Attic Space as Basstrap?

JoeMilner · Post by **JoeMilner** » Fri Oct 31, 2003 9:29 pm

Hi all
I'm partway through construction of an ADR (film dialog) recording stage. My building is a movie theater from 1930, and, in the area where the stage is being built, the existing ceiling is 16' 7", and it is painted 5/8" drywall. The attic space above is about 6 feet in the center of the building (bowstring trusses), and the ADR stage runs parallel to the trusses. I've treated the side walls (from 3' above the floor, to the ceiling) with fabric-covered 1 1/2" mineral wool, the back (control room) wall with 1" Owens-Corning Select Sound (Black 703), and the front wall (where the 16'x8' movie screen is) with a mix of 703 and fabric/mineral wool. The floor is carpet w/ 1/2" pad. As a rule, ADR stages need to be quite dead, but only in the sense that you don't hear any room reflections. You need to be able to put a few feet of distance between the actor and the mic, but you just want to hear the distance, not the room.
So much for the highs, now for the lows.
My question is this: the attic space is currently un-insulated, with no plywood or anything above the ceiling joists. I was planning to fill the joist cavities with batts, and cover with plywood/drywall,but before I do I'm wondering if there's a way to make the attic function as a low-frequency absorber, and how might I go about that? The stage (studio) is 25' 4" long by 23 feet wide, the side walls are each splayed 4 degrees, with the wide end at the screen. To compound the ceiling question, I need to treat the ceiling to control reflections whether or not it functions as a basstrap.Also, the 1st 3 feet from the floor is untreated. Could I do a 8 or 10" deep wainscoting along the walls that could act as 25-foot-long panel absorbers?

Thanks so much, this is a fantastic site!!!

knightfly · Post by **knightfly** » Sat Nov 01, 2003 6:33 am

If you've read Stranger in a Strange Land by Robert Heinlein, you're familiar with the term, TANSTAAFL - There Aint No Such Thing As A Free Lunch...

You can use the attic for a bass trap, but it will lessen the amount of isolation in that direction so you need to weigh that. If the outer roof is both heavy and air tight, it will make up for some of the isolation loss caused by using the attic as a bass trap.

If you decide to do that, one way would be to panel and insulate all but the outer 2-3 feet of the ceiling joists, (let the insulation lay on the panels, damping is good) then use loose hanging panels of Celotex wrapped in house insulation up in the attic, as many as will fit without firm contact - then, panel the 2-3 foot perimeter area of the ceiling with 3-4" rigid fiberglas (703, knauff, etc) - this will put the main "entrance" to the trap in the corners between walls and ceiling, where the pressure is highest, for best efficiency.

Ceiling treatment - 2-3" rigid fiberglas suspended a few inches below ceiling will handle absorption there, with no negative effect on the bass trapping from the attic.

The area next to the floor would be an excellent place for panel traps - again, corners are high pressure areas which is right where traps work best. You can tune these by depth and panel mass, here's the formula -

Fo=170/(sqrt)(md), where
Fo = Resonance Frequency
m = Surface Density of panel in lbs/sq ft
d = depth of air space in inches

1/4" plywood has a density of about .74 lbs/sq ft - other panels you can just weigh with a bathroom scale and divide by their square footage. 1/4" ply on 3.5" studs resonates at about 105 hZ. With a 10" air gap, that goes to about 62 hZ. 3/8" ply on standard 2x4's would come in at 86 hZ, and on 10" deep frames would be at 51 hZ.

It would probably work well to pick one of the room dimensions that might have the worst resonance for your situation and build traps centered on that mode. They would have the most effect if chosen to match the mode that is caused by the dimension that is perpendicular to the traps, and the second most effect on floor-ceiling modes.

I've attached a plot of your first few axial modes for that room, with possible problem areas circled in blue. These are at about 70 hZ and 100 hZ, both good candidates for panel traps. You could actually alternate traps, building every other one for one or the other frequency. If you chose the depth based on thicker panels for the lower freq, you could get the other freq by lighter panels and possibly a "false bottom" inside the higher freq trap. I'll let you play with the formula and a calculator for this while I get to some others' questions.

