Help with deciding acoustic panel design.

[pollock0424]

Hello,
I found someone nearby my location who is willing to make these panels.

I am thinking of these two designs. Can you please help me in picking which one will be the most effective?

<3 inch rockwool> --- <2 inch air gap> --- <3 inch rockwool> <2 inch air gap> wall.

<6inch rockwool> --- <4 inch air gap> wall.

Currently, I use the slatted 2 inch foam panels from Amazon. I stacked 3 of these for a 6 inch layer (including air pockets). These have given very good results for above 200Hz.

 

[kemmler3D]

Hmm, did you mean to write something else in the first example? It looks like 3" rockwool + 5" airgap.

Anyway, I think the 6" + 2" airgap should work best?

 

[pollock0424]

@kemmler3D

My bad, fixed it. Dunno what was I thinking (rather not thinking)

 

[ozzy9832001]

The airgap is effective as much as the thickness of the rockwool. So, if you have 6" rockwool, up to a 6" airgap will be effective.

Keep in mind that the airgap does not increase attenuation; only effective frequency range.

 

[pollock0424]

@ozzy9832001

Good to know. Does the order of air gap and material matter?

 

[ozzy9832001]

@ozzy9832001

Good to know. Does the order of air gap and material matter?

I'm not sure what you mean by order. The air gap would be between the wall and the panel. You could add the air gap into the panel by using larger framing or you could just mount it off the wall.

In my own testing, they were slightly more effective if the entire airgap was enclosed, but it really could have just come down to run to run variations.

 

[pollock0424]

@ozzy9832001

By order, I meant.. between two insulation batts a 2 inch airgap and another 2 inch airgap between the panel rearside and the wall. The other option is simply use all the 4 inches airgap between panel rearside and the wall.

I wanted to know if having a membrane helps ..

I hope that this clarified. I tried to depict this in the first post using text art.

"In my own testing, they were slightly more effective if the entire airgap was enclosed, but it really could have just come down to run to run variations."

I was looking for concrete evidence for this..

 

[ozzy9832001]

@ozzy9832001

By order, I meant.. between two insulation batts a 2 inch airgap and another 2 inch airgap between the panel rearside and the wall. The other option is simply use all the 4 inches airgap between panel rearside and the wall.

I wanted to know if having a membrane helps ..

I hope that this clarified. I tried to depict this in the first post using text art.

"In my own testing, they were slightly more effective if the entire airgap was enclosed, but it really could have just come down to run to run variations."

I was looking for concrete evidence for this..

You definitely don't want an airgap between pieces of insulation or they act as 2 separate pieces. How or why the airgap is important just has to do with where its positioning is relative to the boundary. Lower frequency sound waves are pressure based and at a boundary there is high pressure but low velocity. By moving the panel away from the wall were putting it into an area of higher (relative) velocity. This allow for a standard velocity based absorber to have an effect on the lower frequencies. The amount it will attenuate will depend on the thickness of the material. At roughly 4" thick with a 4" airgap you'd be seeing decent absorption down to about 100-125hz (depending on room size).

In my experience, most people overtreat the room with smaller, thinner panels, when the real goal would be less but thicker ones.

I went from 16x 4" 4'x2' panels to 10x 7.25" 4'x2' panels and the difference was very auditable, especially with regards to the higher mids and treble. The room is far less "dead" but has much, much better low end acoustics.

 

[pollock0424]

Thanks @ozzy9832001
Thanks for confirming my suspicion. Currently, I have 6" panels (+2" airgap) that I madeup stacking the 2" slatted foam panels with zigzagged patterns in them (from amazon). This was an experimental setup and I have good results from upwards of 200Hz. You probably saw my charts from our previous encounters here. I am thinking to upgrade to something that will slightly increase the RT60 at higher frequencies but lower RT60 at lower frequencies (midbass).

I found a guy on Marketplace offering custom made panels for 50$. He uses Rovkwool insulation batts.

 

[ozzy9832001]

Thanks @ozzy9832001
Thanks for confirming my suspicion. Currently, I have 6" panels (+2" airgap) that I madeup stacking the 2" slatted foam panels with zigzagged patterns in them (from amazon). This was an experimental setup and I have good results from upwards of 200Hz. You probably saw my charts from our previous encounters here. I am thinking to upgrade to something that will slightly increase the RT60 at higher frequencies but lower RT60 at lower frequencies (midbass).

I found a guy on Marketplace offering custom made panels for 50$. He uses Rovkwool insulation batts.

NP.

Thicker is better. Less is more and even coverage on all available surfaces (except floor since that's not very practical for most people).

 

[DVDdoug]

I'm not an acoustics expert...

You probably saw my charts from our previous encounters here. I am thinking to upgrade to something that will slightly increase the RT60

Of course, you'll need more hard surfaces to increase RT.

I wanted to know if having a membrane helps ..

There are membrane bass traps. I don't know how they compare to traditional bass traps but they are more practical because they don't take-up a huge chunk of your room. RealTraps sells them and Ethan Winer (one of RealTraps founders) has some information on his website about how to build one. Ethan Winer SEEMS to prefer a dead room with lots of absorption over most of the audio range. And of course, that's more business for him.

