Although I’m now a big fan of using a powered subwoofer or two with a stereo pair of loudspeakers, as I explained in a May 1 article on this site, I’m a recent convert to the practice. It’s only in the last year or so of my 30-year audio journey that I’ve realized two things: 1) The sound produced by all but the most expensive and extreme speakers will benefit from the reinforcement provided by a subwoofer -- few speakers of any stripe can produce useful output down to 20Hz, the lower limit of human hearing. 2) Dollar for dollar, the quality of bass produced by a good subwoofer easily outclasses the quality of bass from a good or even a great speaker. This is due not only to the fact that subs are designed to reproduce only the low bass, but also because their placement in a room can be much more easily optimized to serve their intended purpose.
So -- how does someone who doesn’t know where to start go about this? In this first article on the subject, I discuss the first steps that need to be taken to add a sub to an existing system, or to buy an all-new 2.1-channel system.
Budget
If you’re assembling an audio system from scratch, consider the four things that will most affect its sound quality: 1) how well the music you play has been recorded and mastered, 2) the speakers and subwoofer(s), 3) the room, and 4) equalization (EQ), which these days is usually performed by room-correction software. The effect on the overall sound of every other piece of gear pales by comparison. This is why I recommend dedicating the bulk of your system budget to speakers and sub(s) you can currently afford, then to optimizing your room, and finally to room EQ. After all that’s taken care of, you can use what’s left to select your electronics -- or start saving again to gradually upgrade your system over time.
So if you’ve already got a two-channel system and are looking to add a subwoofer, buy the best sub you can afford, and consider adding a second sub later, when budget again permits -- although, as I learned and explained in that May 1 article, adding a second sub didn’t offer nearly as much improvement in the sound of my system as did adding the first sub. And remember that the best sub for you won’t necessarily be the biggest -- a lot will depend on the size of your room. Work with a reputable subwoofer manufacturer (e.g., HSU Research, JL Audio, Paradigm, REL, SVS) to come up with a model that will suit your needs and room.
Connecting sub to system
Most subwoofers have one of two types of inputs, and by far the more common is line-level, for unbalanced (RCA) or balanced (XLR) interconnects. For a line-level connection, you’ll need to connect the sub to a preamp, integrated amp, or receiver with preamp outputs (usually labeled Pre Out). If you’re installing only one sub, connect the amp’s left- and right-channel pre-outs to the sub’s left and right inputs. If your sub has only a single line-level input, connect only the preamp’s left- or right-channel output to the sub’s input -- DO NOT plug a Y-adapter into the left and right outputs of your preamp, as this will effectively short the outputs. If you’re using two subs, connect the left-channel sub to the left pre-out, and the right-channel sub to the right pre-out. If you’re using a separate preamp and power amp, and the preamp has only a single set of pre-outs, you’ll need two RCA Y-adapters, to split the pre-outs to simultaneously feed power amp and sub. However, if your sub has inputs and outputs, you won’t need Y-adapters -- just feed the pre-out to the sub’s inputs, and the sub’s outputs to the amp’s inputs.
These days, a few subs offer terminals for speaker-level connections. If your sub has such terminals, and your integrated amp or receiver doesn’t have pre-outs, use speaker cables to connect the output terminals of the integrated, receiver, or power amp to the sub’s speaker-level inputs, and the speaker-level sub outputs to your main speakers. The sub’s electronics will pad down the speaker-level input, and use its own internal amplifier to power the sub’s driver.
Positioning a subwoofer: do the sub crawl
Where the subwoofer is in the room can greatly affect the bass response one hears and/or feels at the listening position -- shifting the sub by as little as a foot can significantly change the overall response. Since we often have little choice about where the main listening position should be, we’re left with trying to find the best place to put a sub -- the place that produces the fewest and the least-pronounced peaks and nulls.
To help inform your decision, I strongly advocate taking accurate measurements in your room (see below). If that seems daunting, or you just don’t want to be bothered with it, doing the sub crawl will work, if less precisely. Instead of moving the sub around the room and listening, you do all the moving while the sub sits still. Here’s how it works.
First, place the subwoofer at your listening position. Then play a bass-heavy track on repeat. As the music plays, slowly crawl around the room -- you want to keep your ears low, because subwoofer drivers are almost always close to the floor -- while listening and feeling for the strongest, most even bass response. When you find it, that’s probably where the sub should go.
That said, you might find more than one ideal spot, which will give you a choice (see below). If you choose a spot that isn’t along the front wall, and someday you get serious about bass management and decide to cross over your sub above 80-100Hz due to your room acoustics (as I did -- see below), the bass frequencies may seem to be coming from a different part of the room from the rest of the sound -- that is, the bass may not be part of a coherent soundstage. Therefore, when selecting a sub location, treat the front soundstage preferentially. This may come in handy later on.
Setting the sub’s crossover frequency and volume by ear
Those who connect their speakers and subwoofers to an A/V receiver have a big advantage over those who use an integrated amplifier, or a separate preamp and power amp. An AVR’s built-in bass-management system lets the user set the crossover frequencies: a low-pass filter (LPF) or cutoff for the sub, and a high-pass filter (HPF) or cutoff for the main speakers. But what about the two-channel enthusiast who doesn’t have an AVR, and wants to add a sub to an existing system of pre- and power separates or integrated he or she has already invested a lot of money in? Most such systems lack any sort of built-in bass management. This was my situation.
