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-   -   Multiple Small Subs - Geddes Approach (http://www.diyaudio.com/forums/subwoofers/134568-multiple-small-subs-geddes-approach.html)

Horizons 9th December 2008 05:14 PM

Multiple Small Subs - Geddes Approach
 
After reading about the AudioKinesis "Swarm" sub system and the Geddes paper on loudspeakers, I am fairly convinced that 3-4 randomly placed small subs are well worth trying in both 2-channel and HT rooms. Geddes specifically recommends multiple and different (not matching) subs to force more randomness (and I assume averaging) of room modes. The Swarms are matched drivers/cabinets.

My question is which small commercial subs would be appropriate for this theory? There are hundreds of small 8, 10, and 12 inch subs that are sealed, ported, etc. Has anyone tried this approach and can anyone recommend a combination of subs? I would like to use the smallest possible subs given the need for 3 or 4 or them in my room.

JRace 9th December 2008 05:33 PM

I am currently trying the different/multiple sub idea.
I am using (1) 12" Adire Shiva in a Ported Sonotube tuned to 20Hz, and a Aperion Bravus sealed sub with dual 8" drivers - the manufactures measured -3dB point is 35Hz.

Current setup:
Shiva from 20-40Hz, and the Bravus from 35-100Hz.
At the momement the Shiva is in the back corner of the room, about 5 ft from my listining position. The Bravus is up front, withing 1ft of the main RT speaker.

So far I enjoy the bass, sounds to my ear to me more balanced and even. Not had any chance to measure yet.

Cal Weldon 9th December 2008 06:57 PM

I am yet to hear a multi sub system that didn't sound cluttered and muddy. I will stick with one bass unit thank you.


Drew Eckhardt 9th December 2008 07:01 PM

Re: Multiple Small Subs - Geddes Approach
 
Quote:

Originally posted by Horizons

My question is which small commercial subs would be appropriate for this theory? There are hundreds of small 8, 10, and 12 inch subs that are sealed, ported, etc. Has anyone tried this approach and can anyone recommend a combination of subs? I would like to use the smallest possible subs given the need for 3 or 4 or them in my room. [/B]
You might consider a Linkwitz Transform with a couple of output buffers each having an adjustable level.

The Linkwitz Pluto+ sub woofers (Peerless SLS10 with a Linkwitz Transform) are about half a cubic foot (external dimensions 14 inches square x 7.5" high with 3" feet in a down-firing confiuration)

markus76 9th December 2008 07:46 PM

Re: Multiple Small Subs - Geddes Approach
 
Horizons, multiple subs is the best and only method to smooth the low frequency response when you can't install (any more) low frequency absorber. I've documented it here:
http://www.mehlau.net/audio/multisub_geddes/

Best, Markus

Horizons 9th December 2008 07:50 PM

THANKS Markus :D

That is exactly what I was looking for.

planet10 9th December 2008 08:29 PM

Guess i should try hooking up the spare sub i have in my room already :)

dave

breez 9th December 2008 09:41 PM

Quote:

Originally posted by JRace
Current setup:
Shiva from 20-40Hz, and the Bravus from 35-100Hz.
At the momement the Shiva is in the back corner of the room, about 5 ft from my listining position. The Bravus is up front, withing 1ft of the main RT speaker.

Multiple subs should cover a wider range of frequencies together, otherwise there is only one LF source at a given frequency. I suggest raising the 40 Hz lowpass of Shiva.

Pallas 9th December 2008 10:07 PM

What I use, and I'll leave it to Dr. Geddes or someone else to say whether it's the most theoretically sound approach given the equipment I have on hand, is the following:

Corner ultra-low-frequency sub: Exodus Audio Maelstrom-X in an either a ~100L or ~120L (don't remember, and it doesn't really matter) sealed box, equalized ("Linkwitz Transform" of sorts) and powered by a Crown XTi2000. This subwoofer is near the front-left corner of my room. I believe it plays up to 125Hz or so.

Broadband subs: I use two commercial subwoofers, both M-Design Audio's Eleganza Bella. I chose these for two reasons: aesthetics and their use of Aura's underhung NS12-794-4A drive-unit. The ports are closed off. One is at the right "end" of the front wall (it folds into a hallway), and the other is used as a couch end-table, about 42" from the back wall. They are "tuned" differently simply because of their different relationships to nearby boundaries. The highpass of one of them is maxed out, and the other is set to ~90Hz or so, but I don't remember which is which. Points were chosen by iterative measurement-listening-fiddling-measurement loops.

The ULF sub is considerably more capable than the BB subs, with all that power driving an 18" driver with over 30mm of rated xmax compared to 12" drivers with 15mm of xmax, but I have not noticed any signs of overload from the smaller subs at any volume level. Perhaps the main sub is simply overkill, but better overdone than underspec'd.

Here are some pics, for a general idea of how I did it.
http://img206.imageshack.us/img206/995/p1000584bi4.jpg
ULF sub. The Auralex Gramma was completely useless when used under the sub as intended, but the cat likes to play on it so I put it on top of the sub for her to lie down on. Mains are Tannoy 12 DMT II's in bespoke sealed cabinets, designed with baffles the same size as the originals (considerably less depth, and rounded rather than square) but modified with much larger roundovers in light of some of Dr. Geddes' points about diffraction. The Church's house slippers in front of the sub serve no sonic function. :)

http://img503.imageshack.us/img503/8148/p1000585pi5.jpg
BB sub 1.

http://img142.imageshack.us/img142/2720/p1000581he1.jpg
BB sub 2.

My current setup (unlike my previous one) violates Geddes' dictum that one subwoofer be above the centerline of the room. However, the mains as you can see are well above the room's centerline, so there are three 12" sources playing in the bass.

As for commercial subs, I would look for anything that's relatively small and sealed or 4th order bandpass. (Sealed/4th-order bandpass simply because I think the beginner would be well advised to avoid the possible problems that might come from the steeper LF rolloff of vented subs.) Maybe a 15" for the "ULF" sub, and 10's or 12's for the "Broadband" subs.

But don't forget the other keys of the Geddes approach, which are to run your mains full-range (LFE+Main and DoubleBass are common names for this mode on modern receivers) and set them up along the lines of Markus's excellent page. That includes both the ideas about placement and use of crossover frequencies that seem awfully high by the misguided standards of conventional wisdom.

jlharden 10th December 2008 12:15 AM

Hi Pallas,

I'm interested in your subwoofer setup. I have been thinking about the Maelstom-X sealed and powered by a Crown K2 for ULF duty. It would be in combination with two Rythmik DS15 servo subwoofer kits currently in use. More output isn't needed, but more uniform response throughout the room would be a desirable goal. The Rythmik's are quite capable at the extremes, but may benefit from a little roll-off to limit excursion/distortion with the Maelstrom doing its thing. What's your thoughts on the Maelstrom's sound quality, frequency response, etc. Would you make the same driver selection if you were to do the ULF setup again?
Thanks for your insights!

Jerrod


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