Most home environments can benefit from bass treatment but Floyd Toole prefers 2 or more subwoofers to smooth-out the bass throughout the room.

 

[nathan]

Yeah Welti’s work, quoted by Toole, shows how very effective multiple subwoofers are for making bass consistent throughout a listening area. But the only works as high as the subs are playing, typically 100hz ish. There are still modes up to c. 300 hz in most domestic rooms, and so called “bass traps” like the six inch (plus gap) ones the OP is considering can help in the region.

 

[Kal Rubinson]

Yeah Welti’s work, quoted by Toole, shows how very effective multiple subwoofers are for making bass consistent throughout a listening area. But the only works as high as the subs are playing, typically 100hz ish. There are still modes up to c. 300 hz in most domestic rooms, and so called “bass traps” like the six inch (plus gap) ones the OP is considering can help in the region.

Another approach is DiracLive's A.R.T. which recruits all the capable speakers in the room (main, surround, sub, etc....) for interactive compensation. Currently, it is operating up to 150Hz but they suggest eventual extension to 300Hz or so.

 

[pollock0424]

I use Integra DRX-8.4 but it hasn't received Dirac ART yet. I'm looking forward

 

[Justdafactsmaam]

Hello,
I found someone nearby my location who is willing to make these panels.

I am thinking of these two designs. Can you please help me in picking which one will be the most effective?

<3 inch rockwool> --- <2 inch air gap> --- <3 inch rockwool> <2 inch air gap> wall.

<6inch rockwool> --- <4 inch air gap> wall.

Currently, I use the slatted 2 inch foam panels from Amazon. I stacked 3 of these for a 6 inch layer (including air pockets). These have given very good results for above 200Hz.

Save the space. Just go 4-6” of rockwool. The air gap is great. But more rockwool is better

 

[DonH56]

"Membrane" gets used a couple of ways by acoustic panel manufacturers IME. OC-70x and similar absorption (firewall) panels can be purchased with a foil membrane on one side. Putting that side out (facing the room, away from the wall) will provide LF absorption while reflecting higher frequencies. A true membrane absorber is usually much thinner for the frequency range it covers and uses a series of "holes" and resonators to absorb/dissipate energy rather than the typical absorber that turns velocity (sound waves) into heat (a tiny amount, you'll never feel your panels getting warm).

As @ozzy9832001 said, most absorbers are velocity-based, converting moving sound waves into heat in the panel. Sound waves hitting an ideal wall (perfectly stiff) have zero velocity but maximum pressure right at the surface. Moving the panel away from the wall (air gap) moves the absorber's surface into a region where the sound waves have velocity to be absorbed. You can search for curves, but a small air gap, typically 2"~6", greatly increases panel absorption.

Bass waves are so long that typical absorbers (or diffusors) are not terribly effective at handling low-frequency room modes. You need a number of thick panels to help, thus the standard advice to use properly placed subs to improve in-room deep bass response.

FWIWFM - Don

 

[pollock0424]

I can afford 9 inches of space. So, I'm guessing that 3 x 3 inch rockwool panels will be better than 2 x 3 inch rockwool panels + 3 inch airgap. 9 inch rockwool option will be more expensive than the airgap option obviously.

 

[nathan]

I can afford 9 inches of space. So, I'm guessing that 3 x 3 inch rockwool panels will be better than 2 x 3 inch rockwool panels + 3 inch airgap. 9 inch rockwool option will be more expensive than the airgap option obviously.

Doing all rock wool will capture a little more QUANTITY of sound but not appreciably impact the range of frequencies. The range of frequencies is more about the depth between the front of the panel and the wall surface.

Somewhere between 6 and 12" of rock wool, the effectiveness of using ALL insulation, versus say 6" plus air gap, tends to drop off. If budget is a factor, I'd do six inches of insulation and then add air gap between insulation and wall (up to 6" more inches of air, but really anything at all, even just an inch or two, is gravy).

Since you can comfortably afford nine inches of space, the bang for buck solution is probably 6" rock wool, and 3" air. But you will not harm anything other than your pocket book doing all insulation.

(And, apologies, I don't recall the precise math or where I read it, about where the effectiveness drops off when doing all insulation. )

 

[DonH56]

I can afford 9 inches of space. So, I'm guessing that 3 x 3 inch rockwool panels will be better than 2 x 3 inch rockwool panels + 3 inch airgap. 9 inch rockwool option will be more expensive than the airgap option obviously.

Because there is no velocity right at a boundary, 6" panel plus 3" air gap is likely to provide similar performance.

I find Corning OC-703 panels or similar a little easier to work with than Rockwool FWIWFM (less "crumbly", easier to handle, but still wear gloves and a mask).

 

[nathan]

still wear gloves and a mask)

100% this AND work with it outdoors, in a workshop, or a garage, ie, somwhere you can easily clean up from it, imo and experience.