Rules for matching subwoofers to room size

I am setting up a new music listening room and will be adding a subwoofer for movies and (mostly) to fill out the full audio spectrum for music. Is there a rule of thumb for subwoofer size/output for a given room volume? My room will be about 11x20x8 WxLxH. Thanks in advance.
Think two, rather then one large. Two ten inch, front firing should do the trick. For a discussion of 'two' check out floyd toole's 'sound reproduction' book. I have two 12 inch NHT units and have been able to verify that when only one is used, the 'coverage' is not as great as using two.
I don't know of any rule of thumb for matching room size. Velodyne has a tool on their web site based on the room volume. That may help.

I wouldn't pick a 10" sub(s) unless the room is really tiny -- your room isn't. Better to have a much larger sub than one too small.

I agree that two subs will yield a smoother response over a wider area if you have freedom of placement.
Room size is important but not first on the list.

The first thing to consider when choosing a sub is it's compatibility with your main speakers (you didn't say what they are? ;-)

You need to know/decide where/what the crossover frequency will/must be between the main and sub. It sounds like you're going to run the mains full range and just fill in with the sub (what I always recommend.) Some like to roll the mains off higher than their lowest frequency capability (to put less strain on their main amplifier) and let the sub take over below that.

The one-versus-two subs debate need not be contentious, and has a relativly simple solution: If your sub will be operating under 100Hz (under 70Hz preferred) there will be no directional information (that the human brain can identify) below that frequency anyway; and it's much easier to position a single sub for seamless matching to the main speakers. A good sized multi-driver sub like a MartinLogan Descent is my choice. Why? Plenty of output, but also great transient response with (3) 10" drivers rather than one 12" or 15" driver. The REL and the JL Audio also have great reputations.

It's also much easier to locate a single sub. Just plop it in your listening chair and then wander around your room listening for the spot(s) where the low frequencies sound best. Those are the spot(s) where you should locate the sub when YOU are in the listening chair ;-)
IMHO, If there is any stereo information below 70 Hz (and with an LP there won't be) the SW will be easily located.

I would stay away from 10" subwoofers which achieve their SPL by extreme cone excursion that results in distortion.

A quality 12" or 15" driver will have a much stronger magnet/voice coil than a 10" driver, and so can handle transients just fine.
>I am setting up a new music listening room and will be adding a subwoofer for movies and (mostly) to fill out the full audio spectrum for music. Is there a rule of thumb for subwoofer size/output for a given room volume? My room will be about 11x20x8 WxLxH. Thanks in advance.

As big as possible for output below the fundamental resonance. If you'll be using the same woofer for more than just sub-bass you'll want to pay attention to its output at hiher frequencies.

Above the fundamental resonance you want multiple sub-woofers to minimize the depth of the nulls and height of the peaks, with the references being Todd Welti's paper and Earl Geddes' work.

Earl's solution using three randomly placed woofers with one near the ceiling will provide the best performance; with only one needing to be capable of sub-bass.

>IMHO, If there is any stereo information below 70 Hz (and with an LP there won't be) the SW will be easily located.

With output levels matched with pink noise, a 4th order low-pass filter, freedom from distortion and port noises, and no nearby structural resonances you'll be fine.

If a subwoofer has a steep crossover at 80 Hz or below you can place it anywhere in the room and not be able to hear its location. The reason is, at 80 Hz and below wavelengths are so long that the ear cannot tell what direction they are coming from because the arrival time difference from one ear to the other is too small of a fraction of a wavelength. Now the 80 Hz figure comes from tests done with pure sine waves; in a sound system where you have main speakers reproducing the upper harmonics which are giving you strong directional cues you can probably run the subs above 80 Hz and they won't give away their location as long as the lower midrange energy is filtered out.

Note that we localize the direction of bass instruments (kickdrum, bass guitar, double-bass, organ, synth) from the higher harmomics, not the fundamental.

In my opinion the main argument for using multiple subs has to do with smoothing out the in-room response, as Drew Eckardt says. The roughness of the in-room bass response is approximately inversely proportional to the number of independent bass sources within the room, but that being said not all multisub placement strategies are created equal. Using multiple subs is not a cost-effective way to maximize output level if that's the top priority; a single powerful ubersub will almost always go louder (and deeper) than its dollar equivalent in multiple smaller subs.

