2" vs 4" acoustic panels - RFZ vs Bass Traps

[CaptainNovember7]

So let me get this right. I know I should know the answer, and yes I did do a search.

4" or more for CORNER bass traps

and

2" will suffice for early reflection points

right?

I know we also want the corner traps spaced from the walls to handle low frequencies but to we also want to space the ERZ absorbers from the walls to the same distance?

Thanks. Sorry about potential redundancy. I forgot tho and I'm ready to finally order some traps!

[SaSi_SiDi]

You are right about the corner traps.

For early reflections, I think that 2" is a compromise between effectiveness in the low frequencies and usage of space. The low frequencies that build up in the corners get radiated to the side walls and ceiling above the speakers or instruments.

A 2" trap mounted flush to the wall will not absorb a lot below ~150Hz. A 4" trap mounted at a 4" distance will be many times (3-5) more effective at these low frequencies.

One thing to note however is that to create a RFZ, the traps there need to be FRK free so that they absorb rather than reflect higher frequencies.

[Glenn Kuras]

I know we also want the corner traps spaced from the walls to handle low frequencies but to we also want to space the ERZ absorbers from the walls to the same distance?

No you don't want to space the 4" panels in the corner. Just straddle the panel and let the sides touch the side walls. 2" panels are fine for the early reflection points, but if you can use 4" for the ceiling as it will help with height modes.

[Spiritworks]

[QUOTE=SaSi_SiDi;4602534]
A 2" trap mounted flush to the wall will not absorb a lot below ~150Hz. A 4" trap mounted at a 4" distance will be many times (3-5) more effective at these low frequencies.

How much space should be behind a 2" trap?
How do hang them to provide that space?
A 4" trap needs no space behind?

Thanks

[Glenn Kuras]

For a 2" panel space it 2" if you can.

For the 4" panel when flush mounted you would space it 4" but when in the corner you only need to straddle it with the 2 sides touching the side walls.

[macc]

I'm interested by this - why the convention for having the trap the same distance off the wall as it is thick?

I've been pondering the physics behind it but can't figure it out. Ignoring space/placement considerations in the room, I can understand why having the trap further out - nearer the 1/4 wavelength point of lower freqs - is useful for lowend absorption. But the air in the gap doesn't know how thick the trap is, and the air in the trap doesn't know how thick the gap is. So surely 'as far off the wall as possible' makes sense....?

This has really been bugging me to be honest, it has been 12 years since my last formal acoustics class Any light you could shed would be great. Thanks!

[avare]

Originally Posted by macc

I'm interested by this - why the convention for having the trap the same distance off the wall as it is thick?

It is internet folklore based on the absorption of thin materials when spaced from a reflective surface. In reality, any spacing will result in a dip in absorption at some frequency. With thick materials, what is commonly used ( by groups and people who have studied and tested the absorption) is a gap of around 2 times the thickness of the absorbent material.

What becomes significant, in a good way, is that as the overall thickness of the absorber is increased, lower density material which is a fraction of the cost of 703 etc, can be used. This gives better performance than placing a gap AND is cheaper!

Andre

[dero]

So on a limited budget, in a approx 30' x 20' room using 703, would it generally be more efficient to treat twice as much corner space using 2", or half as much space using 4"

ie, Lets say I get 12 2" panels. Better to treat 96 sqft using 2" or 48 sqft using 4"

[Glenn Kuras]

Well for bass trapping you always want to use 4" in the corners. 2" panels are just for over all decay and early reflection points, so it depends how much of the high end you want to knock down.

[edham]

Originally Posted by avare

It is internet folklore based on the absorption of thin materials when spaced from a reflective surface. In reality, any spacing will result in a dip in absorption at some frequency. With thick materials, what is commonly used ( by groups and people who have studied and tested the absorption) is a gap of around 2 times the thickness of the absorbent material.
What becomes significant, in a good way, is that as the overall thickness of the absorber is increased, lower density material which is a fraction of the cost of 703 etc, can be used. This gives better performance than placing a gap AND is cheaper!
Andre

Andre, could you kindly expand on your post. I have re-read it a few times now and I'm not totally sure of what your saying.
I have been reading your posts for a long time and find them invaluable. Sorry for not "getting" this one.

[SaSi_SiDi]

Originally Posted by edham

Andre, could you kindly expand on your post. I have re-read it a few times now and I'm not totally sure of what your saying.
I have been reading your posts for a long time and find them invaluable. Sorry for not "getting" this one.

