FRK facing used on the rear of fiberglass bass traps

[Gohanto]

I've seen the advice mentioned numerous times about having FRK facing only to the outside of fiberglass bass traps and removing it from the layers closer to the wall. I understand the definite advantages here with the high frequencies being bounced back into the room, but I haven't been able to find an explanation for why to not keep it on the back?

From my understanding, the air gap is present in porous bass traps because they work on sound wave velocity, which is 0 at the wall surface and higher further away from the wall. Then helmholtz and membrane traps work best near a wall since they rely on sound pressure instead of velocity, which is highest directly at the surface of the wall.

So, if the FRK facing acts as a limp mass membrane, then why would having that closer to the wall not help absorb more bass?

Thanks.

[Ethan Winer]

The only way to know for sure is to measure with and without FRK on the rear at varying distances from a surface. As far as I know nobody has done that. FRK on the rear with an air gap should give more bass absorption. But another advantage of an air gap is the rear of the panel can absorb sound that gets back there. And with FRK on the back side of the panel, that will reduce mid/high frequency absorption. So if I were treating a reflection point I'd probably leave off the FRK, even on the rear. But I admit I'm mostly guessing.

--Ethan