Abstract
An investigation was carried out into effects of three types of impression build-up (patting down of impressions, special ear mould-maker build-up and the multistage impression technique) on the dimensions, static pressure seal, degree of acoustic seal and subjective tightness and comfort of ear moulds. Patting down the impression significantly improved the degree of acoustic seal provided by ear moulds 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 especially built-up ear moulds 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 ear mould-maker significantly increased the proportion of ear moulds which provided a static pressure seal but an even higher proportion of ear moulds made from multistage impressions provided a seal. Dimension results indicated that an increase in ear mould-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 ear moulds and that ear moulds with rounder tips are more likely to provide a static pressure seal than ear moulds 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 ear mould made from the impression.