Optimizing absorber/diffuser size?

[Hermit]

I'm doing my first DIY acoustic treatment - also my first 7.1 system.
I'm reading the Master Handbook of Acoustics and I've read lots of website pages - very enlightening, but I'm still a beginner.
My basement room is small (15 X 11 X 7'4), with fairly ridgid side walls, and a front wall half covered (upper half) with heavy mirror (I haven't decided how to cover the mirror, yet). There's just eough room for a comfy couch, a wooden coffee table, CD rack, and the 7.1 system with a Sammy HLR 4667 and two subs.
I'm already constructing triangular, floor-to-ceiling corner traps. Their 16x16x22 footprint is fully filled with layers of OC703 and wrapped in black poly batting and burlap. I plan to hold these freestanding units out from the walls by 2". I'm also building two 43" long corner units I plan to center on the front and rear walls, where the wall meets the ceiling.

The next step will be the freestanding, side-wall panels. The question is -would 6' tall panels would be a more effective size, than the 4' panels I see on the DIY and commercial supplier's websites?
I was planning to use a pair of absorbers on each side wall, at each main speaker reflection point. Are the reflections above and below the shorter panels of little concern OR could it be that the area of larger panels would overwhelm the liveliness of the room OR is the cost to high for a small benefit?
I can easily build 4" thick (by 2x6') OC703 absorbers OR 6x3' polycylindricals (using 1/8" luan door skins with a 703 back pad).

I'd appreciate your input to help me make the best choice possible...

Thanks... Hermit

 

[bpape]

OK.

You need absorption in some certain places (behind your mains and center, reflection points on side walls and ceiling, other scattered through the room for more efficient RT60 decline).

Now, your space can only handle so much mid/high absorption wthout being too dead and becoming unbalanced sounding. That said, you need to figure out how much of what type of absorption (sabins per sq ft at each frequency band) to see how much you have to work with. Figure out IF you made them 6' tall instead of 4', how much area would you have left to work with at other places in the room? If the answer is very little (or negative!) then the answer is make them smaller. Realistically, I'd make them 4' anyway. I can always put up an absorption arran in the rear on the sides that will act like absorption and diffusion if I have leftover.

 

[Hermit]

Thanks...
I'm working on the sabin calculations and rubbing up against having to much absorbtion. Its pretty challenging to balance absorbtion and diffusion in such a small space, and still treat all the considerations in the Acoustic Handbook.
Some conflicting info on whether its OK to use the polys on the sidewall reflections or not. Looks like wide-band absorbers are the preferred choice.

Thanks... so much to do, and so little time!

Hermit

 

[bpape]

In a small space, sometimes it is better to make all of your absorbtion thicker so it does more over a larger range of frequencies.

IMO, use absorbtion at the side wall reflection points. In a small space, diffusion is tough to pull off as you're not really far enough from the diffusors to let it spread out enough to matter.

If you want to deal with absorption and diffusion in a unique way, try using some 6" wide by 4' tall by 2-4" thick absorbers framed in 1x4. Space them out so there's 6-12" between them. If you have too much HF absorbtion and need some more at 250 or so, either use 3.5 inch paper faced wall insulation or just use normal 703 but cover with brown shipping paper before covering with cloth. If you altername every other of these absorbers with and without facing, you'll get good distributed absorpton AND you'll get some diffusion in the rear of the room where your surround field is. Certainly cheaper and easier than building a 3D diffusor and takes up a LOT less depth. On top of it, it lets you have some absorption spread througout the space as oppsed to primarily in the front of the room.

 

[Hermit]

Thanks....
Its a case of theory/learning catching up with my desire to get everything built. So far I've been modeling on what I see in other websites and links (Jon Risch, Bill's Audio Cave, various manufacturers sites).
Finished the Sabin Calculations (on the untreated room) and was surprised to find that all my RT60 decays, from 500 Hz and up, ranged from .22 to .27 (I have a painted drop ceiling and padded heavy carpet). The 125 and 250 Hz were .48 and .50.
Add in the problem of stacked up modes at 150 Hz, 210Hz, and 300 Hz (and tight modes from 260 thru 460) and I better understand that my first project, is a challenging one. But its the room I have, and the best I've got acoustically.
I can see that in my case smaller absorber/diffusers are absolutely necessary, to stay in line with my space.

