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Encyclopaedia Audiophilica I: Room Design & Treatment

Bjorn

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Air gap is never a benefit vs filling it out with the correct type of material.
 

Frgirard

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Air gap is never a benefit vs filling it out with the correct type of material.
The interest of the air gap is financial and in the extension of the low frequency.
The problem with the air gap, is the degradation of performance.
On Acousticmodelling when you add air gap you can see ondulation showing this degradation.
 

JPA

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There are two things I'm unclear about.
1) The benefit of leaving an air gap behind absorption panels. I have 50cm to play around with (at least on the front and back walls). I can fill that with 50cm of rockwool, or do some combination of rockwool and air gap (25-25, 30-20, etc). Assuming I don't learn something that makes me opt for some sort of membrane-absorption panel thing, is there a benefit to having an air gap as opposed to stuffing the entire allotment with rockwool?

2) Is there a maximum absorption depth beyond which you get minimal returns? Using the calculator at http://www.acousticmodelling.com/8layers/porous.php, it seems that 40cm seems to be the upper limit, and adding more, even a metre of absorption, gives no additional bass absorption. Is this because of some exponential increase in the amount of absorption necessary, or is the calculator broken?

1) As abdo123 said, the most important factor is to have absorption as far out into the room as practical. The first few centimeters closest to the wall only affect the very highest frequencies, which are well absorbed by the rest of the material anyway, so leaving an air gap doesn't affect performance much.

2) The calculator is not wrong. As your aborption depth increases you are trying to absorb lower frequencies. However, the lower you go in frequency the less effective fibrous absorption is, so there is a point of diminishing returns.
 

kemmler3D

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50cm of rockwool would be pretty expensive, you may have better bang for buck with membrane traps if you are going that far. A sheet of mass-loaded vinyl and some 2x4s aren't exactly ultra-premium materials.

In general you have to be careful with absorptive treatments because you end up stopping a disproportionate amount of high frequency energy, leading to a relative lack of HF vs. LF, i.e. the room ends up bass-heavy, "too dead". To abdo123's point, it depends on the surface area and porosity of the material. Basically you don't want to cover too much of the walls / ceiling with rockwool unless you have a commensurate amount of bass trapping in the room.




Somewhat related: Can't say if diffusors are called for in your situation vs. absorbers, but they do look pretty cool and are possible to DIY. :)
 

Philbo King

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The physics reasons for an air gap:
1. Porous absorbers are in a class called velocity absorbers. The velocity of air at a reflection point (wall) is zero. The velocity peaks 90 degrees (1/4 wavelength) away from the wall. This, of course, varies with frequency. Rough numbers:
1 kHz - 3 inches
500 Hz - 6 inches
250 Hz - 12 inches
100 Hz - 30 inches
As frequency halves, that quarter wave distance doubles. This means that the further from the reflecting surface the room-facing side of the absorber is, the lower the frequency it will absorb. The velocity varies with the sine of the frequency (which is why it peaks at a 90 degree wavelength distance). So it is a wideband effect since you don't have to hit the exact peak of the sine. Increased absorption for frequencies from 0.707f to 1.414f is reasonably expected, dependent on absorber thickness.

2. A gap behind a porous absorber exposes the rear surface of it to whatever sound waves reach the back of the panel. This results, for some frequencies, in an increase in absorption factor beyond that which can be accounted for by the front area surface alone.

Since this is an evidence-based forum, I don't expect anyone to blindly buy into this theory. So I submit this link from my friend Ethan Winer, who tested the effect for himself:
 
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Bjorn

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Just a quick update. I put my money where my mouth is, and am applying the lessons I think I've learnt:
https://www.audiosciencereview.com/...atment-plan-the-retirement-gift-update.47375/

I'll update this thread with new information, once I've put the theories into practice and have some actual real-world experience.
It's well established now that so much air gap doesn't work and is counter effective. BAD panels are not the best treatment for music. I'm afraid this isn't going to work well.
 
