Rules for matching subwoofers to room size

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 ;-)
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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.

>Knownothing
>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.

>Eldartford
>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.

Duke
dealer/manufacturer

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.