Coupler-related real ear gain. Interaction between subject and hearing aid type

In order to select hearing aids with a highly predictable real ear gain, in situ gain and compliance through the individual ear mould were measured on 8 normal-hearing subjects. The latter were chosen in order to reduce gain variation caused by various pathological conditions of the ear. All behind-the-ear aids commonly used in Sweden and one in-the-ear aid were studied. One sample of each hearing aid type was tested and the subjects had identical ear moulds. Hence, the variables were hearing aid type and subject. The study