703 panel trap construction advice

[googacky]

hello everyone. i'm getting ready to build some panel traps out of owens corning 703 and 705. i've toyed with a few ideas for how best to cover them and hang them. my plan is this: to build a wooden frame for each 2 x 4 ft. piece and figure out the best way to attatch a burlap covering (so i don't suck in all that fiberglass). my questions are these: should i use a piece of 2 x 4 ft. plywood as a backing? it would help the overall structural integrity, but wouldn't it render placement in a corner less efficient? i plan to build a few corner traps with airspace behind them and i'd imagine a wood backing would reflect back a bit of the sound and nulify some of the acoustical effect of the airspace. correct me if i'm wrong. i'm also trying to work out the best way to attatch the burlap. if anyone has built these before or has any links i could check out, i'd appreciate it. also, i'm still shopping for a source for my fabric covering, so if anyone has any leads, throw them my way.
thanks.

[JES]

Hey, I'm working on something similar -- a 4x4 panel to hang at an angle over the mixing area (in lieu of ceiling treatment, which in my basement would be a big headache). For the fabric, I was actually planning to visit Joann Fabrics and find a cool print, and just tack it on over the fiberglass.

Best,
--JES

[Miles]

If you're trying to make a corner bass trap, don't back it with plywood. If you want a mid/high frequency absorber, mounting it directly on the wall or using the plywood backing will work. What exactly are you trying to accomplish with the sound treatment?

You can get the burlap cheap at fabric outlets. Your mom/aunt/grandma probably knows where the nearest outlet is. Some people who are sensitive to fiberglass (or think they are) like to put a layer of cotton batting between the 703 and the burlap/fabric cover. I don't think it's necessary.

[googacky]

thanks for the quick responses. what i'm trying to accomplish is to tame an out of control basement studio. i don't have a separate control room, so any treatment i do to my "control room" will help out the studio as well. i have problems right now with flutter echo between all of the walls. the treatment should take care of that and i hope it'll even out the frequency response of the room. i believe (though it hasn't been empirically tested in any way) that i have some bumps around 500 Hz. i seem to have problems there quite a lot. then again it could be my crappy monitors (that's the next thing to correct). ideally, what i'm going for is a more controlled and accuate space to record and monitor audio. not being an acoustician, i'm sort of flying blind about what specifically needs to be treated, but i figure it can't hurt to make it a bit closer to anechoic down there.

[Kyle]

I just built some traps for my basement and the difference is AMAZING. Use a spray adhesive to adhere the fabric. Before you buy the fabric, get some samples and hold them up to your ear when you listen to music in the room. Check and see if the fabic changes the sound. Different fabrics absorb more or less of certain freq. Pick one that is transparent and inexpensive. Fabirc is not cheap.

If you put a back on the trap, make sure it's 3/4 pylwood and be sure to seal the hell out if it. You can take them with you if you move.

Once your panels are up, you will notice that the bass is more present and very focused. This is because your basement is reflecting all of the waves and you have a ton of phase cancellation happening. The low feq bump that you are hearing is most likely do the size and shape of the room, and where you are standing.

[doctari]

You are on the right track for decreasing bass buildup in your room by treating the corners with 703. I used 2"x2" ripped on a 45 screwed to the walls, far enough out so the rigid fiberglass board completed a triangle with the corner. An inch or so in front of that I attached 1/4" pegboard. Its a southern fried version of a Helmholtz resonator.I don't have any test equipment so numbers I can't give, but the room sounded better after this treatment. Try broadcloth or any cheap material you can breath or blow through to wrap the 703. You can find it at the local discount fabric outlet sometimes as wide as 110". I also made frames, really more like trays that you can rest the covered 703 on, from pegboard and 2"x2", to hang from the ceiling. The Home Depot will cut the pegboard for easier handling.You might want to compare the cost of materials and time for making these items with the price of comercially produced acoustic products. For ideas and useful acoustic information check out,"Sound Studio Construction on a Budget" and "How to Build a Small Budget Recording Studio", both books are by F.Alton Everest. Download "Acoustics 101" from the Auralex website, its free and filled with information

[goldstar]

check out ethanwiner.com for advice and principals on homemade bass traps. He was on the panel for budget acoustics at the last conference and knows what he's talking about. The deeper the frame, the lower the freq. absorbed, as long as the insul. is pulled out toward the front panel without touching it. The Everest books are good, also.

Frank

[rhythm ranch]

For Ethan Winer's info click here.

