Acoustics of open fittings

Abstract

The use of so-called "open fittings" instead of individual ear shells has become very popular in hearing aid fitting, in particular because the open fittings avoid the occlusion effect, but also because they are more comfortable, easier to manufacture, and cosmetically preferred. On the other hand however, there are acoustical issues with open fittings, including the mixture of direct and amplified sound, a poor low-frequency performance, an increased risk of feedback and a supposedly less reproducible position of the sound delivering device (tubing or speaker) in the ear canal, which in turn may result in a greater variability of acoustic parameters such as RECD and REOG. The two latter issues are addressed here in a study with 20 subjects, for individual shell and a number of open fittings, comprising closed and open domes of different diameters with tubings as well as with ear canal receivers. It was observed that in comparison to individual shell fittings, the open fittings did not exhibit higher interindividual differences of RECD and REOG, but up to 15 dB (closed domes) to 25 dB (open domes) lower feedback thresholds.