My multi location subwoofer thread inspired by Welti,Devantier,Geddes !!!

I like the Geddes approach. Here is a nice walkthrough: http://mehlau.net/audio/multisub_geddes/

This is for 3 subs, but you can do the same with 4. Just treat every sub beyond your first two like the 3rd. The key is that the subs in the rear of the room produce only a fraction of the total output.

I don't know what drivers you presently have, but you might be best served to keep the four you have up front and simply add 1-2 smaller cheaper subs in the back. Using larger drivers designed for significant low frequency output is a waste on those rear subs.

I don't think that using a single Bassis would be advisable. At a minimum you need level and phase control for each sub location (not per woofer, but per woofer group that is colocated). This is part of the "blending" process that is described in the link above. Using something like the DCX or MiniDSP is ideal IMO, but the phase and level control on some plate amps would be fine.

There might be other approaches, but I think this is a good practical and feasible route.

I just run mine as a total unit. If the 4 subs are identical I would just treat them as one giant sub and process all of them at the same time as a unit. IMHO.

Since mine are all within 2ft of the same distance from the main listening position and identical to each other I don't adjust their distance differently, I also run them all at the same output (gain) level so as to maintain maximum headroom. I also run them all over the same bandwidth. I EQ and level them all as a unit. This works for me and the response and power distribution over a large area of the room is pretty good. Even when I am sitting significantly closer to one over the other or at some other non optimum position I don't notice anything being off as far as a smearing in the time domain or whatever, this is probably helped by the fact that as you get closer to a sub it's response will dominate more and also that even in the worst position in my room the maximum difference in distance to any one sub of the 4 is only 6 or 7 ft probably. If you get into delaying subwoofers to match the time arrival at one position, and you move from that position to another the difference in time arrival can be more exaggerated due to the delay introduced to align at the main position. Keep that in mind if you have multiple listening positions spread over a decent sized area and want to get the best sound over a wide area. Also consider that even if you have one subwoofer 10 feet further away than another that is 10ms, or less than 1 cycle of delay in the arrival at 100Hz to the listening position not counting all of the many delayed reflections from the room boundaries. At even lower frequencies a few ms difference in arrival is probably completely inaudible. If you or you and a partner are the only ones listening and you only care about the response at those one or 2 positions by all means time align since those are the only spots that matter. Either way with identical subs there is no need to limit range or output level of rear or side placed subs which would handicap the system somewhat from a standpoint of headroom and dynamics, again IMHO, YMMV, FWIW ,etc. Being surrounded on 4 sides by subwoofers the bass is about as omnidirectional as it gets. I'm sure plenty will disagree with what I am saying here. I'm just throwing out a different viewpoint from someone who has 4 identical subs on 4 different walls and how I run them.


If you do have disimiliar subs then yes you will need to go much further into tweaking each individual sub to play the best together with the others as a total system. I prefer identical subs for that reason.

I like Room EQ Wizard (REW) because it works well and it is free.

For a mic, Dayton EMM-6 ($40) with a preamp like the Behringer Xenyx 802 (~$50). You connect the mic to the preamp and the preamp to the line-in of your computer's sound card. From there your software, such as REW will do the rest.

Another option is the Dayton OmniMic which is $250. It is a complete solution including mic, preamp and software.

Like Ricci points out, people have different approaches. Which you want to try is up to you.

+1, but....

With a laptop, you don't normally have a true sound card with a line input. Mic inputs are equalized. You may need an external DAC; I use a Behringer UFO202.
http://www.parts-express.com/pe/show...inger%20preamp

I've got a nice pair of 3 cu ft boxes if you'd like to test different locations....

HAve fun,
Frank

Another vote for the OmniMic system. I think it's still $50 off, even. I got mine and it's pretty damn cool but I haven't used it much... yet. Seems pretty easy to use and got mine set up in minutes. No soundcard hoopla to worry about either.

I'm of the mind of treating subs as Ricci mentioned but I know that's not the best approach for every person, every room or subwoofer 'system'. In your case, KG, I'd keep the quad Tumult's up front and then maybe try out a Geddes style multi-sub system as Coctostan mentions with two more lesser subs. That's what I'd do. Keeps your Bassis in-line doing it's thing on the proper subs and then you could use a couple smaller subs to smooth out the bass if you need to.

