i was wondering what i could do to treat an acoustic tile ceiling (one of the "grid" ceilings). the ceiling has the tile at 9'6" from the ground and above the ceiling there is another concrete ceiling (17' from the floor) i was planning on just laying r-25 or r-30 o.c. (fluffy stuff) on top of the tile...would this work fine? there is also an a/c unit between the tile and the concrete ceiling...your help would be appreciated. thanks

Use some duct board and and maybe some MDF to try to 'box in' the AC unit as much as possible (still needs room to breathe). This will help cut the noise a little.

Filling the cavity with 'fluffy stuff' will certainly help and provide some bass absorbtion.

Depending on what kind of tiles are up there, you may also get some improvement in iso and absorbtion from changing to a better quality tile.

Bryan