[tINY]

When you get to the centerline of the room, both sides of the ceiling are the same distance from your ears or mic and cause odd sounds. It's a bit hard to describe.

The Master Handbook of Acoustics - Google Book Search

You can put mass loaded vinyl behind the T&G for some more isolation. Barrium Ferrite is the normal "mass" it's loaded with.

Soundproofing Mass Loaded Vinyl Barrier, Super Soundproofing Co
Mass Loaded Vinyl - SoundProofing America



-tINY