Sealed bass traps

[pandamonk]

For my drum booth, I'm thinking of having sealed, membrane?, bass traps. Is it best to use 1/4" plywood, and does the insulation need to be rigid, or can it be loft stuff? Should it fill the trap, or just hang in there? Is there supposed to be a gap between the insulation and plywood? Any more info i should know about?

Thanks

[mixsit]

Check out Ethan's site. He did panel traps and the info may still be up there.

[pandamonk]

Originally Posted by mixsit

Check out Ethan's site. He did panel traps and the info may still be up there.

Yeah I saw it. I just had a quick glance through it though. As far as i saw, he just explains how he did it, and I was just asking these questions which i didn't see answer in my glance.

[bpape]

Fiberglass should be rigid. It should be affixed inside the panel to be close to the membrane but not touch - the plan allows for 1/2". Thickness of the front panel (mass specifically) determines how the panel is tuned. In a drum room, you'll likely want 2-3 different tunings. Depth of the cavity also is a determining factor in tuning the absorber.

Just remember that these are only good for about 2-3 octaves at most.

Bryan

[pandamonk]

Originally Posted by bpape

Fiberglass should be rigid. It should be affixed inside the panel to be close to the membrane but not touch - the plan allows for 1/2". Thickness of the front panel (mass specifically) determines how the panel is tuned. In a drum room, you'll likely want 2-3 different tunings. Depth of the cavity also is a determining factor in tuning the absorber.

Just remember that these are only good for about 2-3 octaves at most.

Bryan

Will it work across a corner?

[bpape]

It can but then you'll have to make a really, really rigid back for it. Normally, we don't do sealed tuned absorbers like that in corners. We save the corners for broadband control. Also, there's no benefit to straddling a corner with a sealed panel - you get no additional extension from the spacing.

Bryan

[pandamonk]

Originally Posted by bpape

It can but then you'll have to make a really, really rigid back for it. Normally, we don't do sealed tuned absorbers like that in corners. We save the corners for broadband control. Also, there's no benefit to straddling a corner with a sealed panel - you get no additional extension from the spacing.

Bryan

A really really rigid back? Like 3 layers of brick? I need the to straddel the corner like this anyway, so was wondering if it could be adapted to be of any benefit acoustically. Would it be ok to use acoustic foam instead of rigid fiberglass? And could i place the foam/fiberglass like the first pic, or does it have to be liek the second?

[knightfly]

I agree with Glen on this - corners are better used for broadband absorption. If, after taking care of the corners, you still have "bumps" in your response, panel traps placed in the RIGHT places may be a help. However, keep in mind that the "right" places for broadband absorption are not necessarily the right places for resonators, since panels need to be in high pressure areas while absorbent traps need to be in high VELOCITY areas - not to mention TUNING the panels, etc... Steve

[bpape]

Exactly. Normally the best place for the tuned absorbers are flat on a wall at one end of where you're getting a peak/null based on the bounce. It's a much more targeted approach. If it's a length issue and you put them on the side walls, they won't be nearly as effective for instance.

As for the corner construction, they're most efficient when the fiberglass (I wouldn't use foam) is as close as possible to the front panel - usually 1/2". The back should be like double MDF. Also, if you're not making a rectangle but doing the triangle cavity as you showed, then you need to calculate the average depth of cavity and use that for your figuring.

Bryan

[knightfly]

I'd go further than that, Bryan; I think putting panels across corners is too much of a "crap shoot" to be sensible. First, you're wasting valuable broadband space and second, if you're trying to target specific frequencies you've just made your panel into a less effective broadband unit. Kind of a "lose-lose" situation near as I can see... Steve

[TexRoadkill]

Is there any benefit to using more dense material on a corner trap? For instance OC705 vs 703. Will a 2' to 3' corner cavity (it's tighter then 90 degrees) be just as effective with the 703?

[bpape]

The thicker you make the absorbers, the less density plays a significant part. Once you get to 6" thick, 703 does just as good a job as 703 down to say 50Hz or so (and it costs half the price).

However, it should be noted that the same amount of fiberglass it takes to make a 2'x4'x6" straddling panel will also make the same height 17"x17"x24" solid triangular chunk style absorber that actually takes up less space out in the room and performs better down low.

Bryan

[pandamonk]

Originally Posted by knightfly

I'd go further than that, Bryan; I think putting panels across corners is too much of a "crap shoot" to be sensible. First, you're wasting valuable broadband space and second, if you're trying to target specific frequencies you've just made your panel into a less effective broadband unit. Kind of a "lose-lose" situation near as I can see... Steve

I agree with you guys. But from a practical perspective, my dad and I had decided building across the 2 corners like this. So it's being done anyway, and i just wondered how it would perform as a bass trap, and how i could build it to work better as a bass trap, while still being across the corner. I will have lots of movable broadband absorption in this room, and it's so small(9'x5'x6' drum booth) that i will try and make it reasonably dead.

Will it work at all, even if i use the foam instead of rigid fiberglass? I know it'd be better flat and with fiberglass, but i need to build across the corner like this anyway, and to get the fiberglass i have to travel etc, and would have to wait a while, whereas i already have the foam, and am wanting to get rid of it, behind walls etc, as much as i can. :P

Thanks

[Farview]

I don't think the foam will do anything behind the resonator. It doesn't have the density to dampen the movement of the front panel.