Since most of my units will be freestanding I plan to use a simple design like Jon Risch's, for the side walls and rear wall. However, I thought I would put a reflective facing on one side of each panel, so I can turn the unit around (and vary the distance from the wall) to tune the room response. I already have a couple of cases of unfaced OC703, so I can simply add any reflective material I need under the poly/burlap finish.
Also, I can understand the a larger number of smaller units, would give me a lot more reflective/diffusive surfaces, without adding so many sabin's that it would bury the room. This would be especially useful because there are very few furnishings/decorations. In your experience, is the 6" width for the panel particularly well suited to such a small room? Would 8 or 12 " be equally, or nearly as, effective?

I figure my corner traps will help considerably, however, I have a concern there also. The first 2 units turned out nicely at 15x15x20" and 43" tall (stacking 2 units will be 1" from my ceiling). I used 2x2"s to build an open-sided frame, rounded over the corners to about a 3" radius (sort of mini polycylinder diffusing edges), and stuffed the box with layers of 703. Looking at the absorbtion coefficients (thickness ranges from 3" to 10 " within the trap) this should be an effective wide-band absorber.
However, I notice that many installations stack a smaller trap on top of the larger trap. I plan to use a pair of stackable traps in each corner, and I thought I would build them all to the same dimensions listed above. Would there be an advantage to scaling down the upper traps?

Well, its my day off and time for me to get to work on the fun stuff = the acoustic panels. I hope to complete the corner traps in the next week or so.

Thanks very much for your input... I'm sure it will help me be more satisfied with the results... Hermit

 

[bpape]

Without knowing what you want to use the room FOR (2 channel, multi-channel music, HT, recording, etc.) it's impossible to say what your 500 and up numbers are in terms of correctness ( IMO .22 is awfully low for many of those applications). I'm assuming that you'll be doing some 2 channel listening and some HT listening as the 2 primary uses. For those 2 things, a compromise is in order both in terms of RT60 and in terms of where and what type of treatments you use. However, it's obvious (and quite common) that you still need quite a bit from 250Hz down.

Just a couple things real quick...

When you do the calculations for your room volume, since you have a suspended ceiling, make sure you use the REAL ceiling above that for your height measurement - NOT the suspended ceiling height.

Second, how many of the absorbers do you have now and where? To block off ALL of the corners with a reflector MIGHT not be the best way to handle it - again, without knowing more details it's impossible to say. However, you're between a rock and a hard place. You need broadband bass absorption but also are already too dead in the highs.

Third, don't forget to account for seating, people, and air in the room when you do your calculations. They can make a big difference.

 

[Hermit]

Thanks for the insights...
I'm totally stoked .... without any experience I couldn't appreciate what I was trying to do - so I started playing. I finished 4 wedge corner units (one pair for each front corner), and jury-rigged four flat absorbers at 2"x16x48 for testing the axial reflections.
The corner absorbers acted like a magnet, attracting the sound field to wherever they were placed. A friend and I took turns moving the top trap from front corner to the back corner, and we could "VERY" clearly hear the way the absorber sharpened the sound around it (I assume this was by absorbing the interferring resonances around the panel).
We used the poly wrapped 703 boards, to experiment with the effect on the side wall reflections and were pretty much shocked at the strong results. Just using a third to half of the needed treatment gave us outstanding improvements.

I now realize I can't expect "best possible" results in one try - and that some experimentation needs to be done. I plan to build a slightly thinner corner wedge (about 20 % smaller) that I can test on top of the larger lower wedge. I can use the results to help choose a solution.

