More than you think? The Floating Mass Transducer (FMT) as a microphone – temporal bone measurements

Introduction:

Research using an earcanal-eardrum-model showed that the FMT possibly works as a microphone, so to say "reverse". Temporal bone measurements were performed to determine the sensitivity and frequency range. The FMT working as a middle ear microphone might lead to a fully implantable CI.

Material and Methods:

In a fresh human temporal bone a FMT was coupled to the long incus and to the short incus process to serve as a microphone. As a control for normal middle-ear function, the tympanic membrane was first stimulated acoustically, and the vibration of the stapes footplate was measured using laser Doppler vibrometry. Signals were presented via an intra-aural earphone (EAR3A). A Chirp was used to measure frequency range and sensitivity. Different speech and music signals were recorded to acoustically evaluate the sound quality. The FMT was plugged into the microphone input of an external sound card and signals were recorded digitally. A flat microphone probe was placed in the ear canal to measure the presented levels and frequencies. Vibrations of the ossicular chain were detected via laser Doppler vibrometry.

Results:

Both couplings lead to understandable recordings of speech signals. Coupling to the short incus process led to higher sensitivity. Frequency range of the FMT (0,5 – 5 kHz) was not flat and sensitivity was low. Sound editing could compensate both. Finally, words spoken in the test cabin could be recorded understandable via the temporal bone model.

Discussion:

The temporal bone model showed that the FMT works well as a microphone in both couplings. Coupling to the short incus process was favorable in our test setup. If the technique used for recording can be miniaturized it would be an interesting issue to use the FMT as a middle ear microphone.

Publication History

Publication Date:
23 April 2019 (online)