Encyclopaedia Audiophilica I: Room Design & Treatment

[Bjorn]

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

 

[Frgirard]

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]

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]

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.