What he says is that you can build a 6" absorber using less dense material than 703 and that will absorb better than 2" or 703. So, if you can "sacrifice" some space and decide on thicker panels, then you can relax on the meterial density and have a cheaper solution.

[avare]

Originally Posted by edham

Andre, could you kindly expand on your post. I have re-read it a few times now and I'm not totally sure of what your saying.
I have been reading your posts for a long time and find them invaluable. Sorry for not "getting" this one.

Where is the clarification desired? SaSi expanded on one point. Is there another thing you want expanded?

Andre

[tigheguy]

I was told that when it comes to spacing the material off of the wall, that a 2" panel spaced 2" off the wall is equally as effective as a 4" panel mounted flush.

[edham]

Originally Posted by avare

With thick materials, what is commonly used is a gap of around 2 times the thickness of the absorbent material.

So 4"703 should use a gap of 8" ??

Originally Posted by avare

What becomes significant, in a good way, is that as the overall thickness of the absorber is increased, lower density material which is a fraction of the cost of 703 etc, can be used. This gives better performance than placing a gap AND is cheaper!
Andre

What material are you speaking of?
and at what thickness do you no longer use a gap?
Or does this thicker, less dense material still benefit from using a gap?
If a gap is still benificial - does it still need to be 2X the thickness of the material?

thnks

[avare]

Thanks for the specific questions. IT makes much easier to answer your query.

Originally Posted by edham

So 4"703 should use a gap of 8" ??

If you are making a 12" thck trap and want to use 703, then yes, use 4" of 703 with an 8" gap.

What material are you speaking of?

Porous absorbers

and at what thickness do you no longer use a gap?

When the gas flow reisitance would esceed ~1600 Rayls.

Or does this thicker, less dense material still benefit from using a gap?

A thicker less dense material may be significantly cheaper! IT would use a smaller gap to acheive the same absorption characteristics with less of an absorption dip.

If a gap is still benificial - does it still need to be 2X the thickness of the material?

The gap depends on the material gas flow resistance. UP TO 2x the material thickness can be used with no significant absorption dip.

[edham]

Andre,
Thanks for the extra info. Very helpful.

Last question -
Porous absorbers ?? - can you give some examples (either commercial finished products or the materials that are most appropriate.)

thanks again.

[avare]

Originally Posted by edham

Porous absorbers ?? - can you give some examples (either commercial finished products or the materials that are most appropriate.)

Fiberglas, mineral wool, rockwool, wool fibre are just a few.

Andre

[edham]

Sorry Andre, I should have again been more specific.

I am wondering specifically what "less dense" material you are thinking of as an alternative to 703.

I have the space for deeper traps.
What absorber material do you like to use in this instance.

Again, Thanks for all your posts over the years (and in other forums also).

[Glenn Kuras]

If you are making a 12" thck trap and want to use 703, then yes, use 4" of 703 with an 8" gap.

I have always been told that you should space it off the wall the thickness of the panels. So if 12" you would use 6" with a 6" gap. Once you go beyond that the absorption becomes more narrow it is response. So 2X is still ok? And if so it would not start to become narrow in response?

[avare]

Originally Posted by edham

Sorry Andre, I should have again been more specific.

I am wondering specifically what "less dense" material you are thinking of as an alternative to 703.

I have the space for deeper traps.
What absorber material do you like to use in this instance.

That one is easy, and cheap, assuming you mean 12" or thicker. Regular wall insulation filling the space. Of Roxul Safe n Sound type, depending ont eh better price in your area. Cheaper AND more effective acoustically than gapped 703.

Again, Thanks for all your posts over the years (and in other forums also).

You are welcome.

Blushing,
Andre

[a zombie]

I'm still confused on something

here it says the absorbtion coefficient for 2" is .76 at 250 hz

by spacing that 2" panel out from the wall by 2" or 1.5" with a 2x2, would the coefficient for 250hz go up, or would it be the same absorbtion at 250hz but more effective at lower frequencies?

[FredYeah]

So just to hijack this thread a little:

If i would make an absorber with Rockwool with 50 Kg/m, 4" thick, 4" airgap, how would this compare to an absorber with Rockwool 25 kg/m, 4" thick, no airgap?

Since the 25 kg/m stuff is mad cheap here. I would like to place this absorber on my frontwall just behind my speakers. Is this wise? Will it help? Of should I go with the first option?

