Physical and physiological constraints on the use of bone-conduction speech audiometry

Several authors have recommended the use of bone-conduction speech audiometry, and the literature supports the clinical value of this procedure. It has been claimed that bone-conduction output for speech can be increased to 110-dB HL with the Radioear B-70-A vibrator through supplementary amplification, but this claim is unsubstantiated by objective measurements. Available technical data indicate that the maximum output level attainable with this virbator without incurring serious distortion is 65- to 70-dB HL at midfrequencies and substantially less at lower frequencies. Both behavioral and electromechanical data are presented which show, not only that 70-dB HL is the absolute maximum hearing level for speech attainable through the B-70-A vibrator without serious deterioration of speech-discrimination scores in normal listeners, but also that this appears to be very close to the maximum vibratory level that human observers can comfortably tolerate.