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Angle of Incidence for membrane absorber
Old 30th January 2014
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TalkingTree's Avatar

Angle of Incidence for membrane absorber


Need some help on choosing either MLV or plywood for membrane traps.

Going back to the drawing board in dealing with a +10 - 15 peak, compared to Mix position. 60 - 80 hz is significantly resonant in both the Mix R window cavity alcove and Mix L sliding door fixed pane side alcove.

Both of these cavities are cut approx 10" deeper into splayed side wall plane. The worst spots of these cavities are directly behind the 1st Reflection panels, which are both 4" Rockboard 80 with 3" Roxul AFB behind, and which are 32" x 80" and are free standing frames, and angled off of splayed wall so as to leave a 2" gap from back of panel to wall. Basically, there is a huge pressure node as the 60 degree front corner walls - sheetrock/stud walls - come to a corner at the front wall edge of the alcoves, then jump out even wider into these cavities, which are built into the 12 degree splayed sheetrock/stud side walls.

I could try building Helmholtz boxes. Could do the Mix R box directly into the lower 4 feet of the splayed sheetrock/stud wall, as horizontal box. Could do the Mix L as a vertical box and put it in the open alcove space in between the fixed pane of the sliding glass door and the back of that 1st Ref panel. But Helmholtz at 70 hz might be too narrow??

But, does anyone think a 70 hz helmholtz can have atleast 10 db absorption with a Q set to range from 60 - 80 hz? I can build both boxes to atleast 8 cubic feet.

OR....I can do a membrane trap on each side, behind the 1st Ref panels. I would have to rebuild part of the Mix R wall, but no big deal, and need to get in there to put more absorption in part of the wall cavity anyway.

My question is in regard to the ANGLE OF INCIDENCE of membrane traps. If they need a 90 degree hit, then I will have to cut PORTS into the 1st Ref panels. Probably 2 - 3 ports that are 4-5" diameter each. Is this the way to go? Which material has the more "flexible" angle of incidence, MLV or ply?

And, considering the membrane traps are behind the 1st Ref panels, better to go with 9mm plywood or 2 lb sq ft MLV? Would build the box to 7.5" depth x 2 ft x 6 ft. Sealed all around and doubled, all 3/4" board.

See a few pics of space.

Thanks for HELP!
Attached Thumbnails
Angle of Incidence for membrane absorber-img_7104.jpg   Angle of Incidence for membrane absorber-img_7114.jpg   Angle of Incidence for membrane absorber-img_7100.jpg  
Old 31st January 2014
Here for the gear
TalkingTree's Avatar

Anyone have any insight here?
Old 31st January 2014
Gear Guru

A GS here, G.E., did extensive testing and ended up with a ply panel as optimal if I remember correctly. BBC tests in the past have ended up with 'hardboard' (pegboard without the holes)
VPR's seem very interesting IMO

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