Yeah, pretty much but you don't need uber subs for the Geddes treatment. Seriously, just pick up two 250w plate amps, make two 15" cubes, put a single 10" Dayton RSS HF in each one and try that. Hell... you could just buy a couple of cheap-ass whatever subs from PE and try with those first. No fuss, no muss. Or however that goes.

I forget about that. I have a Creative USB something or other that is a soundcard and mic preamp. I think it was about $100 but I've had it for a while.

The OmniMic doesn't look like it is on sale right now. I think it is a significant premium to pay, but others obviously think the simplicity is worth the extra cost.


In fact, a single sub in the rear might be all that is needed. Ideally, two of the Tumults would be in the corners and two would be somewhere across the front of the room. The rear sub would be best placed higher than half the wall height.

Those are just generic ideals in my experience. They won't be ideal for all rooms and even if they are optimal for a specific room, compromises for practical reasons aren't drastic compromises.

To start, I'd just leave the Tumults as is across the front and add something like the single RSS HF sealed for the back. Just build something that fits your space.

back up to post #3.

http://www.harman.com/EN-US/OurCompa...s/multsubs.pdf

With OM you play the CD in your AV system disk player.

"Ok, i'll try to digest this, whow 30 pages......."

you asked a very complicated question. if you just want the answer...

I'm using #3 in the picture above. Unfortunately the listening position is close to the center of the room for me so that causes some big response variations but it's what I (and many others) have to work with.

Nah. That's the price no one pays. It's always been $300.

As an OmniMic owner for over a year, I'd be glad to chime in. I'd measure close mic each enclosure prior to proceeding with anything else. Pot down everyone but the DUT, and then begin one sub at a time. Just a good practice to possibly flesh out potential issues you may need to address.

Then, proceed with LP measurements.


Good luck

I flank my mains with the dual subs I have now, have measured the results and the improvements made are significant, especially with respect to dealing with the main's floor bounce nulls. I plan to add two more following the Geddes or Romeyn method while retaining the flanks.

A good discussion describing the positive effects of flanking and multiple subs. http://www.diyaudio.com/forums/subwo...ubwoofers.html

Device Under Test

I think that all 3 approaches discussed in the diyaudio thread are valid. The end result is the important part. Reducing the seat to seat variance WITH as little EQ use as possible is the goal.

With the Welti approach (symmetrically placed subs), it is done using standing wave theory that assumes a rectangular room with essentially identical walls. The drawback is that rooms rarely fit this category, but the approach works quite well in many instances. The drawback is that although the response is nearly identical at each seat, lots of EQ must be used for a flat response, and the overall LF contribution of multiple subs is not always realized, especially with 4 subs at wall midpoints.

The Geddes approach is more experimental, and also provides for cancellation of some floor/ceiling modes. By experimentation and measurement, a solution can be had with less subs.

The Parham approach is between the two, with an overall increase in LF radiators.

All three approaches are valid. The endpoint is of course measurement. Although none of the approaches breaches the subject at length, absorption on front and rear walls can significantly affect 1/4 wave suckouts in the 100-300Hz region. To a certain extent, absorption can also help with <100Hz content, but not nearly as much as mode cancellation with multiple LF sources. A combination of the two + DSP control (absorption + multiple sources + DSP control) can yield a very flat freq response and little seat to seat variation, as Toole experienced in his own living room (Room 3 at the end of the paper):

http://www.aes.org/tmpFiles/elib/20111227/13680.pdf

Start at section 6 of the paper, and this is the basic algorithm used by the JBL BassQ unit. It allows for optimized use of delays, levels and a single band of EQ per sub to reach a solution that allows for the least variance seat to seat plus as flat a freq response as possible using minimum EQ. A disadvantage to the BassQ system is that it only allows for four measurement locations (you use four mics simultaneously during setup). It also does nothing below 20Hz, a problem for the ULF-loving people here.

Since there is no established algorithm on how to do a BassQ type optimization with a laptop and single mic, experimentation and measurement are key.


JSS

As far as I can tell, the Welti paper discussion is confined to two dimensions.

The height mode might be addressed the same way by placing the opposite subs at floor/ceiling.

Or half way up the wall, as that would then fail to excite the first floor/ceiling mode.