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Danaxus

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It's well established now that so much air gap doesn't work and is counter effective. BAD panels are not the best treatment for music. I'm afraid this isn't going to work well.
Thanks for the feedback, but without your knowledge, or better understanding of acoustic science in general, I don't know what do with that information. If you have the time and patience to help me learn, I'd be interested in learning why air gaps don't help, and why BAD panels aren't a good diffusion solution.

Currently, my understanding is porous/velocity absorbers only work where there's movement of air, and not where there is pressure build up. i.e. it was explained to me (primarily by Jesco Lohan of acoustics insider), that putting say 20cm absorption away from the wall works just as well in theory, and better in practice, than simply putting 40cm of absorption. I've also heard that BAD panels aren't particularly effective as far as diffusion goes, but they do have 3 benefits:
1) They are incredible simple and easy to build
2) Unlike quadratic diffusers, they will work well if placed close to the listening positions/speakers. Quadratic on the other hand, require you and your speakers be placed some distance away from them (the deeper the diffusion, the greater the distance). They also get increasingly more complex the build, the more layers you use.
3) You can combine binary diffusion with porous absorption. Using quadratic diffusion would require a trade-off by lowering the amount of absorption I can use in the room.

If the logic is faulty, or inferior to other approaches (considering the size of my room), I'd really appreciate some guidance.


Thanks!
 

Bjorn

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The calculation with air gap is not trustworthy. The reason is that energy flows out and and looses effect. Besides, there's no good reason to choose an airgap since filling it 100% works far better. The only reason would be to save some financially. But in this case, the cost of filling it with correct material is cheap and the result would be much better. For 40 cm material one could achieve very high absorption down to 100 Hz area and decent effect to 40-50 Hz.

Flat BAD panels will cause audible specular reflections. It's a treatment that's fine for speech, but not for music. Perhaps it can work ok for multichannel movies, if that's the only usage of the room. The absorption is also high with BADs. It's possible to combine improved diffusion and low frequency absorption. That being said, I would not recommend only the usage of diffusion at reflection points unless one doesn't like good imaging and accuracy, and one only desires a more spatial and immersive representation and which will color all the mixes to some degree.

Also, covering the whole ceiling with absorption would create a dead environment and especially remove too much energy for music.
 
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Danaxus

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The calculation with air gap is not trustworthy. The reason is that energy flows out and and looses effect. Besides, there's no good reason to choose an airgap since filling it 100% works far better. The only reason would be to save some financially. But in this case, the cost of filling it with correct material is cheap and the result would be much better. For 40 cm material one could achieve very high absorption down to 100 Hz area and decent effect to 40-50 Hz.

Flat BAD panels will cause audible specular reflections. It's a treatment that's fine for speech, but not for music. Perhaps it can work ok for multichannel movies, if that's the only usage of the room. The absorption is also high with BADs. It's possible to combine improved diffusion and low frequency absorption. That being said, I would not recommend only the usage of diffusion at reflection points unless one doesn't like good imaging and accuracy, and one only desires a more spatial and immersive representation and which will color all the mixes to some degree.

Also, covering the whole ceiling with absorption would create a dead environment and especially remove too much energy for music.
From what I understand, your recommendation is to use quadratic diffusers instead of BAD, and not in locations of first reflections. First reflection locations should have pure absorption, but being careful not to over-damp (i.e. treat first reflections on the walls and ceiling, but don't overkill beyond that - maybe on the front wall behind the speakers and subwoofer). Then I assume you deal with room modes using membrane traps in the corners. Is that about right?
 

Bjorn

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First one needs to decide on the goal and the design principles follows.

A standard QRD is never something I would recommend. It's outdated.

Pressure based trapping isn't something one does based on guesses. One needs to measure and apply it appropriately. Corners in acoustics isn't only tricorners And only treating those would generally not do much.
 
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First one needs to decide on the goal and the design principles follows.

A standard QRD is never something I would recommend. It's outdated.

Pressure based trapping isn't something one does based on guesses. One needs to measure and apply it appropriately. Corners in acoustics isn't only tricoerners And only treating those would generally not do much.
I wish you strength when you try to spread some reason here :).
 
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