Join Date: Jan 2013
Location: Stockholm, Sweden
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I’m about to start a build in a room with similar dimensions, mine is 300x430cm.
If your room is on ground level and you have a level above consider soundproofing the ceiling as well. I would do it in any case since sound will escape through the ceiling and get to surrounding rooms if you only soundproof the walls.
So if you skip that step your efforts with the walls might be in vain.
Is your floor on a slab?