Others experience re: subs and Magmepan 20

I designed the Swarm subwoofer system mentioned above, and worked with Jim Romeyn on his conceptually-similar Distributed Bass Array system.

The Swarm was originally designed with dipole speakers in mind, specifically Maggies and Quads. Briefly, dipoles have smoother in-room response in the bass region than monopoles do (according to a peer-reviewed paper by James M. Kates). Smooth bass is fast bass, subjectively and literally - in the bass region, in-room peaks happen where the energy decays more slowly, and vice versa. In fact, the two are so interconnected in the bass region that if you fix the frequency response, you have also fixed the time-domain response. So the key to matching the "speed" of dipoles is, match their in-room smoothness.

Two subs intelligently positioned will be roughly twice as smooth as one sub. Still not as smooth as two dipole main speakers, but definitely better than one sub. Four intelligently positioned subs will be twice as smooth as two, and comparable in smoothness to a pair of dipoles.

The improved in-room smoothness that arises from using multiple subs intelligently distributed may seem counter-intuitive, so let me try to explain: Imagine you are looking at the in-room frequency response of a single sub. You see a few big peaks and dips. The problem is not only their magnitude, but also how far apart they are - you see, the ear/brain system tends to "average out" peaks and dips that are within about 1/3 octave of one another, but these are almost always going to be further apart than that. Small rooms are typically worse than big rooms in this respect.

(Also, the ear is especially sensitive to changes in loudness - peaks and dips! - in the bass region. This is shown by the way that equal-loudness curves bunch up south of 100 Hz. So there is usually a LOT of room for improvement in the bass region.)

You can move the sub or move the microphone location and those big peaks and dips get shuffled and changed, but they do not go away. Now imagine that you add a second subwoofer in a different location. It too generates a big peak-and-dip pattern. But unless the second sub’s peaks and dips overlay the first’s (which can only happen if the two subs are in the exact same spot), the sum of the two will be significantly smoother than either one alone. Mathematically, it will tend to sum to half as much average deviation (or "twice as smooth"). The same trend holds as we add more subs. And not only are the peaks and dips smaller in the summed response, but they are also more numerous and therefore closer together, so the ear/brain system’s averaging characteristic comes into play, and the subjective improvement is probably greater than one might think from eyeballing the summed curve.

Anyway my point is, there is some solid acoustic and psychoacoustic science behind the distributed bass array concept. Credit to Earl Geddes for sharing his ideas with me and allowing me to use them.

Duke

Thank you very much, Tim! Glad the Debra is working well for you!

The only question mark in my mind regarding imdoc’s situation is, whether his room has something going on (large area and/or open floorplan perhaps?) that results in minimal boundary reinforcement in that bottom octave. It sounds to me like his primary issue is a shortfall in bass quantity, to the extent that if there is a qualitative mismatch between subs and his big Maggies, it hasn’t been obvious yet.

Both the Debra and the Swarm are "voiced" with the expectation of about 3 dB per octave of gain from boundary reinforcement below 80 Hz or so. There is a fair amount of adjustability built in, between the equalization available in the amp and the other setup options, but it is possible that he needs more bottom-octave energy than our systems provide in their "stock" form.

So while I think we have a good solution in the room-interaction domain, in this particular situation I don’t know whether we’d meet the quantitative requirements. Not that the four 10" subs in the Debra and Swarm systems are wimpy (their motor strength is unusually high, and many of our customers have measured in-room -3 dB points around or below 20 Hz), but maybe he’d be better served by four 15" subs that are voiced "flat" rather than voiced with typical boundary reinforcement taken into account.

Duke

Thank you for your kind words, imdoc! I enjoyed talking with you very much. One thing I forgot to mention - you’ll probably have to EQ in a fair amount of low-end boost in your room. I suspect that the big opening behind the listening area is acting sort of like a bass trap and reducing your in-room bass. You might also experiment with putting the two subs in phase quadrature (90 degrees apart) - it may be beneficial.

Tim, if you look up imdoc’s system (just click on his name), you’ll see a photo of his room. It is a beautiful space that is not a dedicated audio room, and in my opinion multiple subs distributed around the room would ruin the aesthetic he has achieved.

The room is open at the end that’s "behind your back" from where the photo is taken, so that room dimension is not distinct. The other room dimensions (width and height) don’t support room modes below 30 Hz, and he’s only looking to augment the Maggies south of 30 Hz, so the net value of the modal-region-smoothing we get from a distributed multi-sub system doesn’t apply as much. In this situation, I think the more cost-effective and aesthetically practical solution would be two high-output subs that fit in the areas behind his Maggies.

At first I was thinking that a pair of tall two-woofer subs dimensioned to fit behind the Maggies (and shaped to be Maggie-backwave-friendly) might work well, as with one woofer high and one low, we’d at least have our bass sources distributed in two dimensions (height and width). But then I thought about his room dimensions and the modes those dimensions would support and decided that the benefits would be minor, and that it would be more cost-effective for imdoc to just get two powerful conventional subs rather than paying to have expensive custom enclosures built. So I suggested he look at Rhythmik, though they are by no means the only viable choice out there.

Duke