Diffusion vs. Absorption for back wall (13feet from seat)

[sirlordcomic]

Hi all! Glad to find this forum. I'm hoping to get a little insight into placement of absorbers and diffusion in my critical listening room (2channel).

I will post room info map, but it is not up to date and plans have changed. I live in the Middle East where wood is expensive but I have been building 4' x 2' absorbers 4" thick out of 70kg/m density rockwool covered in poly batting then "breathable" cotton.

I pretty much have RFZ covered but now I am considered what to do on the back wall, which is about 13ft behind my head so I think diffusion is ok - and some of the demos I heard at Real Traps have given me pause to utilize diffusion.

Since there is a sliding glass door there, covered by curtain, I am deciding on absorption ( (6) aforementioned 4" thick panels) vs. 2D diffusion to cover the entire space 12ft wide and 5ft down from back wall/ceiling.

I have REW and an MPR1 cal. mic kit and have shot the room without sub. with no treatments. Replacement amp just arrived for sub. so I will integrate shortly and re-shoot room with some of the treatments minus back wall.

I think I have a mode issue at 50hz, as predicted by calculators, and also shown in waterfall etc. I can't really do a ton of bass trapping other than I have a Helmholtz in the works to mitigate it. I'd be hard pressed to do a super-chunk in the one back corner available since my wife is being very patient, and this isn't a man-cave. I know the 4" absorbers aren't going to really help down low but I wonder if I should stick to my original plan of absorption on the back wall vs. diffusion.

If the large rectangle of 2D was up there, my plan was to move (2) 4 x 2 absorbers off the the rear-side wall into a 4 x 4 FOOT square on each side, then add about (4) 2' x 2' 2" absorbers on the rear-ceiling, effectively "surrounding" the diffusion zone.

I'll post .mdats and more info shortly. Interested in perspectives on diffusion given my relatively large room size (12foot ceiling).

Room mode calc. results were not so good IIRC:

24) A worst case scenario calculated by RPG - 1:1.075:1.868 (attached below)

cheers!
SLC

[Soundman2020]

Hi "SLC", and welcome to the forum!

I have been building 4' x 2' absorbers 4" thick out of 70kg/m density rockwool covered in poly batting then "breathable" cotton.

That's fine for mid and high frequency, but it is a bit too dense for really good absorption of very low frequencies: Can you get less dense mineral wool? Or maybe fiber glass insulation?

I pretty much have RFZ covered

I'm not sure that you do! Your diagram doesn't seem to show that: none of the front surfaces are angled!

I have REW and an MPR1 cal. mic kit and have shot the room without sub. with no treatments.

Excellent! Please post the MDAT file here, or if it is too big then upload it someplace and post the link. The graphs you posted are not detailed enough to see how the room is performing.

I think I have a mode issue at 50hz, ... and also shown in waterfall etc.

That's hard to say, from the graphs you posted. And you clearly have a LOT more problems than just one modal issue!

I can't really do a ton of bass trapping other than I have a Helmholtz in the works to mitigate it

I would not recommend that. Helmholtz resonators are great on paper, but hard to do in reality, especially for low frequencies. Tuning it to exactly your modal frequency is a major challenge, much harder than it sounds, and then locating it correctly in the room to actually treat that mode is another challenge. It would be much better to use deep absorption.

Why do you say that you cannot do bass trapping? All small rooms need bass trapping, an a LOT of it.

I wonder if I should stick to my original plan of absorption on the back wall vs. diffusion.

Diffusing low frequencies requires very large devices. If you want to go with diffusion instead of absorption, I would suggest large poly-cylindrical diffusers on the rear and side walls, but you'll also need absorption. Diffusion alone is not going to be enough.

So please provide the MDAT file, so we can take a closer look and see where your problems are.


- Stuart -

[sirlordcomic]

Thanks for your reply.

I have 2" thick overhead in (3) 4' x 4' squares. Back wall has (5) panels 4" thick, and some simple tri-corners in the back. There are some other absorbers as noted in the diagram, but the wall behind the speakers currently has no absorption, just some wood decoration my wife and I put up. I was thinking of adding some 1" thick panels behind them.

I can't do much more because it is not a studio, but rather a shared living room. The sound is greatly improved right now but of course there some issues I was hoping to address with Convolution using Audiolense + J. River. Granted, this may not mitigate the bass issues...but overall I'm pretty happy given what I was hearing before.

Here are the new measurements after treatments:

https://docs.google.com/file/d/0B4DCwTN ... sp=sharing
https://docs.google.com/file/d/0B4DCwTN ... sp=sharing
https://docs.google.com/file/d/0B4DCwTN ... sp=sharing
https://docs.google.com/file/d/0B4DCwTN ... sp=sharing

Naked Room Measurements:

https://docs.google.com/file/d/0B4DCwTN ... sp=sharing
https://docs.google.com/file/d/0B4DCwTN ... sp=sharing
https://docs.google.com/file/d/0B4DCwTN ... sp=sharing

Subwoofer was broken for naked room testing.