A New Probe Noise Approach For Acoustic Feedback Cancellation In Hearing Aids

Acoustic feedback is a big challenge in hearing aids. If not appropriately treated, the feedback limits the maximum possible amplification and may lead to significant sound distortions. In a state-of-the-art hearing aid, an acoustic feedback cancellation (AFC) system is used to compensate the artifacts caused by acoustic feedback. The AFC is usually carried out by adaptively estimating the feedback paths (from receiver to micro-phones) and using these estimates to reduce the artifacts introduced by the acoustic feedback. One major limitation in the widely used adaptive filter technique for AFC systems is the biased adaptive filter estimation problem, especially when tonal signals such as music and alarm tones enter the hearing aid microphones. The consequences of this biased estimation might be significant sound distortion or even worse, howling.
In principle, unbiased adaptive filter estimation can be achieved by adding a probe noise signal to the receiver signal and basing the estimation on the probe noise signal. However, the traditional probe noise approach requires a high-level probe noise signal, which is clearly audible and annoying for the hearing aid user. Hence, this high probe noise level makes the traditional probe noise approach less useful in hearing aid applica-tions.
We present a new probe noise approach which utilizes a low-level probe noise signal, which is inaudible in the presence of the receiver signal even for people with normal hearing. The probe noise signal is generated based on the short-time spectral envelope of receiver signal. Furthermore, this probe noise signal is generated with a specif-ic characteristic so that it can facilitate unbiased adaptive filter estimation with fast tracking of feedback path variations/changes despite its low signal level, which is not possible with the tradi-tional probe noise approach. We show simulation results of a challenging situation for AFC sys-tems, where the acoustic feedback path changes momentarily while the hearing aid user is listen-ing to music. The traditional AFC system fails completely with significant sound distortions and howling as consequences, whereas the new probe noise based AFC approach is able to remove feedback artifacts caused by the feedback path change in no more than a few hundred milliseconds.