soundproofing a ceiling

[livinginhope]

I'm trying to soundproof the ceilings of 2 ground floor rooms in our house, mainly against airborne noise. The rooms are each around 3.5m x 3.5m. A friend has told us that if we put two layers of normal plasterboard below the existing ceiling (thus dropping the ceiling height by perhaps 2-3 inches) and fill the gap above the new plasterboard with thermal insulation wool, that would probably provide an effective solution, a lot cheaper than buying in specialist materials, and he's order-of-magnitude guessed about £1000 to get someone in to do the job.

I'd like to know if this advice is sensible. Does anyone have any opinions? As a non-builder, I'm also curious how, if we did go this route, the new plasterboard would be held up (since the only thing to attach it to in the middle of the room would be the existing ceiling, which I would assume is not designed to support any significant weight). Does anyone know?

Thanks

 

[Deluks]

Lose the existing plasterboard, and hang your 2 layers on resilient bar, 100mm of rockwool between the joists.

http://www.british-gypsum.com/products/Gypframe-sound-insulating-bars