As far as subwoofer output capability, the rule of thumb is that you want your sub(s) to be able to keep up with your mains, but overkill isn't worth paying for. If your mains audibly distort at 105 dB and you have no plans to replace them, there is little point in getting a subwoofer system that can do 115 dB.

Now we are talking - thanks all for your input.

I actually do not have the mains (or really much of any of the equipment) I will use in this room yet. Most of my current two channel system will go to my new modest sized living room and I will start from scratch with gear for the new larger audio/video room. I will do my best to set the new room up with as few acoustic problems as possible from the start, and then demo/buy/sell/trade until I find the speakers I like in that space. I am definitely leaning towards floor standing speakers (and amps that can drive them) that can honestly go down to at least 40Hz without straining, and then supplementing lower bass with a sub - thus this thread.

I have a small Klipsch sub in my current small (12'x14') theater room, and while it is a clean little sucker, it will not go much below 30Hz nor pressurize the new larger planned listening room to the levels I seek. I have worked to integrate my current sub with various configurations of speakers and amplifiers - so am pretty handy with a spl meter and test disk. To be honest, even with cutoffs above 80Hz I have a hard time locating the source of the bass once the sub's position and phase is dialed in properly.

The reason I asked the original question is that I believe I have read that there is an inherent limitation on the bass frequencies that can be reproduced related to room length and to a lesser degree room volume. I guess what I really meant - is in a 20 foot long room, how low of bass notes can I reasonably expect to reproduce? And, if there is a limitation, what is the point of getting a sub that can in theory go down to 19Hz if my room can only allow me to hear or resolve 25Hz? I am way off here? Your continued patience and input much appreciated.
This business about not being able to locate a subwoofer is simply untrue in my experience.

Perhaps it is based on experiments with pure sine waves. With the possible exception of a flute, musical instruments don't produce pure sine waves. A drum is often cited as an instrument that energizes a subwoofer, but a drum also produces percussive sounds at much higher frequency. These sounds should eminate from the same location as the deep thump.

Of course, you don't need to decide on a SW location until you get one, and when that happens you can try out various locations and decide for yourself.
Eldartford, note that I specified "steep crossover". The typical 12 dB per octave lowpass filter is too shallow; you need a 24 dB per octave filter if the crossover point is going to be up around 80 Hz or so.

I repeat that the ear cannot localize sounds below 80 Hz; it is physiologically nearly impossible. When you hear the location of a kickdrum, your ears are using the upper harmonics and skin tone to determine its location. The bottom end of the kickdrum sound will seem to come from the same location as the upper frequencies, but it doesn't matter whether it really does or not.

Let me explain a bit further. The ears localize sound from the first .68 milliseconds of a sound impulse, which corresponds to the roughly nine inch path length around your head from one ear to another. At 80 Hz, your ears can barely detect less than 1 wavelength of sound. If a system were to play 1/2 of a wavelength at 80 Hz, with no overshoot or ringing at the end of that 1/2 cycle, you would not even hear it even if it was very loud. One cycle at 80 Hz takes 12.5 milliseconds to complete. So by the time the ear/brain system even begins to detect the presence of a low bass tone, we are way, way past the .68 millisecond sound localization time window.

You are correct that pure sine waves do not exist in music, with the possible exception of the flute. But the only way to study the ear's response at low frequencies in isolation is to use sine waves, which contain zero harmonic energy. There are no other waveforms which contain zero harmonic energy.

Anytime you can hear the location of a subwoofer, the system is either set up incorrectly or the subwoofer's lowpass filter is too shallow.

Knownothing, the ear's ability to hear low frequency energy is unrelated to room size. If it were, you wouldn't even hear midrange energy, much less bass, from headphones - wherein the room is formed by the headphone cupping around your ear.
One other situation that would allow you to hear the location of a subwoofer would be if the system was poorly designed, such that the subwoofer had high harmonic distortion (or a buzz or rattle) or port noises. Drew Eckhardt mentioned this in his post above, and he is absolutely correct.
Thanks, helpful.
Audiokinesis...I think it "really does matter" that the thump of the drum and the higher frequency percussive sounds come fromn the same location...for phase coherency if nothing else. Contrary to what you said I think that most subwoofers have 24 dB crossovers, not 12. Mine do.

