Fixed filter implementation of feedback cancellation for in-the-ear hearing aids

Abstract

Acoustic feedback oscillation in hearing aids is a problem which often prevents an adequate amount of gain from being realized for some hearing aid users. A study is presented in which a feedback cancellation technique is applied to hearing aids in 13 ears. The technique consists of modeling the feedback path around the amplifier and duplicating it with an FIR (finite impulse response) filter. The output of this filter is inverted and summed with the input signal at the input to the amplifier, canceling the signal from the feedback path around the amplifier. This technique is applied in its most optimized form and also in several variations of a simplified form. The optimized form employs a long floating point FIR filter which is generated from the impulse response of each specific subject's feedback path around the amplifier. These feedback paths include the microphone and receiver responses in a vented in-the-ear (ITE) earmold. Comparisons between feedback path transfer functions are shown to reveal that the phase responses are quite similar, but that the magnitude responses are not. An FIR filter is constructed which is an average of the 13 measured impulse responses. This FIR is implemented in three forms: a long floating point FIR filter, a short floating point FIR filter, and a short fixed point FIR filter. All three are shown to be effective in reducing feedback, provided the overall gain of the filter may be increased or decreased in order to optimally suit the magnitude response of the filter to the needs of each person's ear. Using this technique a fixed filter may be implemented in ITE hearing aids for all hearing aid wearers to allow more gain without feedback oscillation.