Standing wave issues in a home theater

[highfigh]

I'm helping a friend do a home theater and the room has a few problems between 63.5Hz and 100.8Hz (I'm using TrueRTA at 1/6 octave resolution). Here's a plot from this afternoon. The room has some intentional irregularities in the front corners where the speakers are mounted, it has a front row of seats with a higher platform that covers about 2/3 of the floor area and a small soffit along the sides and back, with a larger soffit across the front. The bottom of the soffits is parallel to the ceiling and the inner sides are angled at about 20 degrees. The dimensions are 17'-4" x 24'-6" x 8'-4" and the frequencies that are problematic are very close to the room modes. I have made some panels with Owens-Corning 703 and they aren't doing enough. I'm trying to avoid making some kind of bass traps that look like a major afterthought.

Any help is appreciated.

 

[6L6X4]

One of the Behringer units might be able to smooth out the response. Or Audyssey.

 

[highfigh]

One of the Behringer units might be able to smooth out the response. Or Audyssey.

The receiver is a Denon AVR-3805 and Audyssey only has EQ bands at 63, 125, 250, etc. There's no way to use an outboard EQ with it unless he scraps the whole receiver and gets separates, which isn't going to happen right now, The speakers are fixed but if bass traps prove difficult to do without problems, I'm thinking about using a passive HP filter for the L-C-R speakers so they excite the trouble frequencies less.

 

[bandphan]

I like the idea of using the traps, if WAF can be met.

 

[bpape]

How thick and how many 703 panels? To deal with 60ish hz, you're going to want something at least 6" thick.

Where is the seating position in relation to the room length?

Bryan

 

[highfigh]

I like the idea of using the traps, if WAF can be met.

I do too but she's not my wife.

 

[highfigh]

How thick and how many 703 panels? To deal with 60ish hz, you're going to want something at least 6" thick.

Where is the seating position in relation to the room length?

Bryan

I made up some 2" thick 2'x4' panels and placed them on each wall of the back corners at a right angle to each other, added a 2" thick 1'x4' panel at the corner while the others were there, stacked the smaller panels on top, held the panels up to the wall/ceiling corners and just about every other combination possible. I also stacked panels to make them 4", 6" and 8" thick. I have a first mode for the height at about 68Hz, a second mode for width at about 66Hz and a third mode for length at about 70Hz. The issue is in the two middle chairs of the back row which, of course, are the two most important seats because they're eye level and closest to center. The seats to the outside of these have great response with little EQ, even with no traps or other treatment. The response above 200Hz is outstanding with no equalization at all.

I really can't change the position of the seating or the decor. I'm going to play with speaker polarity and a passive filter on the main speakers today, as well as placing the panels in a way that they bisect the rear corners, 4" thick and 6" thick. The rear seats are about 16'-6" from the front speakers, which are about 20" forward of the wall with the screen. The front speakers are angled inward and the center is above the screen in a soffit, angled downward.

I know formulas exist for trapping bass WRT to length, width and height of the material but haven't been able to find them. The RT60 is within limits for this kind of space because of the carpet, chairs and other decor and they may be OK with some kind of pillar at the rear corners but it would have to look like it belongs.

You mentioned that the material would need to be at least 6" thick- is that for a triangular trap? How about a 6"-12" radiused column section?

 

[highfigh]

I'll post some photos later, if possible.

 

[bpape]

From the sounds of things and your comments on the other seats, your primary culprit down low appears to be the width. That's where you need to concentrate.

I highly doubt that the decay time below 200Hz or so is anywhere close to right with nothing thicker than 2" in the room. If you can rig pillar type treatments and use the 2" on the face you can fill with fluffy to get the additional thickness.

If you want to test this theory, use 3 panel thicknesses on the side walls directly beside the seating area. While I'm sure it's not the entire issue, it appears to be the best way to tackle it.

Bryan

 

[highfigh]

From the sounds of things and your comments on the other seats, your primary culprit down low appears to be the width. That's where you need to concentrate.

I highly doubt that the decay time below 200Hz or so is anywhere close to right with nothing thicker than 2" in the room. If you can rig pillar type treatments and use the 2" on the face you can fill with fluffy to get the additional thickness.

If you want to test this theory, use 3 panel thicknesses on the side walls directly beside the seating area. While I'm sure it's not the entire issue, it appears to be the best way to tackle it.

Bryan

Re: decay time, I really meant above 200Hz. I'm basically having flutter in the range I mentioned, right? It's not audible as discreet repeats but I can use a sine wave generator in my RTA software, too. How much area? Full height x 2' wide?

Thanks for the info.

 

[bpape]

That would be a minimal amount (2'x4'). 4'x4' would be better but doubt it will pass WAF - more of a proof of concept than anything else.

Bryan

 

[highfigh]

That would be a minimal amount (2'x4'). 4'x4' would be better but doubt it will pass WAF - more of a proof of concept than anything else.

Bryan

The proof is what I'm going to go for in the short term. I made another panel that's thicker and will stack that with some of the others, but that's all the material I had left and will need to go to the fabric store for more when I do the actual installation. What I made so far is strictly for testing and my own use. I'll go 4'x4' x 6" first and then try 2'x4' x 6" with another 2'x4' x 6" laying horizontally on the top of the bottom one on each side. That should make a clear difference, IMO. I may be able to get away with a 6" bump-out at the side of the upper seats but the lower row doesn't have the same problems and they're across from the steps to the upper, so that won't fly.

