Air gap or low density absorber behind a bass trap?

[azzzy]

After reading pretty much every thread I could find on the subject I am still not clear on what density rockwool I should go with for my bass traps. The consensus appears to be that higher density works OK in thinner traps but if you can spare space a thicker trap with lower density absorber works better (even at the low frequencies). Is that correct?

Now I have another question. What if instead of an air gap behind a high density absorber (4" 6-8 pcf) I were to place a low density absorber (fluffy stuff)? Would that work better than air or worse? Or same? Perhaps it would make the absorber more broadband? (as in the high density part absorbed lower freqs and the fluffy stuff worked on the mids and highs)?

[SAC]

Fx of filling the gap...(Very) marginally better.

Porous bass traps work on the principle of converting wave velocity to heat.

The velocity of the wave is greatest at its quarter wavelength and approaches zero at and near the boundary, while the pressure is greatest at the boundary. Thus the greatest 'return' is at or near the quarter wavelength spacing and diminishing the closer to the boundary you become.

Thus, while more absorptive material will add a small/diminishing advantage, the return is marginal as the velocity is less.

But hey, if you have it, use it.

[azzzy]

No, I don't have it. I don't have anything yet. I am trying to figure out what to get. I live in a major metropolitan area and pretty much any density is readily available. The most economic route for me would be Thermafiber Safing 40 (4pcf) that I can pick up at a local Menards for around $8 per 4"x24"x48". I can also get Roxul 60 (6pcf) for around $9 per 4"x24"x48". But I can't decide whether I should go 4" thick with 6 pcf or 8" thick with 4 pcf. Or a foot thick with fluffy stuff. Or make a combination/layered absorber with higher density in front and lower density in the back and then an air gap.

[avare]

Originally Posted by azzzy

Or a foot thick with fluffy stuff.

That is the best acoustically of all the constructions you described.

Andre

[azzzy]

Originally Posted by avare

That is the best acoustically of all the constructions you described.

Andre

Thanks for chiming in, Andre. If I do use a foot of fluffy insulation, do I have to leave a foot of air from the wall? What about the corners? I've read that the trap must straddle the corners. With a foot-thick absorber would I have to move it farther away from the corner to the point where the absorber does not touch the walls?

[avare]

Originally Posted by azzzy

If I do use a foot of fluffy insulation, do I have to leave a foot of air from the wall?

Your original options did not include a 2 foot deep absorber. Use 12" of pink fluffy against the wall.

What about the corners? I've read that the trap must straddle the corners.

??? You have not asked about corner mounting for absorbers. For a given depth of absorber, the most absorption will occurr in the corners because that is where the room modes all come togetht\er in phase. There is no "must straddle the corners."

With a foot-thick absorber would I have to move it farther away from the corner to the point where the absorber does not touch the walls?

I think I answered that in the previous two sentences.

Andre

[DanDan]

azzy, absorbent treatments including practical examples can be seen at the home of the SuperChunk
studiotips - tips on studio design, acoustics, and wiring

DD

[azzzy]

Originally Posted by avare

Your original options did not include a 2 foot deep absorber. Use 12" of pink fluffy against the wall.

From what I've read leaving an air gap between the absorber and the wall greatly increases the absorber's efficiency. Is that not the case with the fluffy insulation? Also would a foot of pink fluff flush against the wall perform better (at low freqs in particular) then a 6"-thick 3-6 pcf absorber with a 6" air gap?

Originally Posted by avare

??? You have not asked about corner mounting for absorbers. For a given depth of absorber, the most absorption will occurr in the corners because that is where the room modes all come togetht\er in phase. There is no "must straddle the corners."

I am trying to treat the whole room, which will include (at least at the moment) 4 floor to ceiling corner absorbers, 10 absorbers mounted diagonally along the ceiling (to treat the corners where the wall meets the ceiling), 4 absorbers on the ceiling, and several on the walls.

I cannot go thicker than 4" (with a 4" air gap) for the ceiling absorbers, so those will probably be 6 pcf density. Or do you think 4 pcf density would be better? Or would 8" of pink fluff flush against the ceiling be better?

As far as the corners go, I read the absorber "must straddle the corners" and "must touch the walls". Initially I was going to move my absorbers a few inches into the room to make some air gap between the absorber edges and the wall but after reading several threads this appears to be wrong. My question in short - what's better for the corners: a foot of fluffy stuff placed across the corner or 6" of 6 pcf rockwool? Also does the absorber (in either case) have to touch the walls or is it better not to touch the walls?

[avare]

You are not being consistent in your thought process in the thread. Giving (broad) generalizations, use:

up to 4" thick absorbers use 3 lb/ft3 fiberglass or 4 lb/ft3 mineral wool;
4-6" thick absorbers use 4" 3 lb/ft3 fiberglass or 4 lb/ft3 mineral wool with a gap to complete the depth;
6-12" thick use 4" 3 lb/ft3 fiberglass or 4 lb/ft3 mineral wool with a gap to complete the depth, or fill fill the space with ~2.5 lb/ft3 mineral wool like Roxul Safe N Sound;
12" and up fill the space with pink fluffy.

That is it in a nutshell. A gap may, or may not be usefull, depending ont eh properties of the insualtion and the thickness of it. There is no simple relationship.

