Sound treatment

acoustic wall panel
accoustic ceiling panel
foam panels
create a room in a room, touching each other as little as possible to prevent low frequencies propagating note

absorb sound	reflect sound
lower densities	higher densities
thicker materials

you can perforate hard materials to make them more absorbant
this site has cool gifs on sound wave research, and found that the best shape is that of a standard accoustic panel. A row of triangles on a square. each square is rotated 90 degrees.

good materials for soundproofing (source)