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3000 or 5000 Pa*s/m2 for 50-60 cm bass traps?
Old 5th March 2014
  #1
Gear Maniac
 
🎧 5 years
3000 or 5000 Pa*s/m2 for 50-60 cm bass traps?

Hello

I need 50 or 60 cm deep bass traps (rectangular parallelepiped traps flush to walls, not triangular-base corner traps) for the rear and front walls of a small room.
I can choose between 3000 and 5000 Pa*s/m2 absorbents.

For 50 cm, at 0° incidence, the calculators show that 3000 Pa*s/m2 is quite better above ~40 Hz and slightly worse below. Above 60°, 5000 Pa*s/m2 is better for all frequencies.
For 60 cm the situation is similar, though with different frequency and angle values.

Which flow resistivity should I use? [to be honest I guess it's nearly the same, but I'm not a pro]

On the Internet I could not find examples of bass traps using 3000 Pa*s/m2 absorbent, while 5000 Pa*s/m2 is common, probably because of material availability.
Is there any specific reason to avoid 3000 Pa*s/m2 for 50-60 cm traps?

Cheers
Julien
Old 6th March 2014
  #2
Gear Maniac
 
🎧 5 years
I did some additional research still without finding a definitive answer. I hope I am not taking it too seriously, maybe it's just same-same in practice (?)

These bass traps need to be broadband and work as good as possible for low frequency absorption. In this rectangular room the lowest mode related to length is about 37 Hz. As far as I understand this wave has a normal incidence angle to the rear and front walls.
Here is a simulation of 500 mm porous absorbent flush to wall, 0° incidence angle (represents normal incidence), comparing 3 kPa.s/m2 and 5 kPa.s/m2:
http://www.acousticmodelling.com/mli...1=500&v21=5000
37 Hz is better absorbed by 5 kPa.s/m2, but not so much; above 40 Hz, 5 kPa.s/m2 is worse. Which is why I think 3 kPa.s/m2 is better, but I am not sure in practice.

As a reference, same with 30° and 66° incidence angles:
http://www.acousticmodelling.com/mli...1=500&v21=5000
http://www.acousticmodelling.com/mli...1=500&v21=5000
(as far as I understand 30° and 66° simulations are not relevant for the 37 Hz mode)

I do not trust the simulations blindly, but this is the only tool I have for choosing between 3 and 5 kPa.s/m2

Julien
Old 7th March 2014 | Show parent
  #3
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avare's Avatar
 
🎧 15 years
Quote:
Originally Posted by julien79 ➡️
Which is why I think 3 kPa.s/m2 is better, but I am not sure in practice.
Correct.

BTW, the most accurate modeling is with the Komatsu porous model.

A great model,
Andre
Old 7th March 2014 | Show parent
  #4
Gear Maniac
 
🎧 5 years
Quote:
Originally Posted by avare ➡️
Correct.

BTW, the most accurate modeling is with the Komatsu porous model.
Thanks for your answer Andre

I checked with the Komatsu model:
Multi-layer Absorber Calculator

But with this model, it looks like 5 kPa.s/m2 is quite better below 60 Hz. Above 60 Hz, 3 kPa.s/m2 is still better, but both are above 0.8 so it's not bad anyway

That said I trust your experience and will go for 3 kPa.s/m2 (it should also be slightly cheaper)

Julien
Old 7th April 2014
  #5
nms
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2 Reviews written
🎧 10 years
You should set the calculator to random incidence.

On the topic of models, the developer updated the default to Allard and Champoux which he feels is the most accurate.
Old 7th April 2014 | Show parent
  #6
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Jens Eklund's Avatar
 
🎧 10 years
Quote:
Originally Posted by nms ➡️
You should set the calculator to random incidence.
Modes and SBIR (bass issues) does not feature random incidence behaviour at the boundary (an axial mode for instance will hit the surface at normal incidence). Use the angle of incidence of the mode or reflection (SBIR) you're trying to absorb.
Old 7th April 2014 | Show parent
  #7
nms
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🎧 10 years
Quote:
Originally Posted by Jens Eklund ➡️
Modes and SBIR (bass issues) does not feature random incidence behaviour at the boundary (an axial mode for instance will hit the surface at normal incidence). Use the angle of incidence of the mode or reflection (SBIR) you're trying to absorb.
A speaker pointing directly at something in open air would be 0° angle of incidence. A pair of stereo speakers pushing a full spectrum of audio from the directional HF's down to the low end which wraps around everything would really not be IMO. The predictions that porous absorber spewed out really didn't hold up whatsoever to real world results when I had it set to 0°. In particular it said that my rear wall (which is 16" of fluffy with 3" of Roxul in front) would perform better if the Roxul was removed. In reality it takes a BIG performance hit as soon as the Roxul layer is removed. Switch to random incident and it changes to something much more in line with what happens in reality. It correctly predicts an improvement by adding that layer.