[knightfly]

Farview, panel traps don't work on damping, other than percentage of fill - they work in two ways.

One, the "effort" required to move the panel itself at resonance.

Two, when the panel is moving, it tries to "pump" air back and forth through the inner insulation - the moving air thru the insulation is restricted by the convoluted path thru the interstices of the insulation, which converts sound energy into (very) low grade heat. The combination is what absorbs the sound.

The fuller the inner cavity (and the denser the insulation) the more resistance, so you get more absorption that way - however, this also widens the range of frequencies that are affected and LOWERS overall absorption at the same time. At no time should the insulation actually TOUCH the inside of the front panel, or it will damp that panel's vibrations and again lessen (or eliminate, in extreme cases) the absorption.

Panda, are you in a part of Scotland where you can get sheeps wool locally? If so, that will work better than your foam, and as well as nearly anything you can travel to buy. Just stuff the cavity with wool, put some chicken wire or similar TIGHTLY across to keep the wool from touching the panel, and put up the panel.

I have to warn you though - I've NO IDEA what frequencies this will absorb, only that it should be broader band than a normal, parallel-faced panel absorber and NOT as efficient... Steve

[Farview]

Originally Posted by knightfly

Two, when the panel is moving, it tries to "pump" air back and forth through the inner insulation - the moving air thru the insulation is restricted by the convoluted path thru the interstices of the insulation, which converts sound energy into (very) low grade heat. The combination is what absorbs the sound. Steve

That's what I meant by damping. Sorry for the confusion.

[pandamonk]

Originally Posted by knightfly

Farview, panel traps don't work on damping, other than percentage of fill - they work in two ways.

One, the "effort" required to move the panel itself at resonance.

Two, when the panel is moving, it tries to "pump" air back and forth through the inner insulation - the moving air thru the insulation is restricted by the convoluted path thru the interstices of the insulation, which converts sound energy into (very) low grade heat. The combination is what absorbs the sound.

The fuller the inner cavity (and the denser the insulation) the more resistance, so you get more absorption that way - however, this also widens the range of frequencies that are affected and LOWERS overall absorption at the same time. At no time should the insulation actually TOUCH the inside of the front panel, or it will damp that panel's vibrations and again lessen (or eliminate, in extreme cases) the absorption.

Panda, are you in a part of Scotland where you can get sheeps wool locally? If so, that will work better than your foam, and as well as nearly anything you can travel to buy. Just stuff the cavity with wool, put some chicken wire or similar TIGHTLY across to keep the wool from touching the panel, and put up the panel.

I have to warn you though - I've NO IDEA what frequencies this will absorb, only that it should be broader band than a normal, parallel-faced panel absorber and NOT as efficient... Steve

Thanks steve. Unfortunately I'm not at that area of Scotland, lol. I might be able to get it if I try, but i don't have a clue where, or how to get it.

I have lots of lost insulation extra from insulating the loft which i could try and compress if you think that'd work better than the foam? I just suggested the foam cause i really wanna get rid of it, and the insulation could just stay in the loft.

I understand that it won't be as efficient, and broader band. I'm just really looking for any benefit i can get, 'cause as i said i'd be doing it anyway. If you think it's a good idea, I'll try and compress the insulation, or if not, just use the foam. Even if i get nothing beneficial, it'll get rid of some hideous foam, haha.

And I'll have a lot more broadband absorption in this drum booth, so It shouldn't matter a great deal... I hope :P

Thanks

Lee

[knightfly]

I don't really think compressing insulation will work, especially for that application - the problem is, how do you keep it CLOSE to the inside of the panel without actually TOUCHING it, which you do NOT want to do.Once you let the insulation actually contact the panel, it's not ANY kind of trap - it's just a beveled corner.

I think you may be better off just doing your OTHER treatments and seeing how that sounds - then if you're still not happy you might wanna glue 3 or 4 thicknesses of that foam together and use it for extra absorption - or, you could just burn it (stand upwind, of course :=) Steve

[pandamonk]

Originally Posted by knightfly

I don't really think compressing insulation will work, especially for that application - the problem is, how do you keep it CLOSE to the inside of the panel without actually TOUCHING it, which you do NOT want to do.Once you let the insulation actually contact the panel, it's not ANY kind of trap - it's just a beveled corner.

I think you may be better off just doing your OTHER treatments and seeing how that sounds - then if you're still not happy you might wanna glue 3 or 4 thicknesses of that foam together and use it for extra absorption - or, you could just burn it (stand upwind, of course :=) Steve

Haha. That's all that foam deserves!!! (don't think it burns too well though ) I'm certainly gonna use the other treatments. I'll shove a bit of the foam behind the corner. If it don't work, "it's just a beveled corner", which i was wanting anyway. I would glue that foam together, but i just want rid of it. I don't wanna see it. I could wrap it in fabric, but feel it'd be a waste of fabric, haha. I'd just shove it behind the walls I've still to do, and it can "help" insulate. (doubt it'll help though, but it get's rid of it, without burning it).

[pandamonk]

Aha, talked to my dad about it, and even though we've begun construction, we're gonna just do normal corners . Cheers for all your help guys!