VERB (1)

1. insulate against noise;
- Example: "Proust had his apartment soundproofed"


ADJECTIVE (1)

1. impervious to, or not penetrable by, sound;
- Example: "a soundproof room"

WordNet (r) 3.0 (2006):

soundproof
    adj 1: impervious to, or not penetrable by, sound; "a soundproof
           room"
    v 1: insulate against noise; "Proust had his apartment
         soundproofed"

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "soundproof":
   airtight, ballproof, bombproof, bulletproof, burglarproof,
   corrosionproof, dampproof, fire-resisting, fireproof, flameproof,
   foolproof, hermetic, holeproof, impervious to, insulate, leakproof,
   noiseproof, proof, proof against, punctureproof, resistant,
   rustproof, shatterproof, shellproof, tight, waterproof, watertight,
   weatherproof