Andre

[azzzy]

Originally Posted by avare

You are not being consistent in your thought process in the thread. Giving (broad) generalizations, use:

up to 4" thick absorbers use 3 lb/ft3 fiberglass or 4 lb/ft3 mineral wool;
4-6" thick absorbers use 4" 3 lb/ft3 fiberglass or 4 lb/ft3 mineral wool with a gap to complete the depth;
6-12" thick use 4" 3 lb/ft3 fiberglass or 4 lb/ft3 mineral wool with a gap to complete the depth, or fill fill the space with ~2.5 lb/ft3 mineral wool like Roxul Safe N Sound;
12" and up fill the space with pink fluffy.

Once again, thanks for the answer. I'm a little confused. For 12" and up do you mean fill the space with pink fluff behind a 4" denser absorber or do you mean use the pink fluff alone in 12" (and up) thickness?

[azzzy]

Originally Posted by DanDan

azzy, absorbent treatments including practical examples can be seen at the home of the SuperChunk
studiotips - tips on studio design, acoustics, and wiring

DD

Thanks for the link, DD. Reading it right now.

[avare]

Originally Posted by azzzy

For 12" and up do you mean fill the space with pink fluff behind a 4" denser absorber or do you mean use the pink fluff alone in 12" (and up) thickness?

I wrote

12" and up fill the space with pink fluffy

Nothing about using any other material. None of the constructions have multiple densities of material in them.

Use the pink stuff alone in 12" and thicker.

Andre

[dabigfrog]

Hi dudes, we just finished treating our control room and a big boomy tracking room.

The main corner traps for both rooms we built skeleton frames (2x4 & 1x2) and wrapped them in .07 mil plastic and a woven sun screen material for strength and then blew in cellulose "soft touch" recycled insulation, filled 'em to the top. Once we tested them and were happy I covered them in natural burlap and they look great I think.

the big ones were 8 feet tall, 34 inches wide and like 18 + inches deep at the corner.... and they work great!



We also made some smaller corner traps with the pink fluffy stuff and they work great too.

just fluffy stuff if you can go 12" deep.

we originally thought we needed a thousand dollars worth of OC 703 but I'm happy we decided to go with the fluffy... saved us a bundle and they work.

we are still going to build some 703 panels wrapped in fabric for the walls and first reflexion points but not a third of what we originally planned.

the control room corners - 1x2 frame + aluminum screen door / window screen, I added the .07 mil plastic after test filling the first one just to keep the dust and blow out down. we also filled the ceiling soffit with blown in cellulose (above the desk)


here are the big 8' traps in the tracking room ( used 10 footer 2x4 for the center so we would have a way to mount it to the wall after we filled them)


here is a trap we made with leftover fluffy stuff from the construction.


so to recap what Andre said - with the fluffy - no air gap.... if you use 4 inches of oc703 to straddle the corners - touching the walls - I believe you do use an air gap, and as Andre informed me in another thread - you should not combine different materials in the same trap. pick one and run with it.

it's not rocket surgerythumbsup

[azzzy]

Thanks for the replies, guys.

Andre, so you don't recommend anything denser than 4 lb/ft3 for 4" thickness? That's a little contradictory to what I've read (including Ethan's page about absorbers). I thought 6 lb/ft3 was better at lower freqs in 4" thickness. Is that incorrect?

Why not combine different materials in the same trap? If higher density works better for lower freqs and lower density is better for mids and highs, wouldn't combining two densities in one absorber make the absorber more broadband?

[avare]

Originally Posted by azzzy

Andre, so you don't recommend anything denser than 4 lb/ft3 for 4" thickness? That's a little contradictory to what I've read (including Ethan's page about absorbers). I thought 6 lb/ft3 was better at lower freqs in 4" thickness. Is that incorrect?

Why not combine different materials in the same trap? If higher density works better for lower freqs and lower density is better for mids and highs, wouldn't combining two densities in one absorber make the absorber more broadband?

I do not know how to put it simple terms. It involves the materials gas flow resistance, and location. This is gives a good an naswer as any. Unfortunately it is 3 pages long.

Gas Flow Properties.

It is difficult, if not impossible, to discuss "That's a little contradictory to what I've read " if you do not link, or at provide a reference to what you have read.


Full of gas,
Andre

[DanDan]

Hi azz. I reckon all of the options you mentioned will work fine. Perhaps that is what is confusing. There is little enough difference between any of the choices within reason.

The big theoretical guidelines are.
Thicker is better.
A layer plus airgap is nearly as good as a full fill.
Thicker traps work best with lower density material, and vice versa within reason.
Fibreglass is generally stiffer than Rockwool at equivalent effective densities. e.g. 703 is 48KG, the Rock equivalent would be 60KG but even at this density it is still floppier than the lighter, longer fibred glass product.



Practical matters.
At the end of pondering, when you go to construct, your choices will likely become very limited. There is (non) availability of chosen materials.
A deep block of the fluffy stuff is quite difficult to frame, contain, and make look good. Also Corner Straddling panels are not so easy. Especially if you decide they need to be intimate with the walls.
Conversely a StudioTips SuperChunk is easily constructed, including the cosmetic finish.
Denser materials can be much easier to handle and install, with minimal acoustic difference. I often use 100KG Rock for simple reflection control. It stays firm, is self supporting, easy to cut, easy to handle. For a small space these conveniences over-ride the slight acoustic benefit of lighter material.


DD