Digital Feedback Suppression (DFS): Characterization of Feedback-margin Improvements in a DFS Hearing Instrument

Abstract

The introduction of a new power behind-the-ear hearing instrument equipped with an integrated digital feedback suppression (DFS) system, based on adaptive, digital signal processing, creates the need for new methods for evaluating the characteristics of this new technology. A special measuring method based on determination of the complex loop gain of the DFS instrument and the associated feedback path is described. This method yields information about the static feedback-margin improvement due to the DFS system, and the method is usable especially in connection with measurements on real ears. It requires fairly advanced test facilities, including a dual-channel FFT analyzer and, by preference, an anechoic room. Loop gain measurements on the new DFS power behind-the-ear hearing instrument show encouraging results. Groups of profoundly hearing-impaired children and adults were tested, and static feedback-margin improvements in the order of 10 dB for the new DFS power hearing instrument were seen. Variations were largest for groups using own ear moulds and individually fitted instruments.

Occlusion Effect of Earmolds with Different Venting Systems

“…One important advance in hearing instrument technology has been the development of feedback suppression systems. Some of these systems provide higher maximum stable gain by the use of feedback cancellation filters (Dyrlund and Bisgaard, 1991;Murray and Hanson, 1992;Dyrlund et al, 1994;Henningsen et al, 1994;Greenberg et al, 2000). While this added feedback margin may be considered as means to protect hearing instrument wearers from ever experiencing feedback, it may also be utilized to provide greater venting than the desired gain settings would otherwise allow.…”

Adaptive feedback cancellation in hearing aids with clipping in the feedback path

“…The filter is applied to the hearing aid output signal in order to form an estimate of the feedback signal, which is then subtracted from the hearing aid input signal to cancel the feedback. This approach has been shown to be effective, both in laboratory studies and in commercial hearing aids ͑Bustamante et al., 1989;Dyrlund and Bisgaard, 1991;Kates, 1991;Engebretson et al, 1993;Joson et al, 1993;Dyrlund et al, 1994;Maxwell and Zurek, 1995;Kaelin et al, 1998;Kates, 1999;Siqueira and Alwan, 2000;Greenberg et al, 2000;Hellgren and Forssell, 2001;Hellgren, 2002;Chi et al, 2003;Spriet et al, 2005;Boukis et al, 2006;Freed and Soli, 2006͒. The use of an adaptive linear filter to model hearing aid feedback is based on the assumption that the feedback path is linear. In the context of digital feedback cancellation, the "feedback path" refers to the entire chain of elements from the output of the digital processing system back to its input, consisting of the digital-to-analog ͑D/A͒ converter, the hearing aid receiver ͑and any associated amplifier͒, the acoustical and/or mechanical feedback path from the receiver to the microphone, the microphone ͑and any associated preamplifier͒, and the analog-to-digital ͑A/D͒ converter.…”