Feedback suppression in a mechanically coupled, short signal path sensor-actuator system for use as a hearing implant

We examine a 10:1 scale model of a recently proposed type of integrated sensor-actuator system to be used as part of a middle-ear hearing implant, and show that this inherently unstable system can be controlled by conventional methods of digital feedback suppression. Sensor and actuator of the model are mechanically coupled by a free-floating hard metal frame about 30 mm in length, corresponding to 3 mm original scale. Due to the direct mechanical coupling, the resulting electroacoustic system exhibits strong feedback and high-frequency oscillation will occur even at small gain. We show that the feedback of this system can be successfully controlled by delaying the signal in the forward path and then applying adaptive and active filtering. Through the use of a least-mean-square adaptive filter combined with active digital filtering, an increase in maximum stable gain (MSG) of up to 43 dB and a functional gain of up to 35 dB (maximum) and 24.7 dB (mean) are achieved.