Dual Subwoofers for your Home Theater
By Oliver Amnuayphol
Home Theater Guru
If you’ve ever setup a subwoofer in your home theater system, chances are you’ve noticed that the bass sounds ripe ‘n rumble-y in some parts of the room but not so good in others. This is due to your room’s acoustic properties and its tendency to reinforce or cancel specific bass frequencies depending on where your subwoofer and listening positions are located in relation to the room and to each other. Carefully locating your seat and sub in the room can minimize this effect a bit, but what if you have a dedicated theater room, or don’t watch movies alone? In this case, getting good bass becomes much more difficult. Since you can only optimize a subwoofer’s placement for a specific part of the room, a couple of well-placed listeners may get good bass, but the rest won’t. Even today’s advanced auto-calibration programs can’t accurately optimize bass for several different areas of the room, and room treatments like bass traps aren’t very effective at the lowest frequencies (sub 50 Hz) unless they are impractically large. What can a bass lover do?
The answer is: Get two subwoofers! That’s right: if you want the smoothest, most even bass across the whole room, having dual subwoofers is the way to go. That’s because two subs, placed at strategic locations in the room, will help to smooth out bass modes and nodes (peaks and dips, respectively) throughout the room for more even bass distribution. You’ll also notice improved low bass extension and greater headroom, as well as more complete reproduction of the whole range of bass sounds for more a realistic, life-like experience. Here’s what you need to know if you’re thinking about going the dual subwoofer route:
Consider Dual Subwoofers if you have:
– Multiple seating positions: While it’s possible to optimize the spatial relationship between your room, subwoofer, and one listening position, it’s near impossible to do this for several seats. Two subwoofers properly placed will provide smoother, more consistent bass coverage to all the listeners in the space.
– A dedicated theater room: For the same reasons as above, only to the nth degree—nothing can replicate the theater experience like two subwoofers can!
– Large overall space: Bass from a single sub tends to get lost in large overall spaces, so if you have a large room or an open floor plan (i.e. no walls between the living room, kitchen, and/or hallways, etc.), two subs are almost a requirement for realistic bass quality and low bass extension.
– A single, primary listening position but are jonesin’ for the smoothest bass response.
Where to place the subs?
Ordinarily we’d suggest the crawl method to snuff out the best location for your subwoofer, but doing this optimizes it for only a single listening position and is irrelevant once you have multiple seats and more than one sub. So then, how do you get two subs to produce great bass in your place? Experimentation is the answer: Since there are no guaranteed best spots when it comes to positioning multiple subwoofers, moving it around and listening over and over is really the only way to get the subs locked-in with your lair. But if you have a rectangular room, you can use some general placement guidelines as starting points. Of course, your listening positions will also affect where in the room the subs sound their best, but once you’ve got that dialed in, check out some of the most popular dual subwoofer placement options below:
If you have a rectangular-shaped, dedicated theater room, multiple rows of seating, or just want excellent bass coverage, placing two subwoofers midway along the lengths of opposing sidewalls is the best recommendation. These locations yield the smoothest and most even bass distribution across the widest space due to something known as axial mode cancellation, thereby maximizing bass impact for the most listeners. You can also try placing your bass bins at opposing midwall points across the width if you just can’t position them along the length.
Corner loading both subwoofers in the front corners of the room provides the most bass because it maximally excites all of the room’s resonances, thereby yielding more bass output for the same given volume setting than one that’s not corner loaded. Moreover, you can lower the overall sub volume to consequently increase headroom and decrease distortion. The downside is that corner placement can also provide too much reinforcement, causing the bass to sound boomy or uneven. If this happens, try bringing them out from the corners little by little, starting with around one foot of space between each sub and each adjacent wall.
At this point you may be asking yourself, "Is it possible to get the best of both worlds, i.e. a blend of corner and midwall placement bass?" The answer is a definite maybe: Try positioning the subs along the front wall with the woofer cone centers at the 1/4 and 3/4 distances of the room width. Rooms that are sympathetic with subs at these locations will yield almost as much bass output as corner placement but with a smoother overall frequency response. Of course, this reduces overall output compared to full corner placement, but most quality subwoofers nowadays have more than enough headroom to compensate.
Unfortunately, we can’t always place the subs and our chairs in ideal parts of the room due to aesthetic or visual constraints: Many of us simply can’t remodel our room around our subwoofers or have open floor plans, irregularly shaped rooms, etc. Luckily there may still be a solution—opposite corner placement. Putting the subs in the opposite front and back corners will maximally reinforce bass output like the front corners but will more evenly distribute bass throughout the room. Try tweaking the subs’ positions little by little to see if there are sweet spots where the bass locks with the room. If you’re a bass freak, try this one even if you can do one of the placement options mentioned earlier: You may find this is your preferred bone-crunching, teeth-rattling option.
Dialing in the Subwoofers:
After you’ve fine tuned your subs according to one of the above options, the last things to do are to tweak the subs’ volume levels, crossovers, and phase controls. To achieve proper phasing on your subs, play a steady test tone centered at your crossover frequency (i.e. 80Hz tone for same crossover setting) and have a friend vary the phase controls on each sub up and down until you hear the strongest tone and your subwoofer will now be in phase. All that’s left is to sit back, relax, and feel the good vibrations!
Though the options listed above will get you almost full sonic nirvana, they are far from exhaustive; If you’d like more in-depth information on bass/room interactions or just want to totally geek-out, check out the following links:
– Subwoofer Placement – The Place for Bass Part 1 – Great technical and scientific guide to the subwoofer/room interactions and thorough explanation of room modes and nodes.
– Home Theater Multiple Subwoofer Set-Up and Calibration Guide – In-Depth look at measuring and calibrating multiple subwoofers.
– Loudspeakers and Rooms for Multichannel Audio Reproduction, Part 3 – Getting the Bass Right – Super technical white paper on optimizing multiple subwoofers for home theater and multichannel audio from The Man, Dr. Floyd E. Toole.
(Totally boss diagrams in this course courtesy Audioholics).