homemade bass trap question

[kayagum]

Here's a question (or two) for DIY acoustics (Ethan?):

* If you build corner traps, do you have to seal the trap (a la membrane) or is it OK to have just slabs of 703/705 cloth-covered? Is the unsealed trap good enough?

* If I used a hard surface facing out (like plywood) instead of cloth, would that help bass trapping performance?

I had a relevatory moment the other night... I am spec'ing out a job to salvage a mix (interesting- a guy found church hymns written by his grandfather literally in the attic, scored the whole thing in Sibelius, and recorded them in a church with him singing with organ accompianment).

We sat in different parts of my basement studio, and it was amazing how different the bass response was based on where we were (standing wave nodes!). I also did the "next room" listening test (sat in the staircase outside the room) and we could hear the upper mid smear disappear, and the balance checked out fine between voice and organ.

More importantly, my girlfriend could hear the difference, and she now understood why I have a stack of 703 fiberglass in the garage.

[tiger vomitt]

Here's a question (or two) for DIY acoustics (Ethan?):

* If you build corner traps, do you have to seal the trap (a la membrane) or is it OK to have just slabs of 703/705 cloth-covered? Is the unsealed trap good enough?

ethan would say no i think. mine are as yet unsealed in the back and they work really well. im sure theyd work A LOT better if they were sealed but i havent had the time or inclination to do it yet. i was short on wood when i first put em together. id say seal em from the the start tho.

wait i just re-read your question. i dont think having cloth covered sheets of 703/705 in the corner would do that much. you gotta but em in a box like in ethan's plans. mine are open in the back but they still work pretty well.

* If I used a hard surface facing out (like plywood) instead of cloth, would that help bass trapping performance?

see ethan's plans on his site. he goes over all that stuff.

i could be wrong about all of this. but ive done it..

good luck

[knmy]

By using burlap or other cloth on the front of 703/705 when placed in a corner, makes them a broadband bass trap. You can use wood to frame the fiberglass in. Using wood panels on the front, depending on the size of the wood, makes them a deep bass trap, mid/high absorber or a high bass trap.

[kayagum]

Maybe this begs another question (or a rephrasing):

Does the wood panel front (by itself- no back or sealed membrane) help with increased low frequency absorption, or does it merely make the trap more selectively low freq by reflecting high and mid freq?

[knmy]

From my understanding if using a wood panel front the thickness of the wood determines (selects) what is absorbs. Take a look at this pic: http://www.ethanwiner.com/BTPlans.gif
You will see the only back they have is the wall itself.

Even having 703 covered with burlap spaced out from the wall and in the corners will make a difference.

[Russian Recording]

yo
you guys are talking about two different traps.

the mini-traps, which are the ones that you would hang in corners, are nothing more than a piece of fiberglass in a frame. The fiberglass does all the work. By putting a solid piece of wood in front or behind it you are actually preventing it from doing it's job because the air must "pass through" for it to absorb. Placing cloth arounf the fiberglass is primarilly for keeping the fiberglass particles from getting into the air. A thin cloth will not alter the high frequency acoustics of your room significantly (not anymore than the sruface of the fiberglass).

Ethan used to sell panel absorbers, but has since discontinued them since the mini traps are so much easier to build and just as effective and over a wider frequency range. The panel traps were sealed on both sides and the plywood is actually what did most of the work rather than the fiberglass. The excessive frequencies in the room would vibrate the plywood, which would dampen that particular freqeuncy and prevent it from being reflected back into the room. These looked like a total pain in the ass to build.

I built my mini traps out of 705 fRK, some 2 x 1s, some mounting hardware and some bungee chords. Super simple and very useful.

mike

[knmy]

Hey Mike. I was just explaining the difference between the two, I really didn't get into the why's of the two. Thanks for explaining, I'm bad at that. I have been procrastinating on building my broadband traps for sometime now as I only have one useable corner in my room. The other corners have a window, closet and door in them. I am going have to do the mic stand and trap method. You got any plans you could whip up or a detailed part list from building your traps?

Ken

[kayagum]

Thanks- I just wanted to get some additional info before I start over-engineering them.

Not that over-engineering is a problem around these parts, eh?

[knmy]

By the way, where did you end up buying your 703/705 from?

I'm so bad at building things, ha-ha.

[kayagum]

I looked through the yellow pages, and found an insulation contractor supply store. You won't find this stuff at Home Depot or Lowe's. There should be a section for insulation contractors in your phone book.

You can buy a box of this stuff (dozen sheets) for around $5/sheet. The store I went to (in St. Paul, MN) didn't sell 705, but sold 703 in 1" and 2" thicknesses.

[Russian Recording]

I bought the 705 FRK in one crate of (6) 2" sheets and also one crate of (4) 3" sheets. This cost me $120. The 705 FRK has the best low frequency absorption (the thicker the better). regular 705 comes in second and 703 isn't quite as absorbent in the lower frequencies, but I believe it is more evenly absorbant accross the entire frequency spectrum. You used to be able to see the aborption coefficients for each on Ethan Winer's page, dont know if you still can. I know if you go to the Owens Corning site you can dig up the papers online. Also, there are other manufacturers that make a similar product, although I dont remember their names off the top of my head.

I think i priced out about another $400 for all the materials I would need to build (8) 2' x 4' traps and (4) 2' x 2' traps. This includes mounting, wood, stain,etc. That's awesome considering it would cost *a lot* more to buy them pre-manufactured.

mike

[knmy]

Thanks for the info everyone. You are going to stain the frames so you are gonna get fancy with them.

I believe Ethan has the coeffeciants on his site somewhere. I have a link for one on Bob Golds site: http://www.bobgolds.com/AbsorptionCoefficients.htm

Ken

[llmonty]

I have bought some 703 from a company called spi - i think the site is www.spi-co.com - it was mentioned on homerecording.com and they have retail stores all over the US. I had mine shipped from Raleigh to Richmond, was about $15 to ship (got it in 2 days!) 5 panels and some other accessories.

Their price for it was $98. a sq ft, which is a bit on the high side, but hell, just finding this stuff had meed hard and a few $$ didn't really bother me.

Other alternatives that are cheaper are Johns Manville and/or semi-rigid rockwool - neither of which the Raleigh store had in stock (but I have heard that others do)

[Russian Recording]

yeah, I had the hardest time finding the 705 FRK anywhere in town. One place I called had an equivalent 2" substitute, and their quote was like $350 for 12 sheets! I figured that was what I would have to settle for. Then I lost their number. I forgot the name of the damn place so I just started calling random insulation places in the phone book. The first one I called said it wasn't them, but the would see what they could do. They said they could special order i as much as I needed... no shipping charge! And then the total was $120 for 6 sheets of OC-705 FRK 2", and 4 sheets of 3". That's the dank shit, and a steal at that!

If anyone wants the number to this place, I can send it to you, just PM me. They are here in Bloomington, IN. I dont know if they'd ship it out, but even if shipping was like $50, it's still a great deal.

mtar

[Russian Recording]

Oh yeah, and in case I didn't make it clear the sheets are all 2' by 4'.
And I think you can even order 4" thickness... Im not quite sure though.