Acoustic panel density

[Jas0_0]

Hi all,

I want to build some DIY 100mm absorption panels for my room - measurements at listening position below.

My question is, which density of Rockwool to use?

I was aiming at RW5 which is 100kg/m3. According to the Rockwool data sheet, at 75mm its absorption coefficients are 0.4 at 125Hz, 0.8 at 1kHz, and 0.85 at 4kHz, which seems pretty uniform (and 100mm will be even better for bass).

But Rockwool doesn't provide data for performance above 4kHz and I've read elsewhere here that anything over about 50kg/m3 will reflect too much treble.

Can anyone tell me if I will run into problems using RW5?

Many thanks,

James

 

[ozzy9832001]

If 125hz or lower are your problem frequencies then I'd recommend thicker traps first. Get bass under control and then add panels as you need them. In my smallish room, Front and ceiling made the biggest difference and 16" traps in the corners. SW didn't really help...made it sound too dead and stereo image very fragile. Rear I have windows so I just leave it alone.

 

[Jas0_0]

Thanks Ozzy,

Sadly, domestic situation means traps or subs aren’t an option. Below 125hz I will just have to use PEQ to kill the peaks and live with the troughs

My main issues are diffuse imaging in music and poor speech intelligibility in films which PEQ can’t fix, which is why I hoped broadband absorption might help.

 

[ozzy9832001]

Do you mind drawing a sketch of the room with windows and doorways. That would give us a better idea of what we are dealing with.

 

[fluid]

I want to build some DIY 100mm absorption panels for my room - measurements at listening position below.

My question is, which density of Rockwool to use?

100mm is not very thick and will struggle to be broadband in any material. Really dense Rockwool is not good for broadband panels. You can get an idea of the sort of absorption in a panel by using the calculator here http://www.acousticmodelling.com/8layers/porous.php

If the absorber isn't broadband it is just acting as an acoustic filter and changing the spectrum of the reflection instead of absorbing it, this might be fine as part of a coordinated treatment scheme but isn't a good idea in basic panels used at home.

As an example 100Kg Rockwool has a Gas Flow Resistivity of somewhere around 53000 MKs Rayls. Fibrous absorbers work by letting the gas in, if air can't travel through them they don't work as well. Look at the difference in a 150mm panel of something more like medium density fibreglass with an air gap.

Air gaps really work, so mounting the panel off the wall is nearly as good as making it double the thickness.

 

[Doodski]

Hi all,

I want to build some DIY 100mm absorption panels for my room - measurements at listening position below.

My question is, which density of Rockwool to use?

I was aiming at RW5 which is 100kg/m3. According to the Rockwool data sheet, at 75mm its absorption coefficients are 0.4 at 125Hz, 0.8 at 1kHz, and 0.85 at 4kHz, which seems pretty uniform (and 100mm will be even better for bass).

But Rockwool doesn't provide data for performance above 4kHz and I've read elsewhere here that anything over about 50kg/m3 will reflect too much treble.

Can anyone tell me if I will run into problems using RW5?

Many thanks,

James


View attachment 277467View attachment 277468

My suggestion is to use those office divider sound absorbing panels. They absorb a lot of frequencies and I've used them for this task and they worked fantastically. They can be found at auctions, at used sales outlets or you can buy new if you don't mind the expense of new.

 

[JSmith]

poor speech intelligibility in films

In many cases it's an unavoidable issue to an extent;

Here's Why Movie Dialogue Has Gotten More Difficult To Understand (And Three Ways To Fix It) - SlashFilm

/Film spoke with several Oscar-winning sound designers, editors, and mixers to learn why it has become tougher to understand what characters are saying.

www.slashfilm.com

JSmith

 

[Jas0_0]

100mm is not very thick and will struggle to be broadband in any material. Really dense Rockwool is not good for broadband panels. You can get an idea of the sort of absorption in a panel by using the calculator here http://www.acousticmodelling.com/8layers/porous.php

If the absorber isn't broadband it is just acting as an acoustic filter and changing the spectrum of the reflection instead of absorbing it, this might be fine as part of a coordinated treatment scheme but isn't a good idea in basic panels used at home.

As an example 100Kg Rockwool has a Gas Flow Resistivity of somewhere around 53000 MKs Rayls. Fibrous absorbers work by letting the gas in, if air can't travel through them they don't work as well. Look at the difference in a 150mm panel of something more like medium density fibreglass with an air gap.

Air gaps really work, so mounting the panel off the wall is nearly as good as making it double the thickness.

View attachment 278036

Thanks - this calculator is just what I need.

Seems to suggest that, no matter what the material’s density, absorption doesn’t ever drop away for higher frequencies, which is what I was worried about.

 

[fluid]

Thanks - this calculator is just what I need.

Seems to suggest that, no matter what the material’s density, absorption doesn’t ever drop away for higher frequencies, which is what I was worried about.

There are plenty of options to deal with that if you only want to address low frequencies. Putting plastic sheet over the outside, using the foil backed side facing out on rigid boards, using wood slats over the front, perforated panels, diffusors etc.

There is the multi layer calculator also that might help

http://www.acousticmodelling.com/8layers/multi.php

 

[chych7]

I used Rockwool 80 in my system (128 kg/m^3) and get improvements down to 20 Hz (my measurement data shows it, even though calculators say it shouldn't, so I don't trust those calculators). For treble, I'd be more concerned with over-absorption, so reflection is good. I put plastic wrap/scatter plates on my bass traps, and even then I have too much treble absorption in my room.