I don't fully trust the calculated predictions though. I'd like to, since it would really make life easier. But it's been a little hit & miss when it comes to what combination of materials should outperform others.
Old 7th April 2014
  #8
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Jens Eklund's Avatar
 
🎧 10 years
I don't know what issue you are trying to slow by the treatment on your rear wall, but if for example an axial mode related to the length of the room; the incidence of each bounce will be 0 degree (more or less). If not axial, but tangential or oblique; the incident angel will be different but still not random but specific, and the same for each bounce on the surface in question (again, assuming modal issue) so a random simulation is not very relevant when designing bass absorbers (since we're assuming modal or SBIR behaviour (assuming the room is not gigantic). If the prediction doesn't match the result, that's another issue.
Old 8th April 2014
  #9
Gear Guru
 
1 Review written
🎧 15 years
Shot

I would go for the 5000. I noticed that Caruso Isobond seems to be one of the best absorbent fibres available, and as it happens, the commonly used one is 5K.
DD
Old 14th April 2014 | Show parent
  #10
Gear Maniac
 
🎧 5 years
Quote:
Originally Posted by Jens Eklund ➡️

Originally posted by nms:
"You should set the calculator to random incidence."


Modes and SBIR (bass issues) does not feature random incidence behaviour at the boundary (an axial mode for instance will hit the surface at normal incidence). Use the angle of incidence of the mode or reflection (SBIR) you're trying to absorb.
Actually I first used 0° because the lowest mode in my room is perpendicular to the front and rear walls.

Then I also checked with a few other angles because I think (and I may be wrong) that unwanted non-resonant interferences can come from many places that are difficult to predict for me (I am a musician / producer, not an acoustician). I only try to get a "general picture" here, and to compare different setups.

To be honest I also check random incidence sometimes even if there is no low frequency diffuse field in a small room. I read several times that the result is "closer to practice" in a small room, which does not make any physical sense to me (except if "SBIR may come from everywhere"), but could still be true for some reason. This seems to remain a debate here. Well I have almost no practice of acoustics so I will tell you how it worked in my room when it is finished

Julien
Old 14th April 2014 | Show parent
  #11
Gear Maniac
 
🎧 5 years
Quote:
Originally Posted by DanDan ➡️
I would go for the 5000. I noticed that Caruso Isobond seems to be one of the best absorbent fibres available, and as it happens, the commonly used one is 5K.
DD
Hi Dan, why would you go for 5000 exactly? André said 3000 was correct, so I don't know anymore

Julien
Old 15th April 2014
  #12
nms
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🎧 10 years
After my recent test experiences I can't take anything that comes from modelling predictions as being very useful at all for guiding treatments in a studio. I tested 3 different combinations of material & thickness. The worst predicted configuration measured best and the best predicted measured worst. I have no doubt there are lab conditions which would have yielded otherwise, but in the real world.. with speakers in a studio and treatments placed near walls.. it seems to be a misleading waste of time.
Old 15th April 2014
  #13
Gear Guru
 
1 Review written
🎧 15 years
Uncertain

+1 nms, the algorithms used in most of these calculators do not take various factors into consideration. e.g. The damped resonant effect of corner straddling dense panel, and even the larger corner bass amplification effect.

Elsewhere we see data stating that there is little or no difference in absorption over a 4:1 range of densities. With such a deep trap, I really don't think it matters much whether you go 3 or 5K.
For some reason I kinda like to use 65 degrees or thereabouts, avoiding the two extremes.
3000 or 5000 Pa*s/m2 for 50-60 cm bass traps?-screen-shot-2014-04-15-01.03.18.jpg
DD
Old 15th April 2014 | Show parent
  #14
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avare's Avatar
 
🎧 15 years
Quote:
Originally Posted by DanDan ➡️
Elsewhere we see data stating that there is little or no difference in absorption over a 4:1 range of densities. With such a deep trap, I really don't think it matters much whether you go 3 or 5K.
+1.

Tersely,
Andre
Old 17th April 2014
  #15
Gear Maniac
 
🎧 5 years
Thanks for your answers Dan and Andre. I have both 3kPa*s/m^2 and 5kPa*s/m^2 absorbents (and no time for building at the moment), so I will use 3K for 50 or 60 cm deep traps, and 5K for 30 cm traps.

Julien
Old 17th April 2014 | Show parent
  #16
Gear Maniac
 
🎧 5 years
Quote:
Originally Posted by nms ➡️
After my recent test experiences I can't take anything that comes from modelling predictions as being very useful at all for guiding treatments in a studio. I tested 3 different combinations of material & thickness. The worst predicted configuration measured best and the best predicted measured worst. I have no doubt there are lab conditions which would have yielded otherwise, but in the real world.. with speakers in a studio and treatments placed near walls.. it seems to be a misleading waste of time.
Are you 100% sure the flow resistivity values you entered are correct? I only ask because you did not mention them in the other thread (https://www.gearslutz.com/board/bass-...cy-issues.html)

I did not spend much time with calculators, because I have a limited choice of materials, and I decided not to use composite sandwiches. I just checked which material was the best for each depth (30, 50 and 60 cm), either fully filled or including air gap. I thought it would give me a quick idea at least, because I never made any acoustic panel before. Measured results with 20 cm of 10000 Pa*s/m^2 absorbent were quite coherent with the prediction of the calculator (I did not check exact values for each frequency, divided by coverage surface, nor did I take corner effect in account, etc). Anyway the depths I plan to use are the maximum possible for my room, I cannot do better (and I will not risk to build advanced bass traps like perforated panels or VPR at this point).

Julien
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