Macrae J H
National Acoustic Laboratories, Chatswood, NSW, Australia.
Br J Audiol. 1991 Jun;25(3):183-99. doi: 10.3109/03005369109079852.
An investigation was carried out into effects of three types of impression build-up (patting down of impressions, special earmould-maker build-up and the multistage impression technique) on the dimensions, static pressure seal, degree of acoustic seal and the subjective tightness and comfort of earmoulds. Patting down the impression significantly improved the degree of acoustic seal provided by earmoulds without making them feel tighter or less comfortable. However, special build-up was much more effective than patting down and the multistage impression technique was slightly more effective than special build-up in improving the degree of acoustic seal. The improvement in acoustic seal provided by both multistage and specially built-up earmoulds can usually be obtained without an unacceptable level of discomfort. Patting down the impression did not improve the chance of obtaining a static pressure seal. Special build-up of the impression by the earmould-maker significantly increased the proportion of earmoulds which provided a static pressure seal but an even higher proportion of earmoulds made from multistage impressions provided a seal. Dimension results indicated that an increase in earmould-maker build-up of the minor axis at the beginning of the canal segment of the impression would improve the acoustic seal provided by specially built-up earmoulds and that earmoulds with rounder tips are more likely to provide a static pressure seal than earmoulds with more elliptical tips. The better the impression material fills the ear canal, the rounder the tip of the impression, and the rounder the tip of the earmould made from the impression.
