subwoofer and, again, read the owner’s manual for advice.)
   
   
5. Now the actual process of integration begins, one channel at a time. What you’ll first do is integrate the left-channel subwoofer with the left main speaker. Then you’ll integrate the left subwoofer with the room. When the left channel is set, you’ll repeat the process for the right-channel subwoofer. Disconnect the right-channel main speaker and subwoofer. Play the pink noise and monitor the RTA to set the level control of the left subwoofer so that the combined frequency response of the left sub and left speaker is as flat as possible. Don’t expect a completely flat line -- there will always be some irregularity in the response, due to the room and to ambient noise. Observe the frequency-response curve above and below the crossover point. Adjust the subwoofer’s level control until the frequency response is as flat as possible. If the RTA’s frequency-response graph shows a valley (or trough) in the vicinity of the crossover frequency, the main speaker and sub may be out of phase. Flip the subwoofer’s Polarity switch to reverse the phase of the sub’s signal. If the sub and main speaker were indeed out of phase, the trough should now vanish, or at least diminish. If your subwoofer has continuously variable Phase (0-180 degrees or 0-270 degrees), adjust this slowly to see if the trough in the crossover vicinity can be further minimized. (Skip this last step if your subs lack Phase controls.)
   
   
6. Some subwoofers have computerized procedures for minimizing the peaks and valleys you observed in step 5. Refer to your sub’s manual and carry out that procedure now. JL Audio calls its version of this feature Automatic Room Optimization (ARO), whereas Paradigm uses their Perfect Bass Kit. If your sub has an adjustment for trimming its frequency response (effectively, a tone control), adjust that now, and evaluate it by viewing the RTA.
   
   
7. Repeat for the right channel.

The combined frequency response of a loudspeaker and a subwoofer. The relatively flat line indicates good integration between sub and main speaker.

Experimentation

To further experiment, try adjusting the crossover frequency up or down in increments of 10Hz. For best results, you’ll need to repeat all the other steps in the process each time. Fortunately, once you’ve gone through this process, it should take you less than an hour to repeat it. Sometimes having two people doing the setup -- one to adjust the controls, one to read the RTA settings -- makes the process go faster.

Ultimately, listening to a music system with great deep-bass response is quite satisfying. Putting in a little more work up front can lead to less frustration and less time wasted trying to achieve the bass you really want.

. . . Vade Forrester