Acoustic treatment, or soundproofing, utilises a number of different methods to reduce levels of unwanted sound in a studio, rehearsal space, or live venue. These methods are absorption, redirection, decoupling, distance, and increased mass; and these should be the first considerations when attempting to soundproof your space.
Absorption reduces sound pressure with porous materials such as foam. Redirection reduces resonance by reflecting or diffusing unwanted sound so it either disappears or adds a neutral 'ambiance'. Decoupling involves placing barriers between a source and a receptor, while increased mass uses multiple insulating layers of different widths and densities to filter a broad range of frequencies. Distance should be obvious but the general rule is the more, the better!