John Sayers' Recording Studio Design Forum Archive

Hello, I've gotten enormous amounts of helpful info from this forum over the years, but this is my first post!

I have a very small (503cm long x 277cm wide x 251cm tall) surround mixing room. Due to the built-in counter and cabinets in one corner of the room, I am unable to move the speakers any closer to the front wall. However, I had previously made a large "wall" of 610mm thick 72 kg/m^3 Rockwool (with a 610mm gap behind to accommodate the upper cabinets), which has somewhat tamed my SBIR null at 118Hz (see first attached layout with green insulation). The null is ~10dB deep, which I'm told is not horrible, but I would love to knock a few dB off of that if possible.

To attempt to do so, I played around with different materials and thicknesses using the absorber calculator at acousticmodelling.com and realized that I could get a significantly improved response in my trouble area by using Owens Corning R30 batts instead of the Rockwool (see attached graph).

I removed all of the Rockwool, and filled every cm^3 I could with the fluffy R30 batts (see second layout with pink insulation). I was surprised to find that my null went from ~10dB deep to ~16dB deep! From everything that I have read, a large mass of insulation with a low Gas Flow Resistivity (OC R30: approx. 5,500Pa.s/m^2) should be better than a thinner panel of insulation with a much higher GFR (Rockwool: approx. 33,000Pa.s/m^2).

Questions:
• What did I do wrong??? I assumed that the worst case would be that it would not yield a significant improvement, but I never expected it to actually make the null worse!
• More specifically: Am I misinterpreting the placement of a thick mass of low GFR insulation vs. an equal (or even thinner) panel of low GFR insulation? In other words, does a thick mass of low GFR insulation only work as a corner bass trap, but somehow not as a first reflection point?
• I'm pretty stumped, so any other thoughts or insights?

Few notes:
• I later tried adding a thick MDF wall between the speakers and the R30 (to make an artificial front wall), but that made the null even worse (also unexpected) and changed the frequency slightly (not unexpected).
• The closest room mode is ~102Hz, but that does not seem to be an issue -- the null moves as expected in relation to the speaker's distance from the front wall, where 102Hz has never caused a peak or a null at my mixing position.
• I do have graphs, but I'm not at my studio computer. I will add those ASAP.

Thanks so much in advance for any help!!!
Brett

Hi there Brett, and Welcome!

Please post the actual MDAT file, so we can analyze that. That's the best way to figure out what is happening in your room.

Also, I noticed that you are treating the FRONT wall of your room, but not the BACK wall???

The closest room mode is ~102Hz, but that does not seem to be an issue --

Actually, in that ares of the spectrum, you have modes at 92.6, 96.9, 102.7, 115.2, 120.1, 123.5, 124.3, .... You have a total of 71 modes below 200 Hz, and 16 of those are below your Schroeder frequency. Both the one at 115.2 and the one at 120.1 are with a few percent of your SBIR issue, which you seem to have calculated or measure to be at 118 Hz.

the null moves as expected in relation to the speaker's distance from the front wall, where 102Hz has never caused a peak or a null at my mixing position.

But what about 115.2 and 120.1? I would suspect 120.1, since it is tangential and related to the front-back axis...

• I do have graphs,

Graphs by themselves are no good: I need the actual MDAT file itself.

Also, please post some photos of the room itself, so we can get a better idea of what you are dealing with.

- Stuart -

Hi Stuart,

Thanks so much for your help on this! My apologies for the delay, been dealing with a mini-emergency the past few days, so I haven't had a chance to follow up or post MDAT files and pics. I will do so ASAP!

In the meantime, yes, I do have 200mm of Rockwool on the rear wall. The ceiling is also 100% covered in 100mm of OC 703 (and the back of the ceiling insulation hangs down to make a huge bass trap between the ceiling and rear wall).

I see what you are saying about the tangential modes -- I should have looked at those more carefully. I assume you are thinking that the SBIR issue is likely lining up with one of those modes?

Thanks again for the help, I should be able to post pics and the MDAT files by tomorrow.

Brett