Fix wooden battens to the walls and ceiling, fill the gaps with mineral wool and cover with 2 layers of acoustic plasterboard, preferably separated from the battens with resilient steel bars.
If the room is not on the ground floor lay accoustic matting under the carpet.
http://www.soundservice.co.uk/resilientbar.htm
Adam
September 2009
Hard smooth materials will reflect sound.
You need absorbent, soft/textured surfaces with low stiffness which will absorb sound and not reflect it. Rockwool is often used for acoustic insulation. The same materials which are good thermal insulators are also usually good acoustic insulators.
JD
September 2009
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