Surface Coefficients in Room Simulation

[Nordo]

I've been dabbling with the Room Sim section of REW.
I did a search for "surface absorption coefficients" to ascertain what coeffs I should be putting in for the various surfaces.

In the Help section of REW, it states that the surface absorption is "independent of angle or frequency".
However all information I found regarding surface absorption indicates that the absorption coefficient IS frequency dependent.

Can John or anyone throw some light on this?

In my case the walls are timber framed with plasterboard (drywall) on both sides, and the ceiling is timber framed with plasterboard on the lower face.
The floor is a carpeted concrete slab.

BTW, I have recently realised that lightweight walls, like I am looking at, become almost transparent below certain frequencies.
i.e. the LF wave velocity is NOT zero at the wall face. Which questions some of the theory behind bass traps.

 

[John Mulcahy]

Actual absorption coefficients are dependent on frequency and angle of incidence but the simulator uses a fixed value.

 

[Nordo]

Thanks John.
Is there any guide I can use to estimate what values I should be using in the Room Sim?

I'm only biding my time until we take possession of our new house that has this room I'm modelling, which will be my (very poor) media room (there's no way I can call it a Home Theatre).

 

[John Mulcahy]

There are useful tables of absorption coefficients here, use the 125 Hz values. The coefficients will mainly affect the height of peaks/depth of dips, the overall distribution won't be affected so for experimenting with positioning options the coefficients aren't that important.

 

[Nordo]

That's great.
I had already downloaded a similar table.
But knowing to use to 125Hz coeff. was what I needed to have confirmed.

Thanks so much for your time - yet again.

 

[VolunteerPin]

Question on this also...I looked over the table above. It has been recommended to me to use two layers of 1" linacoustic with a 3mil plastic between the layers on the front wall (behind my AT screen wall). Any thoughts on what kind of coefficient that would create that I can plug into the simulator? Thanks in advance by a new member (first post).

 

[John Mulcahy]

Around 0.2, perhaps.

 

[Nordo]

Around 0.2, perhaps.

View attachment 42622

Hi John
Just realised I had spelled "Room" incorrectly in the title of this thread.
It doesn't appear that I can edit it, but can you or one of the Monitors/Administrators correct it?

 

[John Wilcox]

There are useful tables of absorption coefficients here, use the 125 Hz values. The coefficients will mainly affect the height of peaks/depth of dips, the overall distribution won't be affected so for experimenting with positioning options the coefficients aren't that important.

John, I've been using vector simulation software (e.g., SoundPLAN) for years to do industrial/environmental noise evaluations and mitigation. We always tried to obtain and use octave band absorption coefficients. Are the coefficients to be entered in the room simulator the same absorption units as typically provided in the octave band absorption data available for many materials? For numerous materials, including some in my listening room, only NRC or CAC coefficients are apparently available. Would those be at all relevant to use in for the room simulator coefficients? Would I need to adjust them with a conversion factor?

The surface of the front wall of my room is approximately 50% totally reflective (e.g., OLED panel) and 50% mineral wool (behind perforated panels and cloth). Would it be beneficial to try to use a weighted average of the absorption coefficients for those materials for the entire wall, or would that mess up the modal calculations too much?

Thanks!

 

[John Mulcahy]

125 Hz octave band coefficients should be reasonable to use. NRC and CAC are not useful as they are based on data at much higher frequencies. An area-weighted average for a surface is the right thing to use.

 

[John Wilcox]

125 Hz octave band coefficients should be reasonable to use. NRC and CAC are not useful as they are based on data at much higher frequencies. An area-weighted average for a surface is the right thing to use.

Thanks

 

[Alpi]

Hi John,

What absorption do I enter for e.g. 0.5m rock wool in the rear area?
or for 1m rock wool?
Special for 20 to 100Hz?
Is there a table

lg

 

[John Mulcahy]

See post #4 above.

 

[luegotelodigo]

I've been using a different approach to set the absorption in room simulator. Rather than chosing theoretical alphas I estimate them with the Sabine equation and the measured RT.

I can provide a longer explanation if needed.

 

[Alpi]

I would be interested

 

[luegotelodigo]

OK, in the image attached you can see the dimensions of my room (6.13*3.34*2.47) meters

Also the RT averaged graph. T30@63Hz=0.87 s
The easiest assumption is all the surfaces in the room have the same alpha (but after that, if you have bass-traps can improve it). So I have a room of 50.6 m^3 with a total surface of 2*(6.13*3.34+6.13*2.47+3.34*2.47)=88 m^2

According to Sabine: T=0.16*V/(alpha*S) as all the surfaces have the same alpha, so Alphaavg=0.16*V/(T*S)

So my avg alpha is 0.10 @ 50 Hz and 0.20 @ 100 Hz

 

[luegotelodigo]

BTW, I have recently realised that lightweight walls, like I am looking at, become almost transparent below certain frequencies.
i.e. the LF wave velocity is NOT zero at the wall face. Which questions some of the theory behind bass traps.

That does not question the theory. In fact, that's how diaphragmatic bass traps work. I've designed several studios and by tuning gypsum walls to the frequencies of interest I have avoided the need for expensive traps.

At home I have to put several resonators and I still don't achieve the EBU 3276 recommendations because in my country heavy brickwalls were the standard.

 

[Alpi]

Sorry I was not in the forum for some time. Thank you for the explanations, the calculations are based on the fact that the same material is used for absorption everywhere in the room?
But what if I have a baffle wall at the front, the front area on the ceiling and the sides with absorbers (10cm approx. 6000 Pa) and diffusers at the back?
Can I enter this in REW with the different values? That means, for example, 0.25?? at the front and ??? at the back ??? diffuser.
I have an RT60 of 100 - 200ms in the room. Is that too little?
My walls are made of concrete 30cm thick.

 

[luegotelodigo]

the calculations are based on the fact that the same material is used for absorption everywhere in the room?

Yes, that simplifies the calculations a lot. I shared an easy approach just to help in take some decisions. The aim is not being accurate.

But what if I have a baffle wall at the front, the front area on the ceiling and the sides with absorbers (10cm approx. 6000 Pa) and diffusers at the back?
Can I enter this in REW with the different values?

Yes, REW allows you to use different coefficients for every pair of surfaces BUT it doesn't allow you tu use different coefficients for different frequencies. The lower the frequency the lower absorption you usually have and the dimulator works well below the Schröder frequency SO don't use it for mid/hi frequencies.

I have an RT60 of 100 - 200ms in the room. Is that too little?
My walls are made of concrete 30cm thick.

I doubt you have such a low RT in the low end with 30 cm of concrete. If I were you I'd try to reach 0,6 s in 50-63 Hz and follow the EBU 3276 recommendations for 0.2 s for the rest of the frequencies.