Of course I have heard all your theoretical arguments before, but perhaps others have not. My bottom line is that Knownothing can decide for himself when he gets the SW.
Eldarford, one of the problems here may be in our definitions of terms. For example, you use the term "thump of the drum", and to me that implies harmonic content sufficient to allow pitch to be discerned. The pitch of bass instruments can be difficult to discern without the harmonics; try listening to a properly low-passed subwoofer with the mains off. Anyway if "thump of the drum" includes harmonics in the region where the ear is good at judging directions, then yes where those harmonics originate from does matter.

Phase coherence has become a buzzword, but is actually a relatively low priority from a perceptual standpoint. You see, the ear does not hear waveforms; rather, the inner ear deconstructs and processes incoming sound energy in a complex way that changes with frequency. The ear is especially poor at resolving phase in the bass region, as can be inferred from one of my previous posts. But the ear is pretty good at hearing
certain types of variation in frequency response, so I would argue that there's where our attention should be focused if high quality is the goal.

My bottom line is that acoustics and psychoacoustics can be highly non-intuitive, but quite a bit of real-world applicable research has been done.
Audiokinesis...You are correct about phase coherency being overrated by audiophiles. I threw that out as one plausible explanation of what I personally hear even if you don't :-)

Many years ago I read an interesting observation about an experiment involving phase. An irregular waveform like a steady audio signal can be subjected to Fourier analysis and represented by a collection of sine waves with different frequencies and amplitudes, and phases. If enough of these frequencies are used the original time domain waveform is very accurately reproduced. You can play back the collection of Fourier components (sine waves) and listen to them. They sound like the original irregular waveform.

Now comes the interesting part. Arbitrarily change the phases of the Fourier sine waves. Although the same frequencies and amplitudes are present the reconstructed time domain waveform is now quite unlike the original, but the sound does not change. This implies that our sense of hearing functions in a rather complex and unexpected way.

As I said, I read about this experiment, and did not hear it for myself. I would sure like to.
Very interesting, Eldartford (spelled your name right this time)! Don't think I've read of that particular experiment before, but it certainly is a vivid illustration.

Best wishes, regardless of whether we ever end up agreeing on subwoofing,

Whats your budget, why not get real full range speakers..I have Revel F52's and they presurize my room everone looks for the sub and their is non
Funny you should mention the Revel F52 - was considering a pair of used F32's for the fronts. Budget for whole 5.1 or 7.1 speaker system including sub around $5K+- used. I have some large and small bookshelf speakers that could serve as surrounds for a while - so really only looking for 3.1 system within that budget.

I think something like the F52 would need a lot of power to accurately recreate the lowest frequencies with the aplomb of a good subwoofer. They are also pretty darn big. But perhaps ditching the subwoofer for larger front speakers and a more capable amp would be a reasonable trade off. Then I a would only be looking for a "3.0" system plus breaking the electronics budget (also $5K not including screen) a bit on a stronger amplifier...
The F52 are realy not that big, or should I say take up floor space, they are about 10" wide and 15 deep. I run them with a PASS X250 that runs in clas A for the first 40 watts, which is 90% of the time and they sound supperb (Best amp I ever owned), when i crank it they do take the power when pushed.

After years of playing the buy and sell game this is the system I have ended up with and have had the longest and I'm not planning on changing soon. Its my nervana (VPI SuperScoutMaster is my front end)

I see F52 on here for 3500 to 4000 used. F32 are around 2000

Good luck

I like the the overall presentation of the Revels - the F52's are very nice and I think a good value in a full range speaker - not very far off the Salon Studio2 IMO. I spent a fair bit of time with the M22's and enjoyed every minute, but they are current hungry. Will probably end up trying a few different speakers to see what works best in my space. I do like that fact that Revels allow you to adjust the tweeter level and optimize sound for placement in the room.
Finlbxn, I'm sure others can shed more light on this, but my understanding is that full range speakers have a fundamental limitation in that the bass can not be decoupled from the rest of the audio band so no matter what placement you choose it will be a compromise (usualy in the bass). Having a decoupled bass driver, i.e., subwoofer, allows one to place that driver in a more ideal location for smooth bass response.
But perhaps ditching the subwoofer for larger front speakers and a more capable amp would be a reasonable trade off.

I think this would be a big mistake. Being able to biamp with a subwoofer plus the placement flexibility is a huge win.