Soundproof rooms of plaster on hollow tile are effective acoustically, but have pronounced disadvantages pertaining to their construction and demolition, and have practically no salvage value. This paper describes soundproof rooms composed of panels consisting of two sheets of steel cemented to two sheets of composition board with a rockwool blanket between. The panels are of a few standard sizes, the combination of which enables rooms of almost any size to be constructed. The rooms are supported on industrial type rubber mountings for the reduction of building vibration. For the most effective attenuation of sound a second room is constructed which completely encloses the first room except that the floor of the second room consists of the floor of the building. The construction and the demounting of the rooms involve little dirt and noise, and there is practically complete salvage value. Eighteen such rooms, comprising both kinds, have been constructed in the new building of Bell Telephone Laboratories at Murray Hill, New Jersey. Single‐frequency attenuation measurements have not been made owing to the pressure of war work, but noise meter measurements show an attenuation of 43 db for the single rooms and more than 57 db for the double rooms.