Questions regarding DIY absorbent panels

[MichaelConway]

Thank you in advance for your replies!

Quick Info: room dimensions 10' 10" x 9' 9" and the the ceiling is 8ft, I am working on building 9x 4" bass/broadband absorbent panels to treat my room and 2X 6" for the back vertical corners


In other threads about the DIY 6",4" broadband/bass absorbent panels, I read that I am not supposed to use the 4" on the early reflection points to my left and right walls? Maybe I misunderstood what was being discussed.

but my question is for early reflection treatment for mid/high frequencies would a panel with 2" suffice on the left,right and ceiling? If so I can squeeze more panels into the room and build more 6" panels for the corners

[SAC]

4" thick panels would be optimum - with a maximum effectiveness down to ~600 Hz at an incident angle of 45 degrees (~843 Hz at a normal 90 degree incidence) - assuming complete absorption.

You might achieve a measure of that with a 2 inch thick panel mounted ~2 inches from the surface in a pinch, but I would still recommend sticking with the 4 inch panels either surface mounted or spaced up to ~4 inches from the wall.

[MichaelConway]

Say that they are hung like picture frames. Would this would mean that the most effective range would be down to 850ish kHz?
Also, these are the same panels that would trap bass in the corners of the room correct? [2x 2" Knauf (oc 703 spec) per panel]

Thank you I can finally start putting these up!

[SAC]

Hz, not kHz!

They will have some extension below their calculated 1/4 wave cutoff.

No, broadband panel absorbers are not the same a corner bass traps, although bass traps can be made in the 'form' of panels optimally used to straddle corners, or in a superchunk triangular fashion designed to fill the corner.

Broadband absorbers are used to control specular boundary reflections which exist above the modal frequencies - typically above ~ 400 Hz.

I am confused by the notion that absorbent panels can be too thick. 4" thick panels work great! The problem is not that they might absorb too well, but that they might not absorb enough! (and also, that many coverings are not transparent enough to avoid reflections at higher frequencies and wth increasing angles of incident.)

Bass traps are used to control modal frequencies typically below ~ 400 Hz. 2" thick Rockwool panels would not insufficiently massive (thick) for bass traps. The prime example of a porous corner bass trap is the 'Superchunk' trap configuration..

They differ primarily in thickness and density and to a degree, in size.

[Glenn Kuras]

In other threads about the DIY 6",4" broadband/bass absorbent panels, I read that I am not supposed to use the 4" on the early reflection points to my left and right walls? Maybe I misunderstood what was being discussed.

4" is fine and if your budget works for it then go for it. Just don't use any kind of limp membrane to front like FRK and so on.

[DanDan]

There is pretty good agreement that 100mm plus 100mm airgap is a fine performer. These side and cloud panels can provide some useful control of the Height and Width modes. As you will see below their absorption in fact reaches down well into the modal region.
Some of the 'classic' views of what is actually happening can be quite misleading. Take a look at Andre Vare's illuminations on this in the Q 4 Avare thread.

e.g.

The classic way that the effect of gaps is explained is by use of graphs of thin absorbent material spaced away from a wall. The graphs show high values of α where the distance from the wall is 1/4 wavelength and 0 when the wavelength is 1/2. In other words the porous material is effective only where the particle velocity is high. These graphs are appropriate for thin material. The porous materials that we consider when discussing the use of gaps are not thin at the wavelengths significant to us. Therefore the graph is not accurate for our use of gapping!

Gapping is used to lower the effective frequencies of the sound absorption. It is usefull to start with the effect of thickness of homogenous porus material against a solid surface. This the mounting of material used in the reporting of the absorption of materials as used traditionally in studios. With typical porous material, using 703 for the example, at 4" thickness, α is 1 down to ~250 Hz and usually considered effective down to ~125 Hz. At 250 Hz the wavelength is 4.52 feet. Dividing the thickness of the material by the wavelength (.333/4.52)gives us a ratio of .0737, or ~7%. So the thickness of a porus absorber has to be at least 7% of the wavelength for flat absorption. If we consider 703 material absorption at 125HZ to be practically 1, this gives a ratio of 3.5%.

Remember, the previous paragraph deals porous material against a solid surface. This is the area where the particle velocity is lowest in the sound wave. The efectiveness of a non thin absorber is not significantly reduced when located in the relatively low region of particle velocity.

DD

[MichaelConway]

Originally Posted by SAC

No, broadband panel absorbers are not the same a corner bass traps, although bass traps can be made in the 'form' of panels optimally used to straddle corners, or in a superchunk triangular fashion designed to fill the corner.

Bass traps are used to control modal frequencies typically below ~ 400 Hz. 2" thick Rockwool panels would not insufficiently massive (thick) for bass traps. The prime example of a porous corner bass trap is the 'Superchunk' trap configuration..

They differ primarily in thickness and density and to a degree, in size.

Thanks again guys!


I am using this tutorial from Youtube YouTube - How to make a Bass Trap Acoustic Panel (Tutorial)

Some differences though,
1.) I'm not using OC 703, I am using the equivalent Knauf
2.)Also I have lined the fabric with a thin batting to help with keeping the insulation fibers in.

Just to make sure, have I been pretty much building bass traps for straddling the corners? What materials might I use for broadband panels?

[DanDan]

703 is one of the best all round performers.
If you have found something close, you are doing well.
Can you link to the Knauf specs? I have been having trouble finding them, not just now, but over time.

The same material (703 equivalent) is equally good for traps intended for bass or side reflection or cloud.

For Bass traps, double or triple it. 150mm straddling a corner will be a spectacular performer. Once you go deeper than 150mm or so, a lighter material would be better. e.g. SuperChunks are fine with 20-30KG fibre.

DD

[MichaelConway]

Originally Posted by DanDan

703 is one of the best all round performers.
If you have found something close, you are doing well.
Can you link to the Knauf specs? I have been having trouble finding them, not just now, but over time.

The same material (703 equivalent) is equally good for traps intended for bass or side reflection or cloud.

For Bass traps, double or triple it. 150mm straddling a corner will be a spectacular performer. Once you go deeper than 150mm or so, a lighter material would be better. e.g. SuperChunks are fine with 20-30KG fibre.

DD

Here is the data for it, Knauf Insulation Board with ECOSE® Technology

the guy at the insulation store said its the same density as the OC which is what the 3 stands for in 703. He also said that the Knauf boards don't have super harsh chemicals and they also don't dust as bad as the OC boards.

[DanDan]

Thanks for that. Very useful, right now as it happens. A client is shopping. The 3 pcf stuff would be equivalent to 703, if not better. I have had a feel of the Ecose stuff and can say it does seem less itchy.
DD