Acoustical Treatments Master Thread - Page 313

this depends on the design requirements for the total specular response you wish to achieve.

the problem with "too much diffusion" it is generally via the over-application of NON-broadband diffusion.

another issue is the construction of large arrays (to cover large sq area surfaces) via repeated applications of the same diffuser type/design (periodicity) - of which specular lobing is exhibited. this is why inverse panels (as you will note is available for calculation in QRDude) are utilized, and the array can be formed via a pseudo-random sequence such as the Barker Code.

QRDude technical guide snips much from Acoustic Absorbers and Diffusers (Cox/D'Antonio):
http://www.subwoofer-builder.com/qrdude-user-guide-lobes.htm

you could also apply Binary Amplitude Diffusers (BAD) panels around the room (which offer spatial but NO temporal dispersion, unlike the phase gratings). you could even curve the BAD surface to form a poly. plenty of options to assist in getting the energy as well mixed as possible.
Edited by localhost127 - 1/28/13 at 2:17pm

Here's some pictures of the QRD diffuser installed on the wall. Also the front wall setup. Sorry about the dark images, the black room is great for movies, not so great for pics.




Edited by HaroldKumar - 1/28/13 at 10:17pm

if you have more of the 2d skyline PRDs, may i suggest lining the outer face of your rear LF corner absorbers with them -

I'm planning on building some 8" tall x 12" wide soffits in my small theater room (11' x 15'). Would that size of soffit be good enough to use as bass traps? Thought about building the frames and covering them in fabric so that the sides and bottom would be open. I have some Roxul AFB that I can use inside them, Would that be a good choice or should I get something different? Thanks.

If you see my post just above and the pic, you will notice the narrow room I have.

I don't want to put up 4" sidewall first reflection absorbers as it makes the room even smaller, so I had a thought to cut out a square of the actual wall and embed the absorber into the wall. This would look good I think. I realize it's only 3.5 inches to work with but maybe they could stick out an inch or two.

Dumb idea? Anyone try this?

These would work pretty well. They aren't super thick so they won't go really low, but they should still provide pretty good absorption around 80-100 Hz and up.

This would work well for treatment, but could be bad for isolation and increase noise transmission into and out of your room.

Thanks for the info. I was just making sure that it wouldnt be a waste of time for me to build the soffits for anything other than looks.

I'm getting ready to build some bass traps for my room using OC703 (2" thick). I'll be picking up some 2' x 4' panels today. My approach was to double up the 703 creating a 4" thick panel and cover them with the black ponte material (speaker cloth) using a 1"x 2" backing frame. I have a very limited space; theater room is only about 9' wide by 13' long – actually only 8’ wide at the screen end of the room. A few questions about size and placement:

1. What are the smallest dimensions that would still be effective, 2’ by 2’ or even 1’ by 1’? It seems like most standard sizes are 2’ by 4’ but I have seen some minis that are 2’ by 2’; and I guess there are some triangular ones out there but in my case there are other obstructions that are in the way of that too. I would like to put the first 2 traps behind the speakers straddling either the wall/wall/ceiling corner or the wall/wall/floor corner, but even if they are 2’ by 2’ that would obstruct part of the screen. They would need to be closer to 1’ by 1’ to fit without. The back corners also have things in the way. Would it be as effective to place them either the wall/floor or wall/ceiling corners? I think there would be room to place them at the floor/wall corners either on the sides or between the speakers. Does it matter which one, side walls or front walls?
2. Should there be any other materials placed in the bass trap like a poly?

Your room is about the same as mine. Mine is 15.5lx8.5wx10.5h. I didn't have any problem getting 2x4' traps in the corners. How high is room? Do you have any pics? Is it a dedicated room?

Why don't you use some super chunk corner traps? I built my traps as corner traps by cutting OC703 into essentially triangles that are about 18" long on the 2 short sides and stacked that almost ceiling high. I was only able to put them in the front 2 corners due to windows and doors near the back corners, but even just the 2 of them made a significant difference in the bass performance of the room.

Yep, it's a dedicated room but there is an uneven soffit over the screen.

Here is the layout. Sorry, I can't seem to size my image properly.



The dashed line is the doglegged soffit shape. The floor to ceiling height is only 6'-4" under that but 7'-3" everywhere else. There is a window (not shown) just to the left and a bit back from the left speaker (typical little basement window but covered with linacoustic).

Here is a picture of that front left corner (sorry it's kind of a dark photo):


Another shot of just the front:

Good point, but I have my speakers pushed into the corners (I know, I know, I shouldn't, but I need to). I suppose I could even put them at an angle and straddle the speaker stands.

Speakers won't mind being put into a chunk of insulation. Just make a hole for them in the trap.

Angryht, not a lot of real estate to work with as the ceiling is so low and variable.

One option is move the seating position up 4 feet and use smaller screen (yikes I know), then you'd free up corner space.

Or maybe flip the room around and put the screen on the 9.5 foot side?

Well, actually, I am considering an AT screen. That would enable me to put some treatments in the corners behind the screen.