This should be a fair start, let me know what else you need... Steve

JoeMilner · Post by **JoeMilner** » Sat Nov 01, 2003 5:31 pm

Steve, that was a VERY informative reply to my pile of questions. Thanks so much!

knightfly · Post by **knightfly** » Sat Nov 01, 2003 9:58 pm

No problem, Joe - I just got a few minutes and ran the numbers for panel traps like you were asking about across the bottom of the wall - if you were to use standard 2x6's (5.5" actual) and 3/8" ply, that would come in at the lower of the two frequencies you need to treat - if you used 1/4" ply for the other trap, (90 hZ region) and put a 1/2" plywood spacer at the rear of that trap, you'd be right on for the higher frequencies too. The fronts would look identical if you were to shave 1/8" off the frames for just the 3/8 ply, and the whole thing would stick out from the wall just under 6".

Should work pretty cool... Steve

JoeMilner · Post by **JoeMilner** » Sun Nov 02, 2003 1:19 am

Thanks Steve
I'm assuming that these traps would all be 8' in length, right? Is there a specific distance I need between each trap? I'm just realizing that the trim board that would run along the top of the whole line of (probably 3) traps would be coupling the traps to each other. Is this a problem? Can the traps rest on the slab floor, or do they need to be wall-mounted or resting on neoprene?
Thanks again, I'll now be heading for the "Donations" page!

JoeMilner · Post by **JoeMilner** » Sun Nov 02, 2003 3:16 am

Hmmmm, PayPal seems to be acting up. I'll try again later

knightfly · Post by **knightfly** » Sun Nov 02, 2003 8:50 am

Joe, the only part of those traps that needs to be "un-fettered", or free to vibrate without outside interference, is the face. You need some "fluffy" insulation inside, NOT touching the front but close (1/2 to 1 inch) you can even use chicken wire or other metal mesh to contain the insulation just to keep it from touching the front panel - do a search on the various words associated like trap, panel, insulation, wire (probably a good one, less hits) - I posted the ideas on inner insulation of traps recently in one of the forums, maybe acoustics.

You can make the panels any length, more is better. For your distance, I'd divide by 4 so you can have two of each, alternating. Not quite as efficient for materials, but better for sound.

Never mind the search, just remembered who I was working with -

http://johnlsayers.com/phpBB2/viewtopic ... 7&start=15

Scroll down to 10-29-03, 8:59 AM, about the middle of the post... Steve

Oh, and thanks for thinking about the Donation thing, you'd be amazed at how few do...

bassman · Post by **bassman** » Mon Nov 03, 2003 7:48 am

Hey Joe,

Ashley Shepherd here. You helped me out on a book for PT and film. Sounds like a nice place you're putting together. I hope to be putting a room together myself ASATMRI (As soon as the money rolls in

).

Great site here isn't it? Thanks again for your help on the book. Here is a link:

Pro Tools for Film, Video and Multimediahttp://store.yahoo.com/muskalipman/159200069x.html

-bassman aka Ashley

JoeMilner · Post by **JoeMilner** » Tue Nov 04, 2003 8:58 am

Hi Ashley
Yeah, this is a fantastic site. Steve has been giving me some great advice, and the ADR stage will be finished soon. Be sure to post here when you start building your place. Congratulations on the book, it's been getting rave reviews!!

JoeMilner · Post by **JoeMilner** » Tue Nov 04, 2003 9:06 am

Hi Steve
So, if the panels themseves must remain "un-fettered" does that mean that a vertical trim board to cover the joint between adjacent traps will screw up the vibration of each panel? Also, since I'm hoping to make these appear as cherry wood wainscoting, can I put flat wall trim pieces on the panels themselves? Since the stage is in a 1930's theater, I'd like to have that vibe carry over to the new room.....

Thanks a million!

knightfly · Post by **knightfly** » Tue Nov 04, 2003 4:46 pm

A vertical trim board between panels should be fine, since the panels need to be sealed all the way around the box. However, anything you put on the panel itself will change the mass, therefore the resonant frequency. If you're talking about making them look like a paneled wainscot, as in dividing each absorber panel into squares with moulding, then they won't work anything like the formula predicts. If the traps are horizontal, for example, and you put vertical mouldings on their faces (say) every 2 feet, then you've just changed the 1/4 or 3/8 ply to effectively much thicker by stiffening it and adding mass.

