what is some good soundproof material (cheap that i coud put in a room
i know this isnt perfect soundproofing, but a mattress against the wall you dont want the sound to go through numbs the sound okay
egg boxes. Put them all over the walls.
egg cartons.. aka those cardboard things that eggs come in + a bunch of tissue in each of the egg holder things
what about for a wooden outdoors treehouse
Styrefoam might work. I have heard about the egg cartons, I believe you only use the egg cup side with the points turned outward from the wall. Thick blankets, maybe even a sheet of bubble wrap. All these things are fire hazards, do so at your own risk.
Buy some of those mattress padding things. They look like the egg shells. You can find them at bedding places.