I hate to appear contraian on the riser issue, particullarly considering I suggested the adhesive.. however I'll do it anyway
I'd put the adhesive only near/in/around the seams (3-4" wide) where the scant sheet was cheated.. if there is a concern for the amount of edge bearing or possible squeaks due to extra spacing in the tongue/groove area. You'll know approximately where the seams will hit on your top sheet, hold back the GG in a 6" wide area when applying to the back of the 2nd sheet, apply the adhesive in the area around the seam and drop the second layer in place...
Why GG???... Maybe I'm fully wacked in my theories but the secret sauce in the "broadband riser" is a combination of both the Vents (where everyone seems to focus) and the construction of the risers top and sides.. Commonly the reoteric says "No.. dont create a helmotz" "too hard to predict".. "very narrow absorbtion range".... all true.. but the vents we cut in create exactly that ... openings to a cavity in a high pressure area.. a helmotz.. no? But lets look at the riser as resonant/panel absorber also, its a very large panel it will resonate tempatically (sp?) with the wave front.. The resonant frequency of a panel is determined by the mass and airspace... the "Q" of this absorber is determined by the dampening and cavity absorbtion. A stiff panel (ie no dampening) will be far more "peakish" in its absorbtion range than one thats damped.. look at limp mass absorbers.. it takes alot more energy to keep the panel moving becasue its so well damped... The more you can dampen the panel (top of riser), combined with the absorbition in the cavity increases the effective range of frequency making it "broadband", and less likely to over absorb in a frequency where you dont necessairly need it.
Lets also look at some well known Designers approach to the top of a riser... 3 layers of 5/8 .. i dont belive this is done to get the additional 1/4 " of mass, as compared to just 2 layers of 3/4.. this is done to get the additional dampening layer.. further increasing the effective range for the "broadband absorbtion" and to mitigate the the peak resonance.
I believe the riser construction is "low hanging fruit" particullarly in the LF areas for the first couple of primary modes where your other options would take very very thick absorbers covering your walls...
This is only what I believe, ... I'm Ok with being wacked on this theory... and would like to hear alternative explanations on why/ how broad band absortion is acheived with a riser.
Apologies JPA for cluttering your thread.. I guess that was far more long winded then necessary... I coulda just said:
+1 on the GG