Home Theater Multiple Subwoofer Set-Up & Calibration Guide
Originally Published: May 4th, 2010
Got a pimple, spray some Windex on it. Got a sore throat, swish some vinegar and honey. Got a cold? Eat some chicken soup. Home remedies, some are good, others are a mixed bag at best. I spent years conjuring up home remedies or tricks for properly calibrating multiple subwoofer theater rooms. In a perfectly rectangular room with the ability to place identical subs in the ideal locations around the room, this isn't such a daunting task. This is especially true since you can use room mode calculators to help determine the best subwoofer locations before actually placing them in a real room. But what about real world scenarios of people that don't have ideal rectangular rooms?
I suggest if at all possible to select a rectangular room for your theater room. This is a proven CEA/CEDIA recommendation for good reason. In rectangular rooms, bass reproduction is more predictable, making it virtually an exact science when choosing the ideal subwoofer locations and doing room analysis. This greatly minimizes trial and error, allowing for a more scientific and methodical approach. There are, of course, no free lunches - which means there are trade-offs for all listening spaces, even perfectly rectangular rooms.
Most of us don't have dedicated rectangular shaped rooms for home theater but instead have multi-purpose recreational rooms that double as a theater room. My hat's off to the fortunate ones, and you probably don't need to read this article since you never get pimples, you never get sick, and your theater room is perfectly calibrated just by plugging it in. For the rest of us, my advice and calibration tips in this article could prove invaluable to you and save you countless hours of frustration. Trust me, I've been down that road before and I didn't come to these conclusions in haste. I spent incalculable hours running sweep tones, testing, moving subs, listening and repeating to the limits of my wife's sanity of having to feel and hear the bass shaking the rafters of our home and rattling the fillings loose in her teeth. Like a dog chasing his tail, I was relentlessly spinning my wheels trying to get the perfect calibration until I came up with a methodical approach that not only saved time but saved my sanity as well. Many of the suggestions and placement tips in this article assume a perfectly rectangular room but also serve as a starting point for any room. Regardless of room shape, these methodologies will work, at least in practice, for most room types.
5 Steps to Better Bass in your Home Theater
In order to take advantage of the information in this article, you will need the following items:
- SPL meter
- RTA or FFT analyzer with at least 1/12th octave resolution
- Patience and perseverance
Radio Shack SPL Meter (left); Sencore SP495 FFT Analyzer (right)
The purpose of this article is to provide a method of achieving the smoothest bass response for all listening seats using proper placement, calibration and active equalization (when needed). The goal is met when measurably better performance is achieved by reducing excessive modal peaks and nodal dips across the entire bass spectrum (15 Hz to 200 Hz). +-10dB or better for this frequency spectrum should be attainable for the primary listening area. This is accomplished via proper placement and set-up of multiple subwoofers resulting in standing wave reduction through destructive interference. It is assumed the reader is knowledgeable on how to properly use an FFT or RTA analyzer to properly measure system response. Instructions about how to use such equipment can fill an entire article on its own, which is outside the scope of this article.
If the subs are properly setup, very few or no low frequency passive room treatments will be required as they tend to be cost prohibitive and space intensive. But, they can be a useful tool in helping to improve bass response, though they go beyond the scope of this article. You can read many articles on bass traps in our Acoustics section of the website.
The Benefits of Multiple Subwoofers for Home Theater
Step 1: Choose your Subs Wisely
Ideally you should use identical subwoofers for the best overall performance, but it is possible to mix and match subwoofer brands and types if you're willing to do the extra work. If using mismatched subs, bear in mind that systems with different low frequency cutoffs may well be in phase and additive over most of their band, but at or below system resonance may well be out of phase. This can put us in the position of having the sub with a higher cut-off frequency reducing overall system output below its cutoff frequency. This is why we usually recommend using identical subs all around.
My primary theater room consists of two ported subs and two sealed servo subs. Had it not been for the fact that my servo subs of choice (Velodyne DD15's) have such an enormous array of configuration options such as adjustable EQ, adjustable high pass and low pass slopes, variable phase, etc, I would have not been able to pull off the integration of all four subs so well. This is a point to consider when using mismatched subs or purchasing a subwoofer.
