Abstract
The properties of a prototype DFS (digital feedback suppression) system have been investigated. 21 ears fitted with behind-the-ear (BTE) hearing instruments and hard acrylic earmoulds and 4 ears fitted with vented in-the-ear (ITE) hearing instruments were selected for the investigation. Two ITE instruments with different venting were employed to one of the ears. Complex loop gain has been measured in an anechoic room, and from these measurements the improvements in acoustic feedback margin due to the DFS system have been determined. For the BTE group, median values of 13.1 and 10.0 dB of improvement were established for two sets of measurements introducting a 180° phase shift in connection with the last set of measurements. For the ITE group, values from 9.8 to 16.1 dB and from 13.7 to 16.3 dB of improvement were observed for the normal and the 180° phase shift conditions respectively. Beyond this the DFS system may improve the sound quality to some extent, because the amplitude distortion, caused by the external feedback signal, is almost completely eliminated.