If that's what you must do, the only way I know of to get the actual frequencies you want would be to build one the way you want it to look, seal the BACK with weatherstrip and screws, and test it by sweeping a sine wave generator thru the range of bass frequencies while lightly touching the front panel, thereby finding the exact resonant frequency of the modified panel. That frequency (there may even be more than one) will most likely be LOWER than the design formula states due to extra stiffness of the panel. You could then "reverse engineer" the formula in order to find out what box depth will get you the frequencies you need.

Starting with a lighter panel weight would get you closer to your target frequencies after adding the mouldings. Welcome to Star Trek - you're "boldly going where no amateur acoustician has ever gone before"... Steve

knightfly · Post by **knightfly** » Tue Nov 04, 2003 5:11 pm

Wish my *$(&* brain had an on/off switch, might get some sleep once in a while -

Another way you might do this would be to build the traps normally, then stain the front panels and build your moulding "frames" stood off the front panels by about 3/8", only fastened at the perimeter (through spacers.) This would let the panel vibrate but still look like paneled wainscoting, only with a "shadow" effect... Steve

JoeMilner · Post by **JoeMilner** » Wed Nov 05, 2003 12:37 am

Wow. I have to be careful what I ask for, I don't want you obsessing over my questions! Well, actually I do, but, get SOME sleep every now and again.
Yeah, my idea for the moulding was kind of a 1'x 3' rectangle made from 1" wide (or so) flat pieces in the center of each 3' x 6' absorber. I kinda figured that adding anything to the panel would screw up the resonant frequency. Hmmmmm, maybe the key is to leave the panels alone, and do my "design" only on the top trim, where it could be invisibly decoupled from the traps. Cool idea with the spacers, though, I may try that one out!

Thanks as always!

JoeMilner · Post by **JoeMilner** » Tue Dec 09, 2003 2:36 am

Hi Steve
So, a month later, I've finally got some time to build my "wainscoting absorbers", per your design recommendations. Couple things:

"..... if you used 1/4" ply for the other trap, (90 hZ region) and put a 1/2" plywood spacer at the rear of that trap, you'd be right on for the higher frequencies too".

What do you mean about the plywood spacer? Inside the frame perimeter?

I'm having a hard time here in LA locating 3/8" ply, at least the birch hardboard type I'm using for the 1/4". Any suppliers come to mind?

Do the traps rest directly on the concrete slab, or should I put a felt or rubber pad underneath them to help with rattle?

Should the batts inside the traps be faced or unfaced?

Thanks!

knightfly · Post by **knightfly** » Tue Dec 09, 2003 8:12 pm

Joe, good to hear from you again - Gotta hit the sack pretty soon or wish I had, but I'll see if I can get yours taken care of first -

"What do you mean about the plywood spacer? Inside the frame perimeter?" -

Yeah - the purpose of the plywood is to shorten the air cavity depth, so it would go inside the trap - or, you could leave the perimeter sides of the trap at constant depth (for uniform wall look) and put cleats in place near the rear of the side panels, so you could use just one sheet of plywood for a back but move it 1/2" closer to the front panel, mounting it to the cleats instead of right at the very back of the frame. Either way, you want 1/2" less distance from the front panel to the back for that frequency.

Finding materials locally can be a real pain - if you can't find anybody that can help, the only other thing I could suggest is to either deepen the traps so you can use 1/4" and still get the frequency you need, moving the back panels of the higher freq traps further forward on cleats so they still match outside dimensions but maintain the correct inside air gap depth - or you might be able to get 3/8" panels by laminating 1/8" luan ply to the 1/4" using full surface contact cement (and lots of weights) - what you're looking for is the same mass per sq ft with no extraneous vibrations (other than the panel at its resonant frequency)

I would rest the traps on felt or rubber - otherwise you'll get buzzes.

Insulation, if you're using soft batts, shouldn't matter which side faces out, or whether there even IS a facing - what does matter, is that you keep it from touching the front panel by at least 1/2", up to maybe 1" - this can be done by mounting two separate layers of chicken wire or hardware cloth (really metal mesh) behind the front panel and "trapping" the insulation between them. They can be held in place with cleats and screws, If this isn't clear just say so and I'll do a diagram.

Running out of steam for now, I'll check back on you tomorrow nite... Steve