To face or not to face?

[JP]

Hi everyone,
I've put some time into this site over the last few days and it is really helping me get my head around this subject and come up with some sort of a sensible plan for treating my room - so nice one!

On to my Q:
Having built a 16'8'' (lenght) x 13'5'' (width) x 9'6'' (avg height) control room\studio I am keenly diving in to acoustically treating the space. First step is bass traps in the corners and having read various articles of Ethan's I am planning on staring with 2 adjacent panel traps in each corner (a deep-bass trap and a high-bass trap as per his plans). Working from Ireland I don't have access to the Owens Corning product range but I believe I have sourced a similar product to OC705.

http://www.rockwool.ie/graphics/RW-GB-i ... _slabs.pdf

(The RW5 product is the one I am looking at - going with 1 inch thick, again as per Ethan's plans - not shown in brochure but it is available at that thickness)

Although I haven't seen the OC705 absortion figures, the unfaced absorption figures of the product above, match quite closely to the performance of Knauf Insulation Board (which I have seen recommended on this site) at the same density.
My question is as follows - I noticed when comparing with the Knauf Insulation Board specs that the low freq performance of of the Knauf board was vastly improved with the FSK (facing) option. The Rockwool boards above can be ordered with an aluminium foil facing on one side. Should I go with this? Will it make a difference even though the insulation will be behind plywood fronts and if so, given that it will be inside the trap, which way around should it go?

Cheers,
John

[Ethan Winer]

John,

> Will it make a difference even though the insulation will be behind plywood fronts <

I've never built wood panel bass traps using faced fiberglass so I can't say for sure. But my guess is the facing is of no use. It might even be bad, but I can't say that for sure either. The facing does help when using fiberglass only, but probably not when inside a sealed panel trap.

--Ethan

[JP]

Thanks Ethan,
One last question then - I have assumed that for the purposes of the panel traps in question, the denser the fibreglass the better and that even though you spec OC703 in your plans (at 48kg/m3) if I used RW5 (at 100Kg/M3) I would get an even better result. Again whilst obviously true (I think!) for a simple fibreglass trap - is it also true for the panel traps in question?

Thanks for sharing,
John

[Ethan Winer]

John,

> even though you spec OC703 <

Again I have to plead ignorance. My guess is it doesn't matter a whole lot which density of rigid fiberglass you use. But since I've never actually built a bunch of traps using each density and tested them in a lab, a guess is all I can offer.

I will say this: if you build the traps exactly as shown in my plans they will work as advertised.

--Ethan

[knightfly]

John, I agree totally with Ethan on this one - the old (in the US anyway) saying, "If a little is good, a lot is better" doesn't always apply to acoustics - I'm talking about things like the number of mass leaves in a wall or door or window, the density of absorbent for different applications, the amount of absorbent by percentage of wall coverage, etc -

Perhaps a better adage for this purpose would be "if it ain't broke, don't fix it" -

Since the absorbent inside a panel bass trap functions as further absorption (generally, the panel resonates, and its vibrations convert the pressure at its surface to moving air inside the trap (in the form of sound waves) which are then further absorbed by the insulation - if insulation is too restrictive to air flow, you start LOSING performance at some frequency ranges. Anyway, the total absorption is a function of the size and density of the front panel and the degree of absorption of the inner material, keeping in mind that there needs to be room between the panel and the absorbent for the "restricted movement of air" thing to work -

Hopefully this crude explanation of the inner workings of this type trap will help you understand more; the bottom line, though, is that Ethan's design works as is. If you want to modify it, I hope it's because you're adventurous and don't care about immediate positive results or minimum budget; not all experiments come out as hoped... Steve

Oh, almost forgot - In Everest's Master Handbook of Acoustics, he mentions that inside a wall it doesn't matter which way the paper facing on normal house insulation is oriented - however, it matters quite a bit when using that material for out-of-wall acoustic treatment. Same for the FRK facing on rigid fiberglass. When used INSIDE a trap, however, I can't help thinking that you would run into the physics that make a 3-leaf wall inferior to a 2-leaf wall at lower frequencies; for that reason, I would REALLY have to have a lot of spare time and money to even try the FRK stuff inside a trap, unless it was placed with the facing glued against the REAR of the trap so as to acoustically disappear... Steve

[JP]

Thanks Ethan and Steve,
I have now almost completed the traps as per Ethan's specs. Thanks for your analysis Steve, I went with the "If it aint broke" philosophy in the end.
One last question though (I know I said that last time!). I have built 2 deep-bass and 2 high-bass floor to ceiling traps as spec'ed (I have acoustic foam in other areas of the room taking care of the high frequencies). I have also built a large corner type trap across the front wall\ceiling perpendicular. The backwall remains untreated but I'll deal with that at a later date when funds allow! I haven't tested it out yet as my speakers are in the repair shop but hopefully soon!

Anyway back to my question which relates to the panel traps. I fronted the deep-bass traps with 6mm plywood (approx 1\4 inch), the plan was to front the high-bass traps with 3 or 4mm ply (approx 1/8 inch) again as per Ethan's plans. However I had trouble getting the thinner ply from my local supplier. I have some 3mm hardboard which I was thinking of using instead. My question is does it make a difference? I assume that although the same thickness, the different qualities of the wood would make them resonate at different frequencies? Does this mean I am better off taking the trouble to get the 3mm ply?

Thanks,
John

[Ethan Winer]

John,

> The backwall remains untreated but I'll deal with that at a later date when funds allow! <

Yes, line the whole thing with alternating traps.

> I have some 3mm hardboard which I was thinking of using instead. <

I believe all that matters is the weight of the hardboard. If it weighs the same as 1/8 inch plywood it should behave about the same in a panel trap. I've never used anything by plywood, but John Sayers told me he's used Masonite and gotten good results. So that's another option, assuming the Masonite you buy also has the same weight as 1/8 inch plywood.

--Ethan

[knightfly]

Again, I'll second Ethan's answer - the only negative thing on hardboard is if you're in a fairly humid or damp environment - this stuff is NOT very durable outside of a DRY environment - it will warp, crumble, and just generally make you cry

However, if you can keep it dry, the mass is the main thing... Steve