Originally Posted by coctostan
FOH, I think you bring up legitimate concerns but ultimately many of these proposals are difficult to implement in a home or even a dedicated HT. PNW's room is about as over the top, from a performance perspective as it gets. Not many people can pull that off.
What I'm suggesting is merely examining the cause of the null.
The course of action I put forth had it's genesis in the fact that the OP didn't want to relocate his IB drivers to the rear, and wanted an economical approach of just a few hundred dollars. This is why I shifted the attention to acoustics. Yes, PNW's Octagon is over the top, however I used it as an example of a planar array LF section, which is essentially what Chris has implemented. That said, PNW's acoustic treatment approach to his rear wall is dirt cheap. Either using roofing felt, or equivalent limp mass membrane, or some plywood (or equiv) diaphragmatic absorber attenuating the problem return frequency. By sizing the plywood appropriately, and with an adjacent 703/705 fiberglass to change the resonating plywood energy into heat. Simple, not too expensive, however it would take some simple experimentation.
It is said that plywood at these thicknesses target these respective frequencies; 1/4"=110 Hz, 3/8"=87 Hz, so increasing to 1/2", or 3/4", or perhaps adding mass would fine tune the native resonant frequency of the panel. One could easily implement this pressure based approach at the wall, then in front of this, a method of velocity based absorption bass trap working in concert but more broadband.
Originally Posted by coctostan
I would still use a multi-sub setup with a treated (meant as any and all options...not just traps) to get smoothing across multiple seats. I've never seen a small treated room that couldn't benefit. Of course I've not come across one that was able to implement all of the aforementioned solutions.
I agree. He's currently using a multi-sub setup, the current IB is not addressing front to back modes, but he is addressing lateral modes by spreading the LF sources side to side. I do agree, a multi-sub set up of distributed sources works wonders in smoothing frequency response aberrations. I just wanted to look at the cause.
The more I look at 1/4 wave cancellation, it does dominate the peaks and valleys of our response.
Originally Posted by coctostan
I still believe Geddes's formula is the most practical setup. The key is to get the majority of your output and subsequent smoothing effect from the two primary subs. The key there is having one in a corner and one at some other point along the front wall near the mains. With a little experimentation, this alone can achieve most of the benefit. These subs need to be capable of full output.
The third and/or fourth sub locations are meant more for reducing variance across seats than for smoothing the response. These do not need to be capable of full output. The key is to keep the levels low. I don't mean to pick on KG, but I believe he was trying to get a "filling" effect out of his rear subs and was asking for too much output from them.
You are going to get most of the filling effect from the first two front full headroom locations. Ideally, you can experiment with both locations until you get the best spatial average across all seats. This obviously doesn't apply with a single manifold IB since you are basically stuck in the mud.
KG should have separate his 4 Tumults into two boxes and placed them based on experimentation and measurements at ideal locations across the front. From there he likely would have filled all major holes and could use the rear sub to reduce variance across his seating positions.
IMO, the inability to experiment with sub locations is the biggest catch with single manifold IBs. Of course, prior to construction, you could experiment and come up with 2 locations across the front wall the yield the best results and then place your manifolds in those 2 spots.
So what should the OP do? Well, one option would be to add a 2nd front wall sub that is nearly as capable as his IB. I have a feeling that is not in the cards due to size and cost. The other option would be a rear sub that has significant capacity from ~30-80hz. This won't get results on par with the Geddes formula but should yield improvement.
Many good points. Again, I agree on the multi-sub approach, Geddes or otherwise. I think keeping things simple, wrt capability and strain, and retaining identical output per sub. Clearly this would be preferable all things being equal. I'll be implementing many of these principles soon, blending an IB with highly capable filler subs. But high in my priority list is phase linearity over as wide a freq range and over as wide a listening area as possible.
Yes, any IB install should ideally measure with whatever method possible prior to committing to a location. Stiff boundaries sharpen the Q, lossy boundaries lessen these effects. My biggest issues in my room aren't front to back, but laterally. This is easily addressed. Actually, whichever axis in which the problem arises, spreading the LF sources across the median point typically addresses the issue with somewhat of a selective mode cancellation.
So what should the OP do?
The highly capable rear sub you suggested would help, just as it did in the experiments. But I've got no experience with blending alignment types and maintaining decent phase response etc., so I don't know how it ultimately would blend and subsequently sound.
I just thought we could start examining the time domain. Since I've began exploring as much acoustics related topics as I can, both in intensive reading and mucho hands on in my room and my LP, everything is so strongly rooted in the importance of the time domain. Although I've really immersed myself into the acoustic properties of my room, all I've learned is essentially how much I don't know. What I do know is all the smart guys consistently point to the time domain, and it's relative importance. So I'm pointing too. Additionally, the more I learn, the more I realize the importance of 1/4 wave interaction.
Myself included, we always are quick to suggest adding a rear sub,..and it does typically work. Instead of just looking at a rear sub, or just examining the rear wall's contribution, a prudent approach would include both elements.