In our previous article, “The Audio Butterfly Effect” (issue 143), we looked at some options for the placement of multiple subwoofers to achieve a smoother in-room low-frequency response. In this article we’ll consider what alternatives we can try if we only have one subwoofer, and some more options to experiment with if we can employ two or even four.
When you are planning out your budget for your next high-end speaker purchase, it’s good to think about accommodating part of the outlay for a subwoofer.
Like many if not most audiophiles, you may be thinking that you don’t need a sub if you want to purchase a pair of floorstanding tower speakers with large drivers, reasoning that the money saved in not buying a sub can be better spent in investing in better main loudspeakers. After all, larger speakers with bigger woofers and cabinets which will move more air than smaller bookshelf speakers. Rather than discussing the pros and cons of tower versus bookshelf speakers per se here, let’s instead briefly consider the relative merits of why budgeting for a sub as part of your stereo system (whether using floorstanders of bookshelves) is advantageous. Or to put it another way, why is it disadvantageous to not use a subwoofer?
One of the main limitations of not incorporating a sub is the fact that your stereo speakers are limited in their effectiveness in undermining where standing waves occur in your room. In “The Audio Butterfly Effect,” we looked at strategies for speaker and subwoofer placement that would minimize the effect of standing waves and their resulting areas of bass cancellation and reinforcement (room modes). However, where you locate the main speakers is always a compromise between achieving the best bass response and other desirable aspects such as best stereo imaging and good soundstaging, which is best-achieved when the main stereo speakers are able to work together for that primary purpose rather than take on the job of suppressing nulls or room modes. In fact, if you succeed in crushing those modes with your main speakers, it’s arguable that they will not be properly placed for overall optimum sonic benefit, and are no longer going to represent the mix and panning as intended by the sound engineer.
A second limitation of being reliant on your main speakers for your bass response is that they may not have the ability to produce as much volume as a subwoofer would (especially bookshelf speakers with woofer sizes of 8 inches or smaller).
Thirdly, they may not be able to reproduce the lower bass frequency ranges down to 20 Hz at all.
Utilizing a subwoofer is a bit like playing the ace card to win the hand by playing it at the right time and particularly in the right place. Using a single subwoofer lets the main stereo speakers do the primary job of delivering the stereo image and soundstage, while the subwoofer will provide bass extension and greater dynamics. (Note: though conventional wisdom states that bass is non-directional below around 80 Hz, facilitating the use of a single subwoofer in a “2.1-channel” audio system, some audiophiles insist that stereo subwoofer bass is better.)
OK, so let’s say our budget allows us to purchase just one sub. A typical placement option would be to put this single sub in the corner of our room. Corner placement supports bass extension and volume, is physically likely less obtrusive in the room and contributes to a fuller sound when we have correctly set the subwoofer’s phase, volume and low-frequency crossover point for proper integration with our main stereo drivers. However, this is not the optimum placement for dealing with standing waves. Corner placement maximally excites room modes, giving the most output but also the most uneven response.
A more effective placement for purely getting rid of the typical standing wave null that occurs in the middle of a rectangular room is to place the sub exactly where the null is occurring. This has the effect of destabilizing the room’s reinforcement of the standing wave, and greatly reduces the polarity of the “pulse” or “swing” of the sound pressure variation on either side of the null. This results in more even bass response. It’s just that for most of us, placing the sub in the middle of the room is impractical, not aesthetically suitable, or may even be something to trip over, but if you have the opportunity to set it up this way, it can give you a new aural reference.
This is why corner placement often wins out over placing the sub in the middle of the room (or close to/on a null point). If your subwoofer is currently in the corner and you have the ability to try placing it in the middle of the room, you may be surprised at just how much “richness” can be added to your room’s bass depth.
If that option leaves you cold because of its aesthetics, you can always try the tried and true “subwoofer crawl” method. Here, you temporarily place your sub in your main seating position, and then, playing some bass-prominent music, crawl around your floor listening for the location where the bass sounds the best. When you have found it, place your sub there. Then when you return to your listening position, the roles are reversed and you will hear better bass. (To try this, you will of course need long-enough speaker wires and power cables for freedom of movement, although you may choose to use a wireless subwoofer system which can reduce some cabling.)
In my own experimentation with this method, combined with a healthy amount of additional trial and error and room measurement, I was surprised at just how good one subwoofer can sound when placed a quarter of the way into the room’s width and one eighth in from the front wall. This was a good working compromise compared to another suggested alternative of placing the sub a quarter of the way into the room both in length and width. However, the latter method also can work very well, again, if such a placement is livable.
Now, let’s consider why the use of multiple subwoofers can be so advantageous. Two (or more) subs can produce more even, smoother and in fact louder bass, by reducing the effect of multiple room modes. In many installations, this also gives you the freedom to places the subs in less visually-obtrusive locations and keep your room footprint clearer. More important for some listeners is that optimally integrating multiple subs allows you perhaps as much as a 5 dB gain increase, but without sounding that much louder.
How so? As this well-integrated bass is now producing a more even response, it exhibits less-aggressive anomalous peaks and troughs. Due to more dips in the response now being “filled in” by the additional subs, the setup is also now more efficient in the way it is exciting the room. It sounds fuller and therefore richer, deeper and more encompassing than one sub played at the same increased gain level but which would very likely be far less smooth, suffering from an undesirable frequency response dip that you may have tried to compensate for by boosting the volume to “get more bass.” Optimized bass is less localized, particularly if you are using a higher crossover frequency (but perhaps less than 100 Hz, when localization can become too evident) and will sound more similar in more listening seats.
Here’s a video with some detail on the subject:
As far as placement options for two subs, placing them in opposite corners of the front wall does a good job of reducing the room’s standing waves at their first and third harmonics.
If you have the room to move these subs a quarter of the way into the room’s length and width, you’ll get the bonus of also reducing the room’s third-harmonic null. Doing the same with a third and fourth sub at the rear quarters of the room serves to further dispel the standing wave’s second harmonic.
Another advantage of multiple subs is that you can use subwoofers with smaller drivers (and a smaller form factor), which can successfully quash the room modes while still maintaining great bass dynamics and punch, without the need for huge subwoofers in your room. Be aware, though, that that there is a tradeoff between output and smoothness. Using two to four subwoofers throughout the room will give you a smoother low-frequency response, but with a similar output level to employing just a single corner-loaded subwoofer.
Header image: SVS SB-1000 Pro subwoofers, available for purchase as a package of two.