Any thoughts on these questions? I am thinking that the 2' by 2' might fit better in the room but I'm not sure how effective they would be.

With an AT screen you have enough room to put in the superchunk traps on your front wall like kertofer suggested. This is the most bang per sq ft probably. The only downside is that takes a lot of material, but you are only doing a few traps should be OK cost wise.

I think if you do a superchunk trap on each front corner (and maybe something on the back wall top corners), you will get a dramatic improvement in bass, LF and also speech in films will be much clearer.

Also, one reason I suggested moving the seating position towards middle of room is seating against the back wall is considered a no no, maybe you could do some OC703 against back wall at first reflection.

What are peoples thoughts on this hybrid OC703/R38 trap?

http://www.gearslutz.com/board/8616119-post30.html

I have straddled OC703 traps in all four corners and I'm thinking I could get more LF performance with R38 behind them? Worth it?

You would get some additional absorption around 60 Hz and lower. Your traps likely absorb pretty good from 80 Hz and up as is, stuffing with fiberglass should lower that down at least a half octave.

A couple of quick points:

Most absorbers are 2' x 4' because
A.) That is the size the absorption cores come in.
B.) It works out to actually be quite a convenient size in most cases.

However, there isn't many reasons that twice as many 2x2s wont work as well as 2x4s. There might be some slight differences in terms of diffraction, but not detrimental. In the case of 1x1s, 8 of them would be more or less be equal to a 2x4 absorber. But I do want to draw on the differences here. When you straddle a corner with a 2 foot wide trap, this means you have a large 12" airgap between the middle of the panel and the corner, getting smaller as you go to the edges of the absorber. A 1x1 absorber will only have a 6" gap (which does make a large difference!). So, they won't work nearly as well for absorbing <100 Hz. Also, you would get some losses due to diffraction on such small panels. To put it simply, a 1x1 panel is really small in terms of a bass wave. They'll just navigate around it instead of through it.

With all that said...will they still work good for mid and high frequencies? Definitely. Not sure how well a 1' x 1' absorber would work in the bass range though.

As usual, great straight info. At this point I am going to go with 2' by 2'. If they won't fit at the front tri corners because of the current screen set up, maybe I could put them between the center and right and left speakers straddling the wall/floor corners. Then, if I do go with an AT sceen, I would be able to put them up in the front tricorner when the speakers are moved slightly forward. The 2' by 2' would probably give me the most flexibility.

Question about placement. If I place my 2' x 2' bass traps (4" thick with OC 703) in the corners, I need to lean them back so that the bottoms are 'kicked out' a bit. Other wise they are too tall and obstruct the screen. I will likely be able to rearrange when I get the AT screen set up but I wanted to ask for the mean time. The picture below shows what I mean. Is this poor placement?

You would want them kicked as little as possible, but it should be okay for temporary placement. For a lot of the tests we do when we try setups in our test room, we'll simply lean our 4" bass traps in the corner (with as little kick as possible). The more it completely straddles the corner the better, but we haven't seen drastically worse results with them a bit off.

Good to know.. My other options for the front of the room would be either put them parallel to the front wall between the center and right & left; or to put them against the side wall (bottom kicked out to straddle the floor/wall corner in both cases). Any obvious do's or don't's? It seems like the typical order of business is to try and treat the main corners of the room but if it makes more sense to put them along the walls, maybe I should just do that, since they don't quite fit to straddle the floor/wall/wall properly.

thanks for showing the pic because it sparks a question in my mind. i thought that acoustic panels should be placed to cover ear height (if you can't get floor to ceiling coverage). the 2' x 2' bass traps appear to be lower than ear height. so is the ear height requirement irrelevant with bass traps?

That's the way I understand it, but I'll wait for confirmation. Bass builds up in the corners, which is why the most effective traps are in the corner (broadband that is).

Corners would be best for bass control.


They should be at ear height for first reflection panels (and other panels that are meant to absorb destructive phase related issues - iow, other early reflections). For purely bass trapping, the height shouldn't affect the absorption significantly. If you were trying to get a combo though, reflection control and bass trapping - they'd need to be at ear height (or speaker height - or better yet, inbetween the two)


Yes. And the reason bass builds up in corners is because sound travels in solids (like drywall) faster than in air. So bass waves tend to propagate along boundaries until they terminate in the corners. Also you get the benefit of dampening two modes at once in the corner.

I'm thinking of doing the same thing. I have OC 705 with the FRK facing on the front straddling four corners in my theater. My traps are trapezoid shaped and sit more flush with the wall, so I don't have as big a gap behind them probably about 10" or so. Have you tried this yet, if so has it improved the sound?

I think its a wonderful idea. You can play with the following calculator to get a feel for the benefits of using this method: http://www.stanleyhallstudios.co.uk/pacalc/
Of course this is simply a calculation, but can still give a good indication to the difference of materials.
Flow resistivity of OC705 is roughly 30,000 - resistivity of pink fluffy is about 5,000
Use the average depth of the corner to get a somewhat more accurate picture.
I also recommend selecting "Miki (1990)" from the "Model" section on the left.

Cheers!