From Actuator to Microphone: Acoustic Characterization of the Floating Mass Transducer in Bidirectional Use

<title>Abstract</title> <p>Background Implantable microphones are a prerequisite for fully implantable hearing implants. A new approach utilizes the electromagnetic actuator Floating Mass Transducer (FMT) inversely, as its underlying principle of induction enables bidirectional operation. Objective This study assessed the reciprocity of the FMT’s frequency response as an actuator and, in inverse operation, as a microphone. Three FMT types – one established actuator design and two newly developed for microphone application – were compared regarding reciprocity, sensitivity and signal-to-noise ratio (SNR). Methods Measurements were performed on the FMTs coupled to an ear canal model, using a chirp stimulus (150 Hz − 7 kHz) to record frequency responses. Sensitivity values and signal-to-noise ratio were additionally assessed using pure-tone stimuli. Results For all FMT variants, actuator and microphone frequency responses were well aligned, with correlations between r = .72 and .83. Maximum sensitivity increased to 0.22 mV/Pa and mean SNR extended to 52 dB in the newly developed FMTs. Conclusion The inverse utilization of the FMT for sound recording could be shown. To assess the microphone quality more accurately, measurements on human temporal bones and speech recordings are crucial.</p>