[Measurement of sound transmitted through the body while drilling and grinding isolated petrous temporal bone...

Measurements of the effect of rotating drills and grinders on isolated fresh temporal bone on the sound transmitted through the body have not previously been made known. In contrast to measurement of air-conducted sound, they include the portion of sound which effects the hearing apparatus of the patient through the bone during the process of drilling the temporal bobe. Measurements were made with a calibrated acceleration pick-up in conjunction with a precision sound level meter (test amplifier with sound frequency analyser). The recording was made continuously between 20 Hz and 20 000 Hz. The range between 250 Hz and 8000 Hz was examined mathematically. Above 8000 Hz the curves dropped markedly, apart from a few exceptions. The level of the sound depends largely on the size of the drill bit, and consequently on the breadth and depth of the rotating cutting edges. The smaller drill heads produce a considerably smaller quantity of sound. The highest level of sound comes from the burrs and wing-cutters. The diamond head lies lower in the scale, but almost equals the effect of the steel drills. The speed of rotation of the drill head plays only a subordinate role. Between 16 000 and 80 000 r.p.m. the values are the same. In the region of 10 000 r.p.m. the sound level is frequently reduced, even if a few loud peaks may still occur here. The type of drilling machine, the handpiece or transmission handpiece used have no effect. Altogether, the rotary drill produces less sound transmitted to the inner ear through the body than through the air.