In Search of the Bone-Conduction Threshold in a Free Sound Field

Abstract

Several investigators have attempted indirect determination of the bone-conduction threshold in a free sound field. It has recently become possible to attenuate sound transmitted through the auditory meatus to a level below that of a sound reaching the ear through other pathways. By varying the properties of sound-attenuating devices, and with the help of resonance phenomena, it could be shown that the remaining hearing was actually controlled by body conduction, if not more specifically by bone conduction. According to these measurements, the “minimum audible field” for body conduction (MAFB) exceeds 80 db re 0.0002 dyne/cm2 at low frequencies and decreases gradually to a minimum of about 45 to 50 db between 1500 and 2000 cps. For higher frequencies it increases somewhat again. The difference between body conduction and air conduction seems to be maximal in the vicinity of 500 cps—approximately 65 db. At frequencies around 100 cps the difference is of the order of 50 db. This difference holds also for the frequency range between 2000 and 6000 cps. Between 1500 and 2000 cps the difference is only about 45 db.