I also plan to build 4 flat panel absorbers at 4" thick to move around the room and get a feel for what kind of effect they have. I'm unsure what dimension I'll end up with. Anything smaller than 12" wide won't cover the side wall reflections enough to give me a decently wide sweet spot on my 7' couch (the listening position). The 6" absorbers sound cool, however, I'd sure like to use a bit larger units and so have to build a few less.

I know my sidewall mirror points are 4' apart and I think using absorbtion there with alternating diffuser/reflectors would work really well. I'm working on my front and back treatment, while I build the freestanding panels, and look forward to getting the room a bit more "together".

I am strongly focusing on absorbtion below 300 Hz and a fair amount of reflection above.

Well I need to get back to work... the money job.... Thanks again... Hermit

p.s... my drop ceiling is only 3 " below my joists, and the joists are fiberglass filled with 4" Kraft faced fiberglass. Not a lot of added room volume, but, possibly acts as a very low frequency panel absorber?

 

[bpape]

Again, without knowing what you're going to use the room for, it's difficult to give advice. For pretty much any applicaation though, absorbtion or diffusion at the ceiling reflection points is a good idea. On the side walls, meh on the diffusion - absorbtion, yes.

Rear wall behind the listening position - depends on how far from the wall you are and if you're doing HT or 2 channel listening or a combination as to whether absorbtion or diffusion is a better solution. The rear reflection point of oft overlooked. Either diffusion or absorbtion should apply there - how much and which is a function of usage and what the rest of the space dictates and allows.

As for the ceiling, the volume you need to consider is to the subfloor of the room above, not the bottom of the joists. Yes. A single layer of drywall with insulation behind does help in the bottom end - quite a bit actually but not enough to really dig down deep and help smooth things out.

The idea behind the 6" panels is that it provides scattered absorbtion throughout the space (good for more efficient decay time drops) AND gives you some diffussion via the frame and the change in medium. Yes. It's some work but realistically, once you've made a couple of these frames, they don't take more than 10 minutes each - maybe 15.

If you want to build less, see if a 4'x4' diamond shape absorber

/\
\/

is wide enough to cover all your points on the side wall. It also looks more interesting. It also puts some absorbtion up further and down lower on the wall where you wouldn't normally have any with normal 2x4 panels.

When you decide on the ceiling absorbtion/diffusion, don't forget there are a couple ways to do it.

1. Do the standard and put the absorbtion facing the room.
2. Do the opposite and hang a section of false ceiling down with drywall on the bottom but with the absorbtion facing UP toward the ceiling. Place the front so that the ceiling bounces can come off the ceiling and bounce INTO the gap and get totally absorbed. This is nice in that it still exposes drywall to the room to keep it from being too dry in the highs but still absorbs the reflections.

Lots of ways to skin a cat...

P.S. By the way, if you find that the room is becoming a bit dry on the top, you can cover some of the broadband bass absorbers surface with either paper (shipping paper) or a foil scrim. This will allow it to still act as a bass absorber but reflect the upper mids and highs.

 

[Hermit]

I appreciate all the help...
I use the room for about 60% multi-channel music and 40% HT.

I'm thinking I'll treat about 60% of the front wall = 15% absorbtion (OC 703 flat panels plus the corner traps) and 45% diffusion (installing some upper shelving with a lower, center poly).

The seating position is about 4' of the back wall in a medium pressure area - there is a null not to far forward and steeply increasing pressure to the rear wall. Any farther forward and I'd have to move out nearly to the room's center.
I was thinking of treating much less rear wall area. I could use two 2'X2'x4" diamonds (below and slightly between the back surrounds), plus the corner traps, plus a continuation of the sidewall treatment (4' x 6" x 4" OC703 panels) The couch will shadow much of the back wall.

You seem confident of using 6" wide panels and I realize I can always close the distance between a couple of the narrow panels, to cover the side reflections.

I think 2'x2' clouds should cover the ceiling reflections.

The boss is calling so I've got to go. I'll be building about 16 6" panels , 2 corner traps, and a few flat traps over the next couple of weeks. I'll be in touch... Thanks.... Hermit

 

[bpape]

I think there was some misunderstanding. The 6" panels are for the back half of the room - not for reflection points.