When you’ve found the best place to put the sub, it’s then time to set the sub’s volume and LPF -- and every good-quality powered sub has these controls. If you don’t want to take your own in-room measurements (though I highly recommend that you do), you’ll need to set the volume and LPF by ear, taking into account your main speakers’ specified frequency response. For example, Bowers & Wilkins specifies my 705 S2 speakers as having a frequency response of 50Hz-28kHz, ±3dB. It’s that low-end figure of 50Hz that matters here. When I listen to my B&Ws full-range -- that is, with no external crossover filter rolling off their low-frequency output -- I set my sub’s LPF to 50Hz, so that the speakers handle almost all of the bass down to that frequency, at which point my two subs take over. If your sub also lets you set the slope of the LPF, which affects how quickly the subwoofer shelves off its output above that frequency, I suggest choosing the steepest slope, typically 24dB/octave -- this should result in less overlap of output at the frequency the speakers and sub hand off to each other. This 50Hz setting is your LPF starting point, but more listening and tweaking will probably be required. (If you no longer have your speakers’ spec sheet and have trouble getting their frequency response from their manufacturer, some subwoofer makers (e.g., SVS) provide online tools that suggest the best LPF and slope to use to mate their subs with many commercially available speaker models.)
Setting the sub’s volume level by ear involves slowly increasing the level until the sub’s contribution becomes very apparent, and dialing it back just a bit -- you want the sub to contribute to the bass output, of course, but not be obvious about it. Again, listen and tweak until you’re satisfied with the sound.
Measuring the room to find the perfect subwoofer position
Those unafraid of the process can take some basic measurements that will make setting up a sub more precise than doing the funky sub crawl, and it’s not expensive. All you need are a decent calibrated microphone, such as miniDSP’s UMIK-1 USB mike ($75 USD with tripod), and to install on a computer John Mulcahy’s free Room EQ Wizard software (aka REW), available for Windows, Mac OS, and Linux. That’s the setup I use and describe here.
REW is a powerful tool capable of performing multivariable acoustical analyses and generating parametric EQ filters based on target curves that can be implemented in the digital domain. A book could be written on this subject, and many tutorials are available online for those who want to learn more. I focus here on the basics: measuring, at the listening position, the sub and main speakers’ reproduction of a frequency sweep.
You’ll need to plug the USB mike into a computer, preferably a laptop for portability, and launch REW. REW then finds and identifies the mike, and asks if you have a calibration file, to make the measurements more precise. If you buy miniDSP’s UMIK-1, your sample’s calibration file is available from the miniDSP website -- just make sure to point your mike toward the ceiling for a 90° calibration file, and straight ahead for a 0° calibration file; for the UMIK-1, the calibration file name indicates 0° or 90°.
Next, you’ll need to get your computer to output audio signals to your two-channel system, either digitally (via HDMI or USB, into a preamp or integrated amp that has a built-in DAC), or with a good old-fashioned analog connector (3.5mm TRRS on the computer end to left/right RCA on the preamp end). Then, on REW’s main screen, click the Measure button, specify a frequency range from 10 to 20,000Hz, and select the default output channel: “l+r.” Once the measurement is done, to produce a smooth frequency-response plot, select “All SPL” above the graph, hit the Controls button at top right, select “1/24 smoothing,” then hit “Apply smoothing.”
You should start by measuring your main speakers without the sub, and with the mike placed at your seated ear height at the listening position. Focus on the frequencies between 20Hz and 200Hz. At what frequency do you see the most significant bass null (i.e., cancellation)? The plot below shows the in-room frequency-response plot of my B&W 705 S2 speakers -- note the significant dip at 132Hz. This null is a function of my room’s dimensions and the speaker positions, and it results in a bass suckout at this frequency -- this is the sort of thing you’re looking for.
In addition to listening, I used this plot to determine where I would finally set the crossover frequencies between my main speakers and sub: an HPF of 120Hz (24dB/octave) for the B&Ws, and an LPF of 130Hz (24dB/octave) for the sub. The idea was to somewhat overlap the crossover points to mitigate the bass null as much as possible, by having the subwoofer run a little higher and help fill it in.
In determining the LPF cutoff frequency for your sub, it may not be a good idea to go above about 150Hz, and certainly not without checking with the sub’s manufacturer to find out how well it might be able to perform at higher frequencies -- most subwoofers are designed to work well only from 150Hz down. In addition, as mentioned earlier, any LPF cutoff frequency above 80-100Hz may cause a problem in your being able to hear the subwoofer’s sound as being separate from the sound produced by your main speakers, especially if you’ve placed your sub outside the part of the room occupied by the front soundstage. If, unlike mine, your speakers’ in-room response reveals no particularly glaring dips between 50 and 150Hz, try experimenting with crossover frequencies between 40 and 80Hz for three-way floorstanding speakers, which are often able to play pretty low; or from 80 to 120Hz for two-way bookshelf models, which can’t generate as much low end.
Next time: bass management and room correction
There’s lots more to learn. In my next article on this subject, I’ll present examples of subwoofer frequency-response measurements, and explain how these can inform your choices of subwoofer position and equalization. I’ll also stress the importance and the various ways of setting and using an HPF for your main speakers, as I’ve done in my system.
For example, in the graph above, in addition to that deep null at 132Hz you’ll see bass peaks at 50 and 80Hz. These can be remedied with equalization, which I’ll tackle next time. I’ve also lined up an interview with the designers of Anthem Room Correction, the software built into various electronic components made by Anthem, as well as many subwoofers from MartinLogan and Paradigm. This sophisticated system can help you seamlessly integrate one or more subwoofers into your system.
. . . Diego Estan
diego@soundstage.com