Combined effects of earmold vents and suboscillatory feedback on hearing aid frequency response.

The effect of setting the gain control of a hearing aid to a position just lower than that required to produce audible oscillation was investigated in the condition where the main feedback pathway was an earmold vent of either diagonal (side-branch) or parallel configuration. Results indicated that, with this gain control setting, suboscillatory feedback effects were superimposed on the filtering effect of the earmold vent. The outcome was the formation of spurious peaks in the frequency response of the hearing aid. Most of the data were collected using a KEMAR manikin and the effects were confirmed by measurements made in real ear canals. It is recommended that this gain control setting be avoided in hearing aid fitting.