A soundproofing device for a drill which has an elongate shaft extending between a bit and a forged collar for attaching it to a pneumatic hammer comprises a sheath formed by a semi-rigid tube of plastics material adapted to surround the shaft without touching it. This sheath extends between sealed first mounting ring near the bit and second mounting ring near the hammer. These mounting ring are free to move relative to the shaft in rotation and in translation. Tests show a reduction in the noise level 5 m from the drill from 102 dBA to 89 dBA.