Acoustic feedback in the hearing aid system
This study concentrated on the problem of instability in a hearing aid system due to acoustic feedback. A mathematical model of feedback in the hearing aid system was developed. It was based on that of Egolf (1989. J. Acoust. Soc. Am. 85:1 pp 454-467), but included numerous feedback paths replacing the single path used by Egolf. A computer based analysis tool was designed and built that could measure feedback in the in-situ hearing aid. The feedback measurement system used two different methods to deliver the signal to the hearing aid: direct (or wired) and telecoil. The two methods were tested on partially hearing children at a routine hearing aid assessment clinic. The results demonstrated that where the in-situ hearing aid had a higher tendency to feedback there was a corresponding increase in the level of feedback signal measured. There was also a decrease in the feedback ratio with increasing age of the subject. The effect of changes to the hearing aid system were studied using both the feedback measurement system and the mathematical model. Several parameters were found to have a significant effect on the feedback levels. Some however where found to have little or no effect. The phase response was similarly altered differently by different parameters. The mathematical model and system were thus tested and validated. The system was unique because it collected phase information as well as the feedback ratio, it could also include the subject's own hearing aid in the analysis. The model was unique because it looked at the numerous sources of feedback. Both were able to evaluate the acoustic properties of earmoulds and tubing.