Below plots show the effect of panels in my room with eight 2' x 4' x 4" bass traps, floor to ceiling on corners diagonally mounted. Also have six 2' x 4' x 2" sidewall absorption panels placed between the seating and front speakers.

Waterfall before treatment:

Waterfall after, notice even <30 Hz sees improvements.

 

[Inner Space]

I used Rockwool 80 in my system (128 kg/m^3) and get improvements down to 20 Hz (my measurement data shows it, even though calculators say it shouldn't, so I don't trust those calculators).

Most calculators rest on some pretty solid and reliable equations - but the results are calculated on the basis of a single pass through, i.e. SPLs in one side and out the other, using methodology derived from soundproofing tests. For acoustic treatments in a real room, though, the sound goes through once and is then reflected at the boundary and passes through a second time, in the reverse direction. Thus your 4-inch absorber acts like an 8-inch absorber, hence the disparity.

 

[Cbdb2]

Low freq absorption needs mass. That calculator shows very little absorbtion below 60hz for that material This might make your room boomy. Its hard to find data on absorber materials below 100hz. And as far as higher freq, if they are a problem, you can add a layer of pourus material between the heavy stuff and the cover. Use the thickest, heaviest rock wool if you want to absorb low end.

 

[Cbdb2]

Most calculators rest on some pretty solid and reliable equations - but the results are calculated on the basis of a single pass through, i.e. SPLs in one side and out the other, using methodology derived from soundproofing tests. For acoustic treatments in a real room, though, the sound goes through once and is then reflected at the boundary and passes through a second time, in the reverse direction. Thus your 4-inch absorber acts like an 8-inch absorber, hence the disparity.

And traps in corners are even better.

Listening Room - Corner Loaded Bass Trap (AES-1985) - ASC

Listening Room - Corner Loaded Bass Trap. Manufacturer of the TubeTrap, StudioTrap, SubTrap, AttackWall & QuickSoundField.

www.acousticsciences.com

 

[dasdoing]

even though calculators say it shouldn't, [....] on corners diagonally mounted

how did you throw this into the calculator? it will have varying airgap varying from 40cm-ish to 0. pretty sure the calculator will show absorbtion for your material with a 20cm gap

Low freq absorption needs mass.

not with porous absorbers

 

[chych7]

how did you throw this into the calculator? it will have varying airgap varying from 40cm-ish to 0. pretty sure the calculator will show absorbtion for your material with a 20cm gap

Yeah it does show some NRC at 20 Hz (assuming 60000 for flow resistivity, using the data table here to estimate it for 128 kg/m3 rockwool). So I overstated that part.

The "0.2" at 20 Hz to me would suggest a small improvement. However, I saw a very large improvement to reverb at low frequencies. Perhaps it's really an interpretation thing, 0.2 may seem small but is not at low freqs? Or is it because these panels are in the corner, they effectively have much higher NRC? See below for RT60 Topt and calculated model to 20 Hz, before/after room treatments. The reduction in reverb at 20 Hz is quite large.


RT60 Topt and Calculated Model Before Treatment

RT60 Topt and Calculated Model After Treatment

 

[Cbdb2]

how did you throw this into the calculator? it will have varying airgap varying from 40cm-ish to 0. pretty sure the calculator will show absorbtion for your material with a 20cm gap



not with porous absorbers

Absorber flow resistivity is directly related to density. When you buy fiberglass or rockwool do you see flow resistivity specified? No you get the density.
Look at these tables. Its pretty obvious (even tho the lowest freq is 125hz) that density does improve low freq absorbtion.

https://www.bobgolds.com/AbsorptionCoefficients.htm

From this great article on the subject and that free simulation tool.

Rock wool, glass wool, hemp - which material is best suited for absorbers? — Jochen Schulz

Are you determined to improve the acoustics in your recording studio or home theater? Then a broadband absorber is the first choice! The only question is which material do you use best? I will use examples for different absorber depths to show you which flow resistance achieves the best absorption a

www.jochenschulz.me

"To classify how well the curves of the free tool compare with professional software, I performed the same simulation with the Soundflow software from AFMG (second graphic). The specific weight is also taken into account in the calculation. I used the density of the respective rock wool here. At low frequencies, the degree of absorption is somewhat higher compared to the calculation without weight."

 

[dasdoing]

Its pretty obvious

please rewatch the data. a 3" 701 absorbs (way) more than a 2" 705 for example.

 

[dasdoing]

the most intresting is that you actually gained SPL below that 40Hz mode. I have no clue why your treatment worked so well actually. 40Hz and 60Hz would be the first 2 modes? so the stuff below 40Hz would be pressure realted? sometimes called mode 0? not really sure

 

[chych7]

the most intresting is that you actually gained SPL below that 40Hz mode. I have no clue why your treatment worked so well actually. 40Hz and 60Hz would be the first 2 modes? so the stuff below 40Hz would be pressure realted? sometimes called mode 0? not really sure

30 Hz and 47 Hz for a (19' x 12' x 9') room. Yeah not sure what the stuff below 30 Hz is, it would imply some mode represented by a much larger room. Maybe the outside of the room/other parts of the house are interacting. This is a closed room on the second floor of a house.

 

[ozzy9832001]

That's a huge improvement for 4" panels. I don't understand your results they seem to defy what I've seen. Are the walls in the room paper thin? Or stuff with crazy insulation?