Force as definition of bone conduction input

Abstract

Previous studies [J. Acoust. Soc. Am. 55, S62(A) (1974), EE 8; 56, S4(A) (1974), B8] showed acceleration to be a poor indicator of bone conduction (BC) responses. Using the same magnetostrictive vibrator, unoccluded BC thresholds were measured in terms of acceleration at the input (forehead) and force for frequencies between 100 and 10 000 Hz and for four different-size contact areas. In terms of force, thresholds were independent of the contact area or its place, while in terms of acceleration they varied considerably. Békésy (1939) had already monitored force at the input, realizing this to be the only quantity defining cochlear input, measurable outside the skull. Being the equivalent of electrical current, it is constant throughout a series circuit: i.e., skin, bone, skull content, cochlea. A second vibrator (Radio-Ear B 70 A), calibrated in terms of force, was also employed. Results, when compared with those obtained with the magnetostrictive vibrator, for a comparable contact area, varied by as much as 20 dB. Reasons for these discrepancies will be discussed. [Supported by NIH grants.]