Limitations on insertion gains with vented earmoulds imposed by oscillatory feedback

When an earmould is vented, either to relieve sensations of blockage or to modify the frequency response of a hearing aid system, the gain setting at which feedback occurs is lowered. Although this principle is widely appreciated, the typical gains at which feedback occurs have received little systematic study and vented earmoulds are still prescribed on largely rule-of-thumb, or trial and error basis. In a laboratory study on a KEMAR mannikin the effects for hearing aids with gains up to 64 dB were studied. The acoustical stimuli were varied (using speech-shaped noise, wide-band noise and narrow bands of noise), as were the size of the parallel vent (0.8 and 2.0 mm) and the orientation of the microphone on the aid (forward-facing and downward-facing). The results suggest that: a forward-facing microphone is less susceptible to oscillatory feedback than a downward-facing one; while a 2 mm vent leads to more feedback limitations than a 0.8 mm vent, the smaller vent is more than a 'pressure vent', having some acoustical effects; and the spectrum of the ambient acoustics can markedly reduce the available gain before the onset of oscillation, e.g. from greater than 40 dB to less than 20 dB gain.