DIY acoustic panels - Rockwool question

[ilcaccillo]

Hello,
I'm building some acoustic panels, they will be around 1,35 meters by 0,6 meters.

Inside the panel I will be using 2 layers if 40mm high density Rockwool and also 1 layer of 40mm Low density rockwool, so a sandwich of 3 layers total but with different densities. (it's the material I have)

My question is,
Should I put the low density layer up front the other 2 high density layers, or should I put the Low density layer on the back (wall side) having the 2 high density layers up front?

I had different opinions on this, so I would like your help

Thank you so much for your help

[resistorman]

That's a good question, hopefully someone will have an answer. My instinct would be to put the lower density in the middle, but I have no scientific rationale for this, just a notion that it might make the assembly less rigid and that would be good?

If I had to place a bet, it won't make an audible difference no matter how you do it

[Wonks]

It may help if you could state the densities used e.g. 30kg/m3 and 60kg/m3.

The lower density rockwool won’t do as much for low frequencies as the higher density rockwool, so for better LF absorption you’d want the higher density stuff further away from the wall so the amplitude of LF standing waves is greatest.

So room > HD RW > HD RW > LD RW > air gap > wall.

But from a constructional point of view it would be easier to have the lower density stuff supported in-between the more rigid high density slabs. A low density rockwool may need to be supported by chicken wire or wires stretched across the face of the panel. Not too onerous, but just an extra step required.

But it all does depend on the actual densities and panel thicknesses used.

E.g. 100kg/m3 is really too dense and doesn’t have enough porosity to let sufficient sound waves through to make it an effective absorber; too many will bounce back. It’s good for heat and sound insulation, but 60kg/m3 is probably the optimum density for effective absorption as well as being pretty self-supporting.

[Drew Stephenson]

What Wonks said.