A modified Floating Mass Transducer (FMT_big) used as a middle ear microphone– Temporal bone measurements

Introduction 

In a preliminary work it could be shown in a temporal bone model that the standard FMT (FMT_stand) could be used in principle as a middle ear microphone. The main limitation is that the sensitivity is too low for practical use. A modified FMT_big (coil and magnet optimizations) was examined with regard to its microphone properties in the temporal bone model and speech recordings were played directly to a patient’s CI via Bluetooth for the first time.

Material and Methods 

Measurements were made in human temporal bones using the FMT_big as a middle ear microphone, coupled via an SP coupler to the short incus process and to a bony rim as a control. For frequency response and sensitivity determination, a chirp signal (100Hz-10kHz, 1s) was presented via a plug-in earphone (EAR3A). In the free field, lists of the Freiburg Number Test were recorded at different volumes. Conversations in the room at speech and whisper volume from about 1.5m away from the temporal bone were recorded and played into a test patient's CI system via Bluetooth.

Results 

The sensitivity of the FMT_big was 0.21 mV/Pa at 1000Hz, higher than that of the FMT_stand (0.041 mV/Pa). The maximum sensitivity was at 1500Hz (0.23 mV/Pa). Numerals could be intelligibly recorded in the free field up to 40 dBHL. The recorded, freely spoken sentences were understood up to whisper volume by the CI patient.

Discussion 

The speech intelligibility threshold with the FMT_big was 40 dB HL, comparable to the pure-tone thresholds of subcutaneous (44.9 dB HL) and external microphone systems (36.4 dB HL) reported in the literature (Lefèbre et al., 2017). The results are encouraging to further investigate the FMT_big as a potential middle ear microphone for practical use.

Conflict of Interest Der Erstautor weist auf folgenden Interessenkonflikt hin Bereitstellung des FMT durch Fa. MEDEL.

Publication History

Article published online:
24 May 2022