Rules for matching subwoofers to room size

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

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.

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.

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,

Duke