Predicting bone conduction device output using a forehead surface microphone: comparison with promontory motion and intracochlear pressure

To evaluate the accuracy of output predictions made by a custom prototype forehead-mounted surface microphone (SM) when used with different bone conduction devices (BCDs). Experimental study comparing SM measurements with three-dimensional cochlear promontory motion (VPROM) and intracochlear pressure in response to stimulation by various BCDs. Six fresh-frozen human cadaver heads with three BCDs (one ADHEAR and two BONEBRIDGE devices implanted at separate positions). A stepped sine stimulus (0.1–10 kHz) was applied. Strong linear relationships were observed between forehead surface sound pressure, promontory motion, and differential intracochlear pressure across all BCDs, particularly between 400 Hz and 2 kHz. At higher frequencies, SM sensitivity declined—above 2 kHz for the ADHEAR and above 3 kHz for the BONEBRIDGE—limiting predictive accuracy. The prototype surface microphone reliably measured device output at low to mid frequencies, indicating its potential for dynamic range estimation in clinical settings by normalising SM output to hearing thresholds. However, reduced sensitivity at higher frequencies suggests that further development is needed to enable reliable output prediction across the entire clinically relevant frequency range.