First thing you want to do is to decide whether you truly want to soundproof a room, or the simply want to deaden the sound in the room? Understanding how sound travels will help you to decide this question.

To give a completely and totally ridiculous answer, sound is the interaction of one molecule with another molecule. Therefore the only way to achieve a soundproof room is to build that room in a vacuum. The most sound resistant room that you can install would-be an anechoic chamber. Soundproofing a room, that is preventing sound from transmitting through the walls, can be accomplished in a slightly different manner.

The idea behind soundproofing a room is simply to stop as many molecules as possible interacting with one another as possible. The most common way of doing this is to install some sort of a tab onto the studs of the room. Then you install your drywall on the studs. Insulating material would go into the wall cavity. And then some sort of material would go on the surface of the wall, preferably inside the room so it doesn’t make your whole house look weird, that would act as a sound deadener. That can get pretty expensive.

Now trying to keep as much sound in a room as possible, or sound deadening. The idea behind this is to line the walls with some much junk that the sound simply can’t escape. Common materials are corkboard, deep shag carpeting and insulation. I have even seen rooms lined with bed mattresses. The idea is to simply add some much mass to the walls that the sound cannot escape.

I’m no sound engineer but that’s it closely in a nutshell. I hope this helps.

God bless