For multi-channel and HT, the design goals are relatively similar. If you want a rough ballpark, shoot for a decay time of about .35 across the spectrum. That's actually still a little high for your space but you can always add more. If you want to shoot for the moon, go for about .29.

Sounds like you have a handle on seating position issues.

Be careful with the poly behind a center channel speaker. You don't really want to disperse that signal too much - you want it to stay locked to the screen. I'd just use straight absorbtion behind the center.

We've never addressed the mirror wall. Is this over a drywall wall or what? If it's over drywall, seriously consider removing it. It would be nice to get back the small bit of bass absorbtion that each square foot of drywall over framing provides.

 

[sploo]

If you want a rough ballpark, shoot for a decay time of about .35 across the spectrum. That's actually still a little high for your space but you can always add more. If you want to shoot for the moon, go for about .29.

This is probably a dumb question but, for RT60 calculations, do the .35 and .29 refer to .35 and .29 seconds (i.e. 350 and 290ms respectively)?

 

[Ethan Winer]

Sploo,

> do the .35 and .29 refer to .35 and .29 seconds <

Yes, exactly.

--Ethan

 

[Hermit]

I'm Very Encouraged....

I thought the larger absorbers for the front reflections were preferred and I'm glad you confirmed this. What you describe is pretty much exactly what I envisioned a few days ago.

Bad news and good news on the mirror. Bad news is the inset mirrored niche is a custom installation and the woodwork and framing make removing it difficult and expensive. The good news is it is a totally flat surface and I could easily build over the 3'8 X 10'10 niche to create tuned absorbtion.
I've been considering several possibilities:
1 - Install the 32" wide shelves to the right and left (there are integrated adjustable shelf supports) AND build an absorber for the center section. Stocking the shelves with various items could provide a lot of diffusion, but, also a source of a lot of unwanted noises.
2 - Frame in the entire niche, use full coverage fiberglass against the glass, and select a facing that would suit the room application. This option allows me to choose cavity volumes and reflective material specifically aimed at tuning my room.
3 - Build several independant diffuser/absorbers installed over the existing glass. This would leave some of the reflective surface of the mirror exposed.

My current goals are to build: (last dimension refers to OC703 thickness)
1 - Four 16X48X4" side absorbers (nicely covers speaker reflections).
2 - Three 16X42X4" front absorbers = right, center and left speakers.
3 - Ten 6X48X4" absorber/diffusers for the side and rear walls.
4 - Four 24X24X2" diamonds = 2 for the high rear + 2 for the clouds.
5 - Four mid size corner traps to top the four larger (half-high) corner traps.
6 - Frame in the upper front wall to create a mid-volume cavity right and left, and a larger volume center cavity. Use 1/4" plywood to face the cavities and possibly perforate the plywood if tuning requires it. I need to check the handbook to see if pegboard would be a good choice for the fascia.

I think this will put my calculated RT60's to the low side, for my mids and highs. However, as a percentage of area I think it will be a good balance between reflective and absorbtive surfaces. The ceiling and carpet calculations are skewing the numbers, however, I don't "hear" the kind of effect I would expect from excessive mid and high absorbtion. Maybe it will be more pronounced when I clean up the rest of the room.

May the force be with me... Hermit...... thanks

 

[Hermit]

Please be patient with me....

I made yesterday's post from work and didn't have any reference material with me. Now that I've taken some time to think, look, and review I have refined my approach a bit.

