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stenerlov 9th August 2020 09:43 AM

Low density mixed with high density

I have a pallet full of 75kg/m3 Rockwool that I would like to use for treating a small control room. I will build floor to ceiling 30cm deep traps on back and side walls. My plan is to put one layer of this rigid rockwool closest to the wall and put a thick layer of fluffy on top of it.

Looks good in the calculator but are there any other issues I should be aware of? Impedance etc?

I know I could do the full depth with fluffy but it would be great if the rigid/semirigid stuff could be used here.


stenerlov 11th August 2020 02:41 PM


stenerlov 13th August 2020 12:45 AM

I’ve read all I can here and cant find an answer to this

radxge 15th August 2020 07:21 PM

In practice, it seems that most folks put the higher density material on the front of the panel... the reverse of what is proposed here.
I personally had good success using a layer (3") of Safe N Sound (GFR of ~10k) in front of 21" of Pink Fluffly.

These calculators may not be very accurate, especially for more complex scenarios such as mixing materials. BTW, I prefer the Komatsu model over the default model of the online calculator.

To be honest, real life testing is the only to be sure.
See here:
Porous Absorber Calculator.. Accuracy issues?l

stenerlov 16th August 2020 09:38 AM

Thanks for your answer!

Doing the opposite, putting the denser material on the front doesnt look good:

Lemon Squeezer 17th August 2020 09:08 AM

Surely the fluffy less dense material is at the front, to capture/absorb the sound's very high frequencies.

What's left then hits the harder stuff, which is more reflective of high frequencies.

Any reflected high frequencies then bounced back and absorbed by the fluffy layer.

That is how I am going to build mine.

No chance of me putting the the hard/dense stuff at the front.

stenerlov 17th August 2020 03:33 PM

We’re on the same page - the calculator shows practically the same result with the inner dense layer as compared to the whole depth with just fluffy.

So I cant see the inner rigid layer making anything worse - maybe even some positive effects? Membrane effect?

The only negative I’ve read is about impedance - is that an issue here?

Lemon Squeezer 17th August 2020 10:04 PM

I reckon the rigid, i.e. denser/heavier layer will absorb more low end - as the graph shows.

avare 18th August 2020 02:39 AM

Having the denser material deeper provides a more gradual transition of the resistance, therefor less reflections

stenerlov 18th August 2020 06:46 AM

2 Attachment(s)
This is the rear wall - 10cm 75kg/m3 (Rockwool RedAir) closest to the wall and then 20cm fluffy (Isover Piano).

stenerlov 18th August 2020 07:00 AM

1 Attachment(s)
One of the side walls with dense inner layer