Thanks Andre. Your posts (and Glenns and Ethans) actually inspired me to build my own homstudio.

[Glenn Kuras]

If i would make an absorber with Rockwool with 50 Kg/m, 4" thick, 4" airgap, how would this compare to an absorber with Rockwool 25 kg/m, 4" thick, no airgap?

The only way to really know is to test it and see. But I can say without a dough that 4" with a 4" gap will perform much better with the air gap.

[takman]

Originally Posted by avare

It is internet folklore based on the absorption of thin materials when spaced from a reflective surface. In reality, any spacing will result in a dip in absorption at some frequency. With thick materials, what is commonly used ( by groups and people who have studied and tested the absorption) is a gap of around 2 times the thickness of the absorbent material.

What becomes significant, in a good way, is that as the overall thickness of the absorber is increased, lower density material which is a fraction of the cost of 703 etc, can be used. This gives better performance than placing a gap AND is cheaper!

Andre

what about for people who dont have cheaper alternative for insulation? i can only basically get one kind here....

So you are saying that i should place my 2" panels 4" from the wall? I dont think its going to be practical in my situation...interesting to know though...

How exactly ( or where exactly) did this internet myth start? Is it like the 38% listening position rule maybe?....just a starting point....like space the panels at a minimum the thickness of the panel, then space them farther if you need better performance?

I was curious about to see what it said in my books...in mitch gallaghers book, he says 1" to 2" makes a big difference, and a larger gap results in even better LF absorption....So he doesnt state that panel gap has to be equal to that of its thickness, like its a rule or something...

In rods book, its the same....you get better effciency with an air gap..but he doesnt say "the gap has to be the thickness of the panel" as i have seen it here...

If it is really a myth...then im guilty of spreading it because i have told others this...

Do you have any information you can link me to about this? im curious...

thanks

[mrhudson]

Originally Posted by avare

That one is easy, and cheap, assuming you mean 12" or thicker. Regular wall insulation filling the space. Of Roxul Safe n Sound type, depending ont eh better price in your area. Cheaper AND more effective acoustically than gapped 703.

Blushing,
Andre

So, just to clarify, if I can afford to sacrifice 12" of depth in a room, say an unfinished basement, I could simply wrap burlap around regular wall insulation, say, and stack those bass trapping blobs around the corners, etc., and use the "insulation in the ceiling with burlap trick" from Ethan's site ....hmmmm.......Do I have that right? I have an unfinished basement that I could EASILY do that in :-) Of course, there would be cement walls behind in some cases, but......beats the cost of finishing a basement - doesn't need to be pretty for me! Is this a crazy scheme?

[Glenn Kuras]

Originally Posted by mrhudson

So, just to clarify, if I can afford to sacrifice 12" of depth in a room, say an unfinished basement, I could simply wrap burlap around regular wall insulation, say, and stack those bass trapping blobs around the corners, etc., and use the "insulation in the ceiling with burlap trick" from Ethan's site ....hmmmm.......Do I have that right? I have an unfinished basement that I could EASILY do that in :-) Of course, there would be cement walls behind in some cases, but......beats the cost of finishing a basement - doesn't need to be pretty for me! Is this a crazy scheme?

12" in the corners wrapped with fabric will work fine.

[mrhudson]

Wow, that opens up all kinds of cheap, easy, and wonderfully/decidedly ugly opportunities for me :-) Fantastic possibilities! Would work fine in front of a cement wall?

[Glenn Kuras]

Originally Posted by mrhudson

Wow, that opens up all kinds of cheap, easy, and wonderfully/decidedly ugly opportunities for me :-) Fantastic possibilities! Would work fine in front of a cement wall?

sure.

[FredYeah]

Originally Posted by Victor_Stoian

4" 50Kg/m³ rockwool + 4" airgap (gives a total of 8") put against 8" (not 4") 25Kg/m³ rockwool + no airgap.
The later should be better in the lows.

Allright, then 8" it is!

[a zombie]

Originally Posted by a zombie

I'm still confused on something

here it says the absorbtion coefficient for 2" is .76 at 250 hz

by spacing that 2" panel out from the wall by 2" or 1.5" with a 2x2, would the coefficient for 250hz go up, or would it be the same absorbtion at 250hz but more effective at lower frequencies?

does anyone have an answer to this?