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Bass Traps - To air or not to air Studio Monitors
Old 15th December 2010
  #1
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Bass Traps - To air or not to air

Hi all,

I will be constucting my own Bass traps for the sides and rear wall of my home theatre. (I already have superchunks planed for the corners). The maximum thickness for these bass traps / acoustic panels will be 4".

I am confused from various articles whether it is best to fill the 4" totally with rockwool or fill it say with 3" rockwool and leave a 1" air gap.
Any help would be great appreciated.
Old 15th December 2010
  #2
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Glenn Kuras's Avatar
4" and leave a gap. It is best to leave a 4" gap off the wall but even a 1 or 2" gap will help a lot.
Old 15th December 2010
  #3
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Glenn, thank you.

Are you suggesting a 4" solid fill panel, that is ideally hung 4" off the wall? So 8" projection into the room?

If so, i am likely to fail miserably as i can only accommodate a total projection off the wall of 4".

I was thinking that a 4" panel, fixed back to the wall, with either 4" of insulation (total fill) or 3" insulation and a 1" air gap within the panel?






Old 15th December 2010
  #4
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johndykstra's Avatar
 

total fill.

air gap is to get added performance from thinner material, but more material is better than air.
Old 15th December 2010
  #5
JWL
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4" and no fill will be better than 2" with a 2" gap. If you only have 4" to work with and this is not negotiable, then fill it.
Old 15th December 2010
  #6
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RonT's Avatar
 

Quote:
Originally Posted by johndykstra View Post
total fill.

air gap is to get added performance from thinner material, but more material is better than air.
Did I help or hurt doing mines this way?



4" of 703 2'wide by 8'tall with the Rolled batt insulation stuff behind the 2X4s and the triangular air pockets on either side of the insulation.




Just curious how much difference it would have made if I would have left the rolled insulation out. I couldn't afford to fill the corners with 703 but I just thought it would be better to put something in the corner?
Old 16th December 2010
  #7
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Quote:
4" and no fill will be better than 2" with a 2" gap. If you only have 4" to work with and this is not negotiable, then fill it.
Thanks, so it is still worth using 4" solid fill panels fixed straight to the wall.

Has anyone got information on the % decrease in performance when fixing straight to the wall as opposed to hanging the panel off the wall. I ask this as i want to know whether it is worth the battle in trying to convince the mrs to allow me to use up more space in the room by hanging the panels off the wall. I don't want to have to do this if there is only a marginal benefit. Thanks
Old 16th December 2010
  #8
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Magickman's Avatar
 

Quote:
Originally Posted by RonT View Post
Just curious how much difference it would have made if I would have left the rolled insulation out. I couldn't afford to fill the corners with 703 but I just thought it would be better to put something in the corner?
Putting the rolls of insulation in is great! Far better than leaving the air space empty.
Old 16th December 2010
  #9
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Glenn Kuras's Avatar
Quote:
Originally Posted by Tinytim View Post
Glenn, thank you.

Are you suggesting a 4" solid fill panel, that is ideally hung 4" off the wall? So 8" projection into the room?

If so, i am likely to fail miserably as i can only accommodate a total projection off the wall of 4".

I was thinking that a 4" panel, fixed back to the wall, with either 4" of insulation (total fill) or 3" insulation and a 1" air gap within the panel?





In that case then go with the total fill of 4"
Old 25th May 2012
  #10
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H, I'm sorry to dig up an old thread, but I'm wondering, does the no gap concept apply for bass traps only, or is 4" with no gap better than 2" with a 2" gap for first reflection panels as well?

Thank you.
Old 25th May 2012
  #11
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Magickman's Avatar
 

Quote:
Originally Posted by Chris Fletcher View Post
H, I'm sorry to dig up an old thread, but I'm wondering, does the no gap concept apply for bass traps only, or is 4" with no gap better than 2" with a 2" gap for first reflection panels as well?

Thank you.
If your goal is only to reduce reflections, then it makes no difference. But honestly, what room can't use more bass trapping???
Old 25th May 2012
  #12
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johndykstra's Avatar
 

Quote:
Originally Posted by Magickman View Post
If your goal is only to reduce reflections, then it makes no difference. But honestly, what room can't use more bass trapping???
I would disagree.

The goal when eliminating reflections should be as broadband as possible. The thicker the treatment, the lower in frequency reduction you have.
Old 25th May 2012
  #13
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Thank you for the replies,

John, do you mean that if there were a midrange problem, 2" with a space would be better at reducing the mids? 4" with no space would be more reflective of hi frequencies? I have just been reading everywhere that you should leave the space so the sound can be better absorbed on the return trip from the wall (I know that's oversimplifying, but hopefully it's somewhat accurate).
Old 25th May 2012
  #14
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Magickman's Avatar
 

I seriously doubt there will be measurable difference at a first reflection point between 4" with no gap and 2" with a 2" gap.

Just sayin...
Old 25th May 2012
  #15
Gear Guru
Size Matters

Bass Traps - To air or not to air-screen-shot-2012-05-25-13.26.35.jpg

dd
Old 25th May 2012
  #16
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Glenn Kuras's Avatar
Quote:
Originally Posted by Magickman View Post
I seriously doubt there will be measurable difference at a first reflection point between 4" with no gap and 2" with a 2" gap.