 

[highfigh]

Well, I put the panels up and saw very little difference on the RTA but it made a significant audible difference. The bass had hot spots all over without them but not with them. The only problem is that I can't see a way to use 6"-8" thick fiberglass panels in that room because instead of doing several things we had recommended, they just steamed on through the job until it was done, other than acoustical analysis/treatment and a few other things. It sounds good, but to me, it's not great. Still needs treatment for bit of flutter with sharp impact, but it's not bad.

Thanks for the advice.

 

[jostenmeat]

Bryan, if 6" would be the "desired" minimum for 703, what would it be for 705?

A lot more money, but maybe in this case it could be worth it?

 

[TLS Guy]

Well, I put the panels up and saw very little difference on the RTA but it made a significant audible difference. The bass had hot spots all over without them but not with them. The only problem is that I can't see a way to use 6"-8" thick fiberglass panels in that room because instead of doing several things we had recommended, they just steamed on through the job until it was done, other than acoustical analysis/treatment and a few other things. It sounds good, but to me, it's not great. Still needs treatment for bit of flutter with sharp impact, but it's not bad.

Thanks for the advice.

I think you could solve this by spreading the crossover frequencies. Set the receiver crossover around 100 to 110 Hz and set the sub crossover to 60 Hz, and experiment from there.

This might not be a room issue but a crossover blend issue anyway, or more likely a combination.

 

[highfigh]

I think you could solve this by spreading the crossover frequencies. Set the receiver crossover around 100 to 110 Hz and set the sub crossover to 60 Hz, and experiment from there.

This might not be a room issue but a crossover blend issue anyway, or more likely a combination.

That's the plan. With no sub, the main speakers' response is very good but when the crossover is set to anything higher than 40Hz, the bump shows up. As a test, I put a cap on the mains and it helped a lot. I really prefer to treat the room because it minimizes strong peaks and dips but since I won't be able to do drastic bass trapping, I'm planning to get a 1/3 octave EQ with variable crossover and subsonic filter so the receiver can be set to LFE, crossover set to 150-200Hz and work with the equalizer to get the most seamless response. Also, it'll be a lot more versatile since it will have 1/3 8va control vs 63, 125, 250Hz bands, which are useless for most bass equalization, IMO. I'm sure a lot of the problem is from not being to place the speakers for best response/sound before the room was finished. The modes show up on the RTA and the unfortunate parts is that treatment isn't looked at as a necessary thing when compared to decor.

 

[bpape]

Unfortunately, 705 isn't going to help much of anything. This is purely a matter of thickness if you need to go broadband. Another option would be to build sealed membrane type absorbers tuned to the area you need to deal with. They'd be good for about 2 octaves. You can get down into that range with about 3.75"-4" of thickness this way.

Bryan

 

[highfigh]

Unfortunately, 705 isn't going to help much of anything. This is purely a matter of thickness if you need to go broadband. Another option would be to build sealed membrane type absorbers tuned to the area you need to deal with. They'd be good for about 2 octaves. You can get down into that range with about 3.75"-4" of thickness this way.

Bryan

New developments-

I discussed the options with the client today and I may be able to remove the drywall, so I would be able to use the stud cavities behind, from floor to ceiling. This would allow me to treat the area next to the row of seats that have the problems without intruding into the room- one wall has 2x4 metal studs and the opposite wall, which is shared by a finished storage room, has 2x6 studs. All of the walls have 3/4" Celotex panels and 5/8" drywall. The front wall was foamed completely and the panels with the speakers has 1/2" MDF, screwed to the studs, foamed from the back with 5/8" drywall. The main speakers and subs are all sealed enclosures, make from 3/4" MDF, screwed, glued and caulked so they would be well sealed.

That, or a Helmholtz absorber, which I can cover with a thin pad and cloth. They knew going in that treatment would be needed- they just want it to look good.

Do you have any sources for calculations to determine the required dimensions for these to affect specific frequency ranges? I have dealt with this before but it was a long time ago and I'm trying to find my information.

 

[bpape]

Here is a link that has a couple pics of the product construction and the formulas

http://www.customaudiodesigns.co.uk/articles/membranes.html

Note that NOTHING can touch the face membrane except where it's attached to the framing. Anything that does will damp it like grabbing a bell.

Bryan

 

[highfigh]

Here is a link that has a couple pics of the product construction and the formulas

http://www.customaudiodesigns.co.uk/articles/membranes.html

Note that NOTHING can touch the face membrane except where it's attached to the framing. Anything that does will damp it like grabbing a bell.

Bryan

Thanks for that link- I read it before I left earlier and may be able to use something like them.

OTOH, I went back to install the glass over the projector housing and went around the room, pounding on things to find out what makes noise. The back, left and front walls were foamed from behind but the wall that has a plain storage room on the other side wasn't. I pounded on that wall and the whole thing vibrates in what I'll assume is the problem region. The others are extremely dead. I wasn't there when that wall was framed and assumed that it was built right but I guess I screwed up by doing that. To add to the goodness, the drywall on the backside wasn't taped or mudded.