Q? 2 Eric et al Corner Bass trap

[ppk]

Hi all, only just found this forum and I'm hoping you guys can assist me in a problem I'm desperate to solve. This is a post originally made over at recording.org. The reason I am posting here is I was lead to believe the DIY panel resonator was the be all and end all of bass trapping, I guess I'm really hoping for a second opinion as I have now run out of funds and REALLY need to get this right!
I have SERIOUSLY trawled all the posts I could find 'but I still haven't found what I'm looking for' (there's a song in there somewhere!)
Ok, brief history, built a studio in a double garage. The whole floated room within a room thang, resting on neoprene. Walls single skin plasterboard (exterior) 4inches of rockwool, double skin plasterboard, all sealed. Isolation wise couldn't ask for more. I really would recommend this method to anyone who wants to work into the small hours or at volume.
Strategically placed foam and diffusers, live end/dead end setup. Clarity and definintion are really good, very happy. So why am I posting?
The dreaded bass!!! I'm using Genelec 1031AMs but found my self guessing when it comes to the sub bass, tried repositioning the monitors but nothing gave me the answer I was looking for, so I bit the financial bullet (again) and bought the Genelec 7070 sub (real bad boy!). Isolated from the floor and resting nearly touching the rear wall (slightly off centre). I thought this would be the answer to my probs but it just gave me a set of new ones!
There's alot of wallowyness (is that even a word!) in the bass. I suspect this is from the use of double skinned walls on the interior (bass reflected, not absorbed). Also I think I'm experiencing major room modes and nodes. Using a swept sine wave there is serious reinforcement of freqs around 30hz>45 then a big hole at 60Hz coming back around 80Hz.
I'm thinking corner traps (I'm not keen on drilling into my sealed walls). The traps I was considering making would be constructed outside of the studio as complete sealed units. I'd make triangluar frames from 2x2 and screw ply or MDF to the faces you wouldn't see, sealing from the inside, line it with the rockwool, attach a batten to the inside to mount the last piece of rockwool and finally seal the vibrating face onto the front. (Hope this makes sense!) I would then take them to and position them in the studio. No drilling, removable if unsuitable. Panel resonators at 45o cutting off the corners, 2foot face. Rockwool on inside of wall faces and an inch gap between the vibrating panel. I plan to use two stacked metre tall traps, one with a face of 3/8 ply and one with 1/8 ply (in all four corners). The theory behind the two different thicknesses of ply are in reference to this post (to someone else) by 'Jazzman in pa':
But if you insist on building a corner trap yourself, here are some standard figures and what you should be able to expect for results, assuming the panel is at a 45-degree angle to each side wall:

Face width, thickness, center frequency
48", 3/8", 48 Hz
48", 1/4", 57 Hz
48", 1/8", 80 Hz

36", 3/8", 55 Hz
36", 1/4", 66 Hz
36", 1/8", 93 Hz

24", 3/8", 67 Hz
24", 1/4", 80 Hz
24", 1/8", 114 Hz

For a more broadband effect, stack traps with different thicknesses.

(This assumes that my calculations are right. I just plugged the numbers into formula published in Everest, for example. Average depth is one fourth of the panel width here.)


End quote.

The rockwool (from travis Perkins U.k) is not backed (i.e fuzzy face both sides) and 50mm thick, 100kg/m2 density. Does it matter that it is not backed or should I keep looking for a supplier?
I know you can't tell me whether this will solve all my problems but is there anything that stands out as being wrong? Is it a good starting point?

I've alot of time (and all my money) on my studio and I so desperately want to be perfect, so any advice or comments would be welcomed,

If you can think of a better solution to these problems I'm open to new ideas!

Thanx in advance for your time and reading this far,

Lee

[ppk]

I wondering if I would be wasting my time making the variable depth broadband absorbers as described above, if I negated the resonating panel and covered the face of the unit with cotton how would this affect the units behaviour and effectiveness? Understand this changes the function of the trap from a 'piston damper' system, letting the waves pass through to the rockwool unchallenged, would this reduce the amount of absorption or increase it?

any replies would be a great help and really appreciated, Knightfly..... anyone...... is this mic on?!

Cheers,

PPK

[Eric_Desart]

Hello ppk,

Just short,

If I understand correctly, you make a standard panel trap but with the backside triangular rather than the traditional rectangular cavity?
If so: Use the standard formulas but see the cavity depth as representing the volume of the cavity.
So a corner trap with the same volume and mass will show about the same tuned frequency as a standard trap with the same volume.
The volume is meanly defining for the dynamic stiffness of the airspring.

Maybe some other aspects come into play, but that's trying.

Why not just using wool (glassfiber or rockwool boards)?
Or with the Helmholz approach often used here?

I don't think for a corner that's necessary: The low frequencies are better absorbed than the high frequencies anyhow. So corner absorption does not really jeopardize too much mid and high absorption.

But I agree many ways lead to the same goal.

Regards
Eric