Limp mass membrane material

Posted: Mon Aug 04, 2008 1:15 pm

Off by 5mm?!!!!! Shocked

That's why I've got the 30db cancellation @ 400hz then!

I've just ordered a laser triangulation system that is accurate to 0.0001mm Wink

(Of course I'm joking)

Yeah I hear what you're saying, chasing rainbows n'all. On a serious note, the room dimensions are shocking so i've got to go pretty hardcore with the treatment.
For small rooms the advice is generally
1 : Make sure speaker setup is symmetrical and along the short wall

2 : Treat RFZ with 703/rockwool covered in fabric at least 2" thick with a 2" airgap behind. Thicker the better (to extend absorption lower)

3 : Diffusion is not applicable in a room this small, it's better to treat the back wall with 4" 703 spaced from wall

4: As much bass trapping as possible, superchunk vertical corners if possible and don't forget the wall/floor and ceiling/wall corners

5 : 38% distance from front wall rule

I've ticked all those boxes and I've made alot of headway from where I was 3 weeks ago (which was probably a great improvement to an empty room). I may, in the future when i can bear to, remove all the treatment and shoot an empty room but for now I'm pretty happy with where it's at, given that I'm not going to get "perfect" in a room of these dimensions.

I will be happy when I get this 370hz problem sorted. That's the trouble with not measuring incrementally, you get an anomoly like that and don't know where it came from.
Also my speaker placement options are limited which doesn't help but I just want to say i do appreciate the help and opinions.

Posted: Mon Aug 04, 2008 11:13 pm

No, you 4.06cm from 38%.
You're 5mm from 39%...

Are you factoring altitude, average humidity and air pressure into your calcs?

Posted: Tue Aug 05, 2008 8:52 am

Great Scott!!! Marty!!!

Actually the flux capacitor needs recalibrating, then I can incorporate altitude and more importantly dark matter inertia.

But am I right though? Dem's the rulez right?

Posted: Tue Aug 05, 2008 1:53 pm

scoobz wrote:

Great Scott!!! Marty!!!

Actually the flux capacitor needs recalibrating, then I can incorporate altitude and more importantly dark matter inertia.

But am I right though? Dem's the rulez right?

Rod Gervais wrote:

38% back into the room from the wall you are facing is a good starting point.

This puts you in between nulls...... then your moves shouldn't be in feet - but in inches........ you have to move very little and test a lot to locate the sweet spot. Inches can make HUGE differences in what you are hearing.

And the move is inches forward and back......

I would think picking the relative speed of air at 1130 ft/s should be close enough.

Posted: Tue Aug 05, 2008 10:40 pm

Maybe I won't factor in tidal variations and curvature of the earth then Wink

Had a chance to take some pics tonight.....

http://i33.tinypic.com/25gvw4k.jpg
http://i33.tinypic.com/2wqz9mt.jpg

(Tried to go too wide with the back shot, it's a flat wall. You've got acoustic tiles, corner trap (with superchunk behind), another trap at the top, then a big 4" thick panel (4" air gap), some more acoustic tiles around the light switch. Then the door obviously, another trap like the big panel and a repeat of the other corner. Just in case it's not clear)

Posted: Wed Aug 06, 2008 9:40 am

I've been thinking, surely the 4" rockwool and 4" air gap in the RFZ is good down way past 400hz, so surely the reflected energy shouldn't be enough to cause a 30db cancellation?
Have I missed a reflection somewhere? Or am I misunderstanding SBIR?

Posted: Wed Aug 06, 2008 10:09 am

scoobz wrote:

I've been thinking, surely the 4" rockwool and 4" air gap in the RFZ is good down way past 400hz, so surely the reflected energy shouldn't be enough to cause a 30db cancellation?
Have I missed a reflection somewhere? Or am I misunderstanding SBIR?

I do not see anything directly behind the speakers - what do you have here?

Rod

Posted: Wed Aug 06, 2008 11:00 am

The dark area is a 4" rockwool panel (60kg/m3) spaced 4" from the wall. The white above and below is the wall itself

Posted: Wed Aug 06, 2008 12:28 pm

Scoobz,

Are the inner / outer speakers for A - B purposes? (In other words - you aren't listening to both at the same time are you?)

Rod

Posted: Wed Aug 06, 2008 10:32 pm

Lmao! Strictly A/B. It would be funny if I were though. Wink

Posted: Fri Aug 08, 2008 10:38 am

Hi Rod,

I'm afraid I don't know the equations or coefficients but I know for each db you reduce the reflected wave by you get an exponential reduction in the null.
My question is would the 4" 60kg/m3 rockwool spaced 4" from the wall reduce the reflected energy enough to make this cancellation due to a direct early reflection, eg from the mixing console?

I'm trying to plan my next course of action, I've read about the Peter Walker from Quad theory (basically about 1/3 distances for those that don't know) and I'm thinking about mounting my speakers a little higher, so the woofer is a third of 2m from the ceiling and 2/3 from the floor.
To my understanding this may help because 2/3 height would help to place the speakers imbetween the vertical modes and reduce standing wave excitation, also it would change the reflection point from the desk, perhaps beyond the edge of the desk.

Does this sound anywhere near plausable?

Posted: Fri Aug 08, 2008 11:49 pm

Yeah well that doesn't work! (lol)

Had an evening with Smaart, seeing the consequences of every little movement in realtime is the way to go!!!
I think I can confirm it was a null, I can shift it around the spectrum but can't eradicate it. Grrr
I tried all sorts of things, speakers up, down, back, out, same with the mic.
It looks like I can't win, helping one problem accentuates another, whether it be a cancellation or comb filtering.
Given the amount of absorption in the room I was really hoping to get a bit closer to 'flat', guess I'm going to have to settle for a compromise.
I've got so few placement options it's going to be a tough one

Ps. out of interest I tried placing the mic BEHIND the treatment to see what was making it thru to the wall. On the back wall things were killed very low down (like 100-200 ish hz) but around the speakers below 700hz was attenuated but not killed by any means, maybe 10db?
Now I know the sound waves have to make it thru the rockwool again so the reduction would be greater than I measured but it's obviously not enough to kill a 30db null! Wink

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