It looks to me that framing over the mirror niche has excellent potential. I am now planning to use 1x4 and 2x4 framing to constuct 5 equally spaced cavities 4" deep. I would shim the 2" thick OC703 3/4" away from the glass. Then use smoothe 1/4" finish ply on the right , center, and left cavities AND 0.79% perforations on the 2 remaining cavities (= ply, perf, ply, perf, ply). If I am understanding the principles and coefficients in "the Handbook" correctly, this would (should, could ?) return a good distribution of absorbtion and liveliness to the upper front wall (and possibly some mode control in the 150 Hz range where I have the most reinforcement). (coefficients of 4" depth with 2" 703 are: ply = .28, .22, .17, .09, .10, .11 AND perf = .4, .84, .4, .16, .14, .12).
The total front wall treatment now includes floor-to-ceiling bass corner traps + "lower wall" front, center, and left channel absorbers + corner-to-corner tuned "upper wall" resonant panels.

On the rear wall, I am eliminating the rear corner traps unless there is some later proof they are needed. I believe six of the 6X4" absorbers you have suggested will provide all the lower wall treatment I need.
The rear diamonds would not be adequate at 2" thick so I will be building them 24X24X4" thick, and have to mount them flush to the wall .

Each of the side walls will use two of the 16X4" panels + four 6X4"panels. I hope this will comfortably knock down the primary speaker reflections, significantly suppress comb-filtering, and provide enough diffusion/reflection for decent spaciousness.
One question here = how much effect does vertical panel height have on performance? Most panels use legs to raise the panel up closer to center of room height. I'm planning 1' legs which will leave the panels 2'4 from the ceiling - any appreacable effect?

Finally, I will install the 4" thick clouds on the ceiling. I don't have the physical head room to suspend them down more than an inch, so I can't create the cavity you suggested. I won't be framing the clouds, instead relying on the ridgid panels to support themselves (just a poly and burlap covering all around).

Looking at the walls, ceiling, and floor seperately, I think this will put me in pretty good shape. Looking at the room, as a whole, I am feeling a bit restricted by my existing acoustical/resource realities. It sure would be interesting if I could custom build my HT from scratch.

At least I am able to "know" how nice it would be to design HT principles into a home - this is definitely a great improvement over ignorance....

Oh well.... its off to the lumber yard to buy materials and get to work on this latest plan.... TTFN.... Hermit

 

[Tom351]

Just as an FYI- I have attached a graph showing the THX tolerance range for a room of your dimensions. I do acoustical simulation/modeling for a living so I just plugged the overall dimensions into my software to see where the tolerance range falls.

TD

 

[Hermit]

Awesome Tom....
I appreciate the info very much. I'm surprised to see reverbs so low and glad there is some room for me to work with.
I could be misreading the chart, since I am unfamiliar with it. Could you please define "red lines, blue lines, solid lines, and dashed lines"? I believe the solid lines are defining the ideal range.
I'm going to run new numbers during my free time at work this week. Since my early calcs show sub .30's from 500hz and above, and I am focusing much of my treatment below 500hz, I suspect I am closing in on some decent performance. Since the coefficients were generated in a different environment I am keeping in mind that my application/constructions could cause some variance from expected results.
I'm in a build, then listen, then modify/refine kind of approach. I'm fairly confidant this first step (described in my previous post), will address the major issues I have. It will also give me many adjustable elements/panels to play with, so I can learn (as well as my friends) to be a more critical judge of sound/acoustic qualities.
I hope I'll only have to take a few small steps after this, to reach acoustic Nirvana.
Your info helps me understand issues central to complementing my receivers capabilities (THX Select - couldn't afford the full package). I will certainly use it to tighten up my goals.

Thanks again... Hermit

 

[Tom351]

No problem at all, here is the THX spec to go with the tolerance chart (see attached).

 

[Tom351]

Also, those lines are not supposed to be dashed (they appear that way in the web browser)- you will get a better look at the chart if you right-click and download the JPG to your PC.

 

[sploo]

Very interesting.

Are these RT tolerance values highly dependant on room size? I.e. would my listening room (25.5' x 9' x 8') be wildly different to Hermit's?

Cheers.

 

[Tom351]

Here is the tolerance for your specs Sploo- the extra volume pushes the whole range up 0.02s. I'm not sure what formula the software uses- most of the rooms that I work with would have much larger volumes with 1k-4k seats and THX ranges of over one second.