soundproof

adj

1. Not allowing sound through.

   “The contestant was placed in a soundproof booth so he could not receive help from the audience.”

   “The electrical activity of the subject's cerebral cortex was recorded in a state of relative rest, after a 5 min adaptation of the subject to a dark soundproof chamber. The biopotentials were recorded from the electroencephaloscope screen […]”

verb

1. To make resistant to transmitting sound.

   “We soundproofed the room so we couldn't hear the road noises; unfortunately we couldn't hear the fire alarm either.”

   “It almost goes without saying that the train is fully soundproofed, with double-glazed windows and highly efficient air-conditioning by the British firm of J. Stone.”

   “Soundproofing a room may be as simple as sealing gaps around a door, or it may require rebuilding an entire wall or lowering a ceiling.”