[googacky]

i've checked out all the resources you guys have mentioned, so i guess i'm on the right track. i haven't really seen any specific schematics for the actual construction of frames, however. until i started thinking about the acoustics of the situation, i never really thought about the frames having backs and that sort of thing. i think i'll go with frames with just a few slats in the back to add structural support. making these things mobile is definately a concern, so i need them to survive a move. thanks for the input and i'll let you guys know what i learn when i build them.

[rhythm ranch]

Sorry, I should have included this site earlier: Go here for plans.
Good luck,
Mark

[joeysimms]

Hey googacky, try covering your frame with 3/4" soundboard before the burlap. When I built my gobo, I did a 4' x 8' frame, put the soundboard on one side, then layed in the stuffing. Then tack the other side with another piece of soundboard. Nice and tight. Wear gloves / goggles / face mask and work where there's fresh air.

[knightfly]

Googacky, not many traps will help your flutter echo, because it's caused by parallel walls and parallel floor/ceiling. For a non-treated room to not have flutter echo, there needs to be at least a 12 degree "splay" between walls, which can be either 6 degrees per wall or all 12 degrees in one wall.

Another way of taming flutter echo in a rectangular room is to build Ethan's traps, but angle the fronts by at least 6 degrees, then place the traps along the parallel walls on both sides, with the thinner edge to your rear as you face the monitor speakers. This will effectively do the same thing as splayed walls, as far as flutter echo is concerned.

If you're building bass traps out of 703, the thickness of the 703 determines the degree of absorption and the distance the 703 is spaced away from solid material determines how low a frequency they will affect. More distance = lower bass. For sounds that hit the trap head on, the lowest frequency affected is 1/4 wavelength. This means that the distance from the face of the absorbent to the first hard surface (wall) is 1/4 of the wavelength of the frequency the trap will be most efficient at absorbing.

Putting solid plywood on the back of the 703 is only good for mids and up. It can also work for "clouds", or plywood backed 703 floated over the mix area. If you hang these so that they are lower toward the front of the room, you will further decrease flutter echoes between floor and ceiling.

Beyond bass trapping all possible corners, the first places you should put absorption are those that, if they had a mirror placed flat on them, would reflect either of the monitor speakers back to the listening position. This includes the ceiling if there isn't already a "cloud" placed there.

After that, if the room is still too "live, it would depend on the size of the room and what else is in it (and where) to decide where/how much absorption you still need.

Smaller rooms should be fairly dead at the rear, so that you don't get early reflections back to the mix position that are less than about 25 milliseconds later than the direct sound. This can cause what's known as "comb filtering", which tends to smear stereo imaging and can cause you to make bad changes to EQ trying to fix it. Since this phenomenon is a TIME error, using EQ will only make it worse so the best thing to do is not let it happen in the first place.

Hope I didn't get too far astray... Steve

[googacky]

thanks for the information and the links guys.

i would think that absorption would help my flutter problem. if less energy is bouncing back, it should be less apparent. not to mention the modicum of diffusion i'd get by putting a few jutting boxes on my walls. splaying the panels is a good idea, though, and i'll look into incorporating it. it's interesting to see everybody pitching in on this topic. it seems like acoustics is something that isn't covered in the magazine as much as many other topics, and i'd imagine many of us need a primer.

[JES]

Putting solid plywood on the back of the 703 is only good for mids and up. It can also work for "clouds", or plywood backed 703 floated over the mix area. If you hang these so that they are lower toward the front of the room, you will further decrease flutter echoes between floor and ceiling.

Steve

Hey Steven

*Great* post. Agree about the need for absorption and diffusion. I'm very interested in your "clouds" idea as I'm thinking of something a little different -- a 4'x4' frame, filled with 705 (for more low frequency absorption) and covered on each side with fabric. Then I'll hang it from the ceiling joists over my mix area. Is this less preferable in your mind than a "cloud"? If so, why? It is a little more involved.

Also, I was under the impression that the lower end of the "cloud" should be toward the back of the room. What's the theory behind that?

Thx.

--JES

[JES]

and another thing. . . .

I want to second the need for more acoustic discussion in Tape Op. As it stands, people get way too hung up on monitor specs, etc. It didn't hit me until I picked up a set of Mackies and discovered that I had huge holes in the low end and huge cancellations in the high end (since I've heard them in a properly tuned room, I know how good they can sound). That's when I realized that some treatment would be money better spent than a new rack unit or anything else.

Best,
--JES