Multiple Small Subs - Geddes Approach

[gedlee]

Originally posted by Pallas
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.

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.

The "above the centerline" is not a dictum, but a recommendation. I've often done without it, it just worked better that way in the simulations. But the real world is more complicated and we can never get such a clean setup as in a simulation. Hence, the actual field adjustment becomes the key. I don't think that you will get the optimum without some measurements, although listening can be sufficient at times. Markus procedure is correct and it should at least be tried. But, each room is different and each time I do this it comes out different. Its always an improvement, but nearly always different levels, etc.

[gedlee]

Originally posted by Nanook
and of course it all depends where one might cross over the sub(s).

Stew

In my setup there is no "crossover" to the subs. All LF sources overlap responses.

[Wayne Parham]

Originally posted by Cal Weldon
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.

There are two or three schools of thought on this, but I believe most everyone agrees that muliple subs in a small room is a good idea.

Outdoors, this would not be the case. You would want everything within 1/4 wavelength, if possible. Where not possible, you would still want to setup where interference were minimized, to put the lobes where you wanted them to be, in the listening area.

Indoors the problem is every sound source is accompanied my multiple virtual sources further than 1/4 wavelength away. They're the reflections from the room boundaries. So the idea is to average the sound field using a handful of sound sources. It is a way of using dense interference to smooth the sound field.

You can position the subs in any number of ways to do this. Welti suggested a handful of arrangements. Geddes suggests a random arrangement, one in a corner, one above mid height and another in a random spot. I like modeling the room to get a rough idea where to put the subs, then measuring to get it just right.

• How many subs are enough?

[markus76]

There can be only one school of though because it's just a physical phenomenon. Modeling the room is IMO a waste of time. You only get the first few modes right and then everything becomes sort of random in a real room. For calculating the first modes a hand calculator is sufficient.
Earls method is trial-and-error but doesn't take much time. You can place the subs anywhere you want and still get best results. At least as good as the location of the subs allow.

The only faster method I know of is that:
http://www.jbl.com/home/products/pro...at=EQU&ser=PER

[Wayne Parham]

The schools of thought I'm referring to are these:

1. Single monopole sub
2. Single dipole sub
3. Multiple subs arranged per Welti
4. Multiple subs arranged per Geddes

I think most everyone agrees that Welti or Geddes multisub placements make more uniform bass response in a small room than single sub installations. But there are differences between Welti and Geddes approaches. The main one is that Welti suggests symmeytrical placement and Geddes suggests pseudo-random placement.

If you use measurements to setup your subs, then I suppose modeling becomes less important. But I think it is a worthwhile step. It is easy to do and can help you know where to start.

[markus76]

If you would have read Welti's paper than you would know that Welti's SFM and Earl's approach are basically the same.

[gedlee]

There are differences in Welti's approach and mine. But the basic fact is that Welti only tried symmetrical placements, not random and I tried both. He recommends four as optimal, I recommned three, but in different setups. Its also true that Welti's modeled room was ideal - no doors or furniture, perfectly rigid walls, etc. and this is never the case in the real world. So how do you do Welti's recommendation? Well you can't really and it ends up being basically random anyways.

And I agree that I wouldn't bother to model the room if I were going to use measurements in the end to set it all up. I never worry about "where" the modes are, I'll find these soon enough when I measure the setup. And then I know exactly where they are.

[markus76]

I guess you too should read Welti's paper
http://www.aes.org/e-lib/download.cf...80&name=harman
Start reading from 6.1

[Wayne Parham]

What's with the attitude, Markus? I've usually considered you to be fairly level headed over the last few months of reading your postings. Your last few posts seem fairly uncharacteristic of you.

I've read both Welti's and Geddes' approaches, studied them in detail, modeled them both, tested them both, measured them both. I've tried various techniques and crossover approaches, more like what Geddes likes to call it, "blending" slopes of overlap.

I'm basically a horn guy, and have been championing uniform directivity loudspeakers for decades. I raised some eyebrows on some of the forums about five years ago when I said basshorns weren't the way to go for home hifi. It was like heresy for a horn guy like me to say I preferred direct radiators for subs, at least for home hifi. I make a pretty massive hornsub, but I never recommend it for home use. I always suggest multiple subs instead, usually made from a simple direct radiating LAB12 woofer.

So please don't mistake me for some guy that just stubled onto this idea. I've been doing the multisub thing for years.

[markus76]

Sorry Wayne, but I really don't see any difference in "SFM" and "Gedlee Multisub".