soundproofing and converting an attached garage

[stealtheatingtunnocks]

DH has aspergers. Three kids, who are getting bigger and noisier. He NEEDS down time to function, and, can't get it, which has resulted in some marital disharmony as he finds it hard to function.

Solution is smother him in his sleep to convert garage into a study/spare room/bolt hole. It's a 60s house, so, the garage is too small for a modern car anyway. Currently houses old tins of congealed paint he collects things like that and my chest freezer my precious.

So, question is - anyone know where I can find out about how to sound proof a new room?

The benefit to this is two fold - he'd have the squabbling family noise muffled so his distress dissipates, and, he could play his piano/guitar after the kids are in bed with abandon, his music helps to manage his stress.

I'd get a bed settee in it, so, space for family to come. Freezer could go in the living room with a doily on the top and masquerade as an occiasional table.

Any ideas about soundproofing?