Attic Space as Basstrap?

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!!!

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

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

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

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!

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

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...

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 :lol: ). 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

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!!

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!

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

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

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!

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!

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

Hi Steve As always, thanks for all the info. Unfortunately, I'm a bit confused. I guess I was not aware that these absorbers would actually have BACKS; I just thought they were to be sealed to the drywall by caulking the frames. Duhhhhhhh. Oops. OK, so the frames are 2x6, the 1/4 or 3/8 ply (front and back) is screwed and glued to the outside of the frames, the chicken wire holds the insulation 1/2" to 1" away from both the front AND back panels, and I shorten the depth of the 1/4" trap by mounting a sheet of 1/2" ply on cleats inside the frame perimeter. That would seem to effectively give a much stiffer back, as opposed to the 1/4" on the front..... Steve, I think I understand everything you've said, and I feel like an idiot for asking, but I think I'm gonna need a diagram. As you obviously know, the absorbers need to come out at the exact same depth for aesthetics. I can rip the frames for whatever depth I need, put whatever thickness of plywood wherever it needs to go, and I apologize for asking but a diagram of the 2 traps would REALLY help. Thanks as always

No prob, Joe, but give me a day or two - I'm kinda "ass-deep in 'gators" momentarily, so only have maybe an hour or two in evenings (other than "morning coffee time") - I'll get you something ASAP... Steve

Joe, here's a first attempt - if a particular trap is slated for a thicker front panel, just reduce the width of the side pieces by the difference so the faces are all the same distance from the wall. Then use the movable inner back as drawn, placing it where needed for the particular air gap required. The "hardware cloth" can be anything from 1/4" mesh to 1" or even 2" mesh, its only purpose is to contain the insulation so it doesn't contact the front panel. I didn't show all the screws needed, it's too much work. Obviously where two screws are shown at right angles to each other, they do NOT occupy the same "Z" axis - as for screws, relatively light weight "bugle head" screws will work, but whatever you use make sure to get a proper pilot bit for the exact screw size. Better ones allow you to adjust a separate bit in the center, to compensate for same diameter screws with different lengths. I don't recommend either flat slotted heads or phillips for power driving, they tear up too easily. Try to get square drive if you can, and buy a few proper size bits for them while you're at it. Either rent or buy a driver or screw gun with adjustable torque and/or depth (Makita makes one with both, possibly some other brands do too) - then, experiment with some scraps to determine proper depth/torque before you go for the real thing. If you don't have/can't afford two separate drills, you can get kits that let you quick-change bits between pilot and driver, so you can drill a few/drive a few - much quicker that way. Let me know if you need any other clarification... Steve

Why does the insulation have to be held off the rear of the resonator? Isn't just the front panel the one that resonates? Just curious... -ashley

You're right, it doesn't have to be held clear of the rear - if you were to pack the box full, the rear of the box could take the place of the lower screening in the section view. It's only if you DON'T pack it full that the batts could sag, giving a less even damping. I'm not even sure THAT would negatively affect the trap, but it might. Don't take this as the only way to build, that drawing was in response to Joe's request for traps that all have the same outer dimensions, to be used as a sort of "wainscoting" around the bottom part of a room, with other absorption above them. He wanted the fronts even, but the frequencies that needed trapping didn't cooperate with available panel densities without using different air gaps... Steve

I've just finished building some gobos that have rockwool inside that is held off the slats by a couple strips of metal cloth. If you don't have to put the two layers of retaining cloth to hold the insulation, don't. What a pain in th a$$ putting in all those cleats and securing the cloth when you could just set it in there and put one layer to hold it in. The stuff ain't that heavy. I would hope that it wouldn't sag. Just MHO. Joe, you going to NAMM? -ashley

Ashley, Joe didn't mention (and I didn't think to ask) WHICH kind of insulation he planned on using - if it's the stiffer stuff like rockwool or 703, then definitely only one cleat and zero chicken wire should be necessary to keep it off the front. If those materials aren't easily available or for some other reason he decides to use spun fiberglas batts, then he WOULD need the hardware cloth at the front, but could just stuff in enough "fluffy stuff" to stay put between the front hardware cloth and the back panel... Steve

How cool, I've got more than enough mineral wool to do the job- I guess I'd heard somewhere before that batts were the way to go. Fab, I've just saved another trip to Home Depot. Steve, thanks so much for the drawing, it's all clear now. Referring back to your original reply, you'd calculated that my problem areas are at 70 and 100hz, After making an Excel version of your panel formula (which I can email if you'd like, but I'm sure you have a much more sophisticated version:)), I've determined that a 1/2" panel over a 4" cavity will give me the 70hz, and a 1/4" panel over a 3 7/8" cavity will give me the 100hz. I'm using 2x6's for the frames, and will either rip them down and caulk the frames directly to the drywall, or install the false backs (of 1/2" ply) to achieve the proper depth. All the panels will be 3ft x 6ft, and my trim only along the edges of the panels. If I've made any mistakes, tell me now 'cause I gotta start building! Again, Steve, thanks a zillion!

PS. Ashley, thanks for the info. I'm not sure if I'll be going to NAMM, January's way busy this year for me. Steve, my last post was at 3:59 am LA time, and your previous post was only an hour before. GO TO SLEEP!

Lookin' good Joe, sounds like you're there - Sleep? Freakin' waste of time, I've got two new Philip Newell books I've not even been through the FIRST time :roll: Besides, I got to bed by about 4:30 and slept clear til 9... Steve