Mechanical impedance of the skin-covered human head at different stimulation positions, static forces, and interface areas

Bone conduction (BC) transducers applied to skin-covered areas of the head differ in their positioning, contact area, and static force, all of which influence output characteristics. This study assesses the mechanical impedance of the skin-covered head under varying conditions. Measurements were conducted on 30 participants at 3 positions, earfront, forehead, and mastoid, using circular interfaces with diameters of 10, 15, and 25 mm, and 6 static forces ranging from 0.5 to 7 N. Results showed that mechanical impedance was stiffness-controlled below the resonance frequency and mass-controlled above it. Low-frequency impedance depended on position and static force, with the forehead producing the highest impedance magnitude and the earfront producing the lowest impedance magnitude. At high frequencies, impedance across positions was similar, except for the mastoid with 25 mm interface. Impedance magnitude increased with interface area below 350 Hz and above resonance frequency. These findings highlight an impedance mismatch between standard artificial mastoids and human mastoids, potentially leading to inaccurate force estimations in BC devices. Additionally, three-element and six-element lumped mechanical models for the earfront, forehead, and mastoid were developed, with parameter values as function of contact area and static force.