Digital feedback suppression (DFS). Clinical experiences when fitting a DFS hearing instrument on children

A new power behind-the-ear hearing instrument with digital feedback suppression (DFS) seems to be an important step towards solving the problems of acoustic feedback in high power instrument fittings. Previous experiences (Dyrlund & Bisgaard, 1991) with a DFS prototype are confirmed in the present work. Ten profoundly hearing-impaired children were fitted with the new DFS instrument and wore it over a trial period of approximately three weeks. with the new instruments the rationale was to supply equivalent low frequency (< 1 kHz) gain compared to the subjects' original hearing instruments, and extra 5-10 dB high frequency (> 1 kHz) gain. The hypothesis was that with the new DFS system it would be possible to provide extra high frequency gain for these children without the annoyance of acoustic feedback. The test included questionnaires before and after the trial period, comparative free-field audiometry between the subjects' original and DFS instruments. The free-field audiometric results show a typical improvement of 5-10 dB at 2000 Hz and even larger improvements at higher frequencies. The increased high frequency gain provided by the new instruments did not give rise to significant complaints of feedback or howling during the trial period, and was generally preferred by 9 of the 10 subjects after the trial period.