Attic Space as Basstrap?

JoeMilner · Post by **JoeMilner** » Wed Dec 10, 2003 12:34 am

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

knightfly · Post by **knightfly** » Wed Dec 10, 2003 5:21 am

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

knightfly · Post by **knightfly** » Wed Dec 10, 2003 1:25 pm

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

bassman · Post by **bassman** » Wed Dec 10, 2003 1:51 pm

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

knightfly · Post by **knightfly** » Wed Dec 10, 2003 2:12 pm

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

bassman · Post by **bassman** » Wed Dec 10, 2003 2:51 pm

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

knightfly · Post by **knightfly** » Wed Dec 10, 2003 9:07 pm

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

JoeMilner · Post by **JoeMilner** » Wed Dec 10, 2003 10:00 pm

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!

JoeMilner · Post by **JoeMilner** » Wed Dec 10, 2003 10:03 pm

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!

knightfly · Post by **knightfly** » Thu Dec 11, 2003 5:44 am

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

Besides, I got to bed by about 4:30 and slept clear til 9... Steve