Just sayin...
I do agree with you but I have a act like I don't or people will flame me. All kidding aside I tested this not so long ago and yes 4" did work better but not by much. 2" is fine with a air gap for the early reflection points.


Glenn Kuras
GIK Acoustics USA
GIK Acoustics Europe
770 986 2789 (USA)
+44 (0) 20 7558 8976 (UK)

See the NEW Scopus Tuned Trap
Old 25th May 2012
  #17
Gear Guru
On Reflection

The action at the first reflection points, i.e. Left and Right and Over the listener to speaker zone, is often considered as HF.
However, the listener position is often mid width and mid height.
At the null of the first modes and peak of the seconds.
A 4 inch trap will have a useful taming effect on at least the second mode and higher. Include a 4 inch airgap and we can achieve absorption of 0.7 at 100Hz
DD
Old 25th May 2012
  #18
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johndykstra's Avatar
 

Quote:
Originally Posted by Magickman View Post
If your goal is only to reduce reflections, then it makes no difference. But honestly, what room can't use more bass trapping???
If the goal is to reduce reflections, than categorically the prescription should be to eliminate as much of the reflection as possible. Why limit one self to 4" total at all?

I take little issue with your answer, only the bit about "making no difference". This may lead the casual reader to believe that 2" of fiber gapped 2" from the surface is the "optimum" first reflection absorber. Not the case.
Old 26th May 2012
  #19
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kasmira's Avatar
 

Quote:
Originally Posted by Chris Fletcher View Post
I have just been reading everywhere that you should leave the space so the sound can be better absorbed on the return trip from the wall (I know that's oversimplifying, but hopefully it's somewhat accurate).
Ok, I've just heard enough of this logic (no offense to you, I'm just gunna throw it out there)

Just think about this:

2" trap + 2" air gap:

The full path of the sound would be:
2" trap -> 2" of air -> wall/reflection -> 2" of air -> 2" trap

4" trap:

4" trap -> wall/reflection -> 4" trap

Why would it be better if there were an airgap in this scenario? The sound is going to reflect from the wall either way, and still pass through more insulation on its second run...its not like just because an absorber is there doesn't mean sound that would pass through won't reflect..

Its like when people think that glass is for some reason so much more extremely prone to reflections than a wall... :P
Old 26th May 2012
  #20
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Tom Higgins's Avatar
Because they are velocity-based absorbers and the velocity is greater further from the wall (so they are more effective).

(I think that's right...)
Old 26th May 2012
  #21
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Another factor is the overall size of the absorber.

IMO bass waves can easier diffract around objects when they are placed at the middle of the wall, hence you need bigger absorbers than 2*4 to have an effect in the low end.

With corner placement, the bass is almost forced to flow threw the material with higher resistance due to the nearby walls.
Old 26th May 2012
  #22
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Magickman's Avatar
 

Quote:
Originally Posted by johndykstra View Post
If the goal is to reduce reflections, than categorically the prescription should be to eliminate as much of the reflection as possible. Why limit one self to 4" total at all?

I take little issue with your answer, only the bit about "making no difference". This may lead the casual reader to believe that 2" of fiber gapped 2" from the surface is the "optimum" first reflection absorber. Not the case.
The OP was asking specifically about 4" vs 2" with a gap at a first reflection point. My answer was based on his specific query. You'll note that I did suggest that more trapping will always be better when I said:

Quote:
Originally Posted by Magickman View Post
But honestly, what room can't use more bass trapping???
Tough crowd around here these days.

I shall now humbly bow out of this conversation.

-MM
Old 26th May 2012
  #23
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johndykstra's Avatar
 

Quote:
Originally Posted by Magickman View Post
Tough crowd
Sorry MM,

I got nothin but love for ya.
Old 26th May 2012
  #24
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Quote:
Originally Posted by hsal View Post
Another factor is the overall size of the absorber.

IMO bass waves can easier diffract around objects when they are placed at the middle of the wall, hence you need bigger absorbers than 2*4 to have an effect in the low end.

With corner placement, the bass is almost forced to flow threw the material with higher resistance due to the nearby walls.
+1
no matter what the models say with respect to absorption, a 24"x48" reflection broadband absorber is not going to do much to a 11ft 100hz wave...
Old 26th May 2012
  #25
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Jens Eklund's Avatar
Quote:
Originally Posted by localhost127 View Post
+1
no matter what the models say with respect to absorption, a 24"x48" reflection broadband absorber is not going to do much to a 11ft 100hz wave...
+1
Especially not if 75 deg incidence.
Old 3rd June 2012
  #26
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It appears to me that 2" of 703 + 2" of Pink Stuff works as well as 4" of 703.

Also, that 4" of "Pink Stuff" does better than 2" of OC703 + 2" air gap.

I am assuming the Rayls value of "Pink Stuff" to be 5000 for this is what I have read.
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Old 12th June 2012
  #27
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Insulation Man's Avatar
 

I don't know how much of a difference it makes, but I would have tried to limit the amount of wood inbetween the 703 and the batt insulation, as well as put the kraft paper facing the corner rather than the inside of the room. Overall, the batt insulation shouldnt hurt your application.
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