Decide how many subwoofers you can afford and logically place in your room. It's usually a good idea to place either two or four subwoofers in your room. Odd multiples of subs don't work out so well in rectangular rooms (research conducted by Devantier, Welti of Harman Labs) but they can work just fine in odd shaped rooms without perfect symmetry. We usually don't recommend more than four subwoofers as it's not only overkill but can cause more problems than it solves, especially if the subs are placed in non optimal locations. For instances where more output is needed, you can exceed the four sub rule as long as you limit your placements to up to four locations. So in such cases you can stack subwoofers on top of each other in the locations that are best suited for optimal performance.
Step 2: Location Location Location
You hear this often from real estate agents that location is everything. Well as an audioholic, I can tell you the same applies for speaker and subwoofer placement. Location is critical and is your first line of defense for minimizing calibration heartache and ensuring the most consistent sound across all of your listening seats. Check the old school mentality of calibrating for a single money seat at the door. Our goal is a state of the art home theater experience for most if not all of the listeners in our room, sans the mother-in-law of course. You can put her in the corner seat closest to the bathroom, and just as long as you keep her happy with snacks and beverages she will love you for it.
We have covered subwoofer placement to death in various articles on this site with the two most recent and relevant being:
However based on recent research from Dr. Floyd Toole's book "Sound Reproduction" as well as the CEA/CEDIA-CEB22 Home Theater Recommendation, we felt it was time to update multi subwoofer placement options for rectangular shaped rooms. Experimentation in placement will be needed for odd shaped rooms however, and even in perfectly rectangular rooms, these diagrams should not be interpreted as a biblical source. You must measure and experiment to determine the ideal locations in your room.
Figures: 1a. Two Sub Midwall; 1b. 4 Corner Placement; 1c. 1/4W Placement
Note: The distance from the walls to the subs should be measured at the acoustical center for each sub which is usually near the dustcap of the drivers.
Figure 1a. Two Sub Midwall Placement
If you are running two subs, the ideal locations for them in a rectangular room are on opposing vertical or horizontal midwalls. Two subs configured in such a method work nearly as good as four subs configured via our 4 Corner Placement recommendation. You will however need to stack two subs for each location to achieve the same output as the 4 Corner Placement unless you chose two subs with twice the output capability of the four subs you were planning on using.
Although it's been incorrectly touted as ideal to place four subs at each mid-wall location (since it offers consistent frequency response for each seat); this is usually NOT recommended because it is significantly acoustically less efficient than just employing a single subwoofer! Thus the maximum available output level of four identical subwoofers placed mid-wall will be at best equal to or at worst LESS than one subwoofer located in the front corner. In our opinion, this is a waste of power and money. We recommend avoiding this option for four subs unless you are stacking them for two locations or if your primary goal is the absolute best seat to seat consistency over maximum output.
Figure 1b. 4 Corner Placement
In you are running four subs, the ideal locations for them in a rectangular room are the corners of the room. Assuming all subs are identical, this will achieve up to 12dB of increased output when compared to a single corner loaded subwoofer. Placing the subs in all 4 corners will maximally reduce standing waves through destructive interference and provide the most linear and consistent frequency response for all seats. Used in conjunction with global equalization, this option provides nearly as good frequency response and seat to seat variation as the midwall placement with the huge advantage in efficiency. This can allow you to use smaller less powerful subs compared to the less efficient 4 midwall placement.
Figure 1c. 1/4 W Placement
Although this isn't spelled out in the CEA recommendation, Dr. Toole references it in his book as a good solution for two subwoofers but suggests additional subwoofers may be needed. In my experience I've had excellent results placing two subs against the front wall at locations of 1/4 the room width. I've had even better results placing two additional subs in a similar manner against the back wall. This configuration can achieve nearly as good frequency response performance as the 4 Corner placement with nearly as much bass gain as well.
Editorial Note about Subwoofer Placement & Combined SPL Output
Placing your subwoofers as symmetrically as possible with respect to the listening area can be useful especially if one favors a particular listening seat but we should attempt to make the bass good for all seats. If your placement options are limited, you will have to use trial and error to compensate by manipulating electrical delay of one or more of the subs (1 ms per foot) via the speaker distance settings (assuming your processor has multiple subwoofer outputs with independent delay settings and channel trims or can artificially add group delay by engaging one of the subwoofers internal crossover slightly (20Hz-30Hz) above or below your processor's crossover frequency.
If you can't achieve proper integration, you will have to consider moving the sub(s) to a more optimal location. If your subs are properly summing together, you will potentially gain up to 6dB for every doubling of subwoofers (corner loaded) used minus the loss for mode cancellations. Realize however that anytime you vary time delay, phase or EQ for just a single sub you potentially reduce the effectiveness of standing wave reduction or modal reduction. Be careful not to cancel the modal reducing benefits of multi subs while also endlessly chasing your tail in search of the best settings! The goal is to time align the subs to the room which is NOT necessarily the distance to the listening position. In order for more than one sub to get "traction" against the same room mode they must be symmetrically placed to the mode.
If you can't move your subs to improve bass performance, try moving your seats a couple of feet forward or backwards. Make sure you keep the seats away from room nulls which are typically located at 1/4" the width and length of the room. We want to achieve the smoothest response possible minimizing modal peaks and nodal dips so it's a worthy effort to experiment with subwoofer and seat placement. This will minimize the amount of EQ required later which is always a less effective band aid solution to proper placement Remember even so called "perfectly" rectangular rooms aren't truly rectangular due to door openings, windows, differences in wall construction, etc. Exact ideal subwoofer placement can't always be calculated, sometimes it must be measured. A good deal of trial and error is often needed.
I recommend using room mode calculators as a starting point but keep in mind they are only useful for parallel walls in non rectangular rooms. There are more sophisticated software modeling tools from company's like Rhintek via their program called Cara that appear promising though we will have to do further testing to understand its potential. I also attached a very useful room mode calculator courtesy of HAA Acoustics.
Also recommended reading: Listening Room Acoustics: Room Modes & Standing Waves
THX Subwoofer Calibration Recommendations
When discussing this article with THX, they suggested the following guidelines to follow for proper subwoofer placement and setup in which apply most closely to rectangular shaped rooms. Odd shaped rooms will have to experiment a bit more to find the best positions.
There are a few variations for subwoofer placement, depending on how many subwoofers you have in your room. Small rooms generally mess up the bass with "room modes." The five things that you can do to control it are:
- Choose a room with dimensions so that the modal frequencies do not overlap
- Place the seats where you are not at a modal peak or dip
- Place the subs so they help control the amplitude of the modes
- Use low frequency absorbers to knock off the peaks of the bass
- Equalize (as a last step when everything else has been optimized)
In general for rectangular rooms; if you have four subs, THX recommends starting with one in each corner, if you have two, put them in opposite mid-wall positions, if you have one place it in one of the corners along the front wall. Next listen to the seat to seat variation and move the subs along the walls until you achieve the smoothest bass coverage. Being able to move a separate subwoofer around the room to find the best location many times outweighs the benefits of a "built-in" sub. Follow subwoofer positioning recommendations, perform the proper measurements and always trust your ears. When determining the best subwoofer placement, you should always move your subs around the room to determine how the room affects bass quality.
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Recent Forum Posts:
gene, post: 715187, member: 4348can we get a video on this process in the future?
Good question. For years, I always measured at the listening area for each sub. It was a pain to match levels that way. Then it occurred to me when reading Dr. Floyd Toole's book that what matters is how the subs balance with respect to the room NOT the listening area. Remember in order to take full advantage of standing wave reduction, all subs must:
- be level matched with respect to each other
- be properly placed
- play exact same mono signal (LFE + all speakers set small)
The easiest way to measure each sub is to place the mic on the floor nearfield to the sub (within a few inches). If all your subs are single driver, then you can measure the SPL at the cone for each. You don't need to turn off the other subs if you do a nearfield measurement b/c the sound from that sub will dominate all others.
After you level match each sub, then go to the listening area and level match the main channels to the combined sub output. Once you flatten the bass of the subs, you can even boost their combined output a few dB over the main channels if you like.
Alternatively you can pick an equadistant point from all subs to measure their SPL individually but that can be difficult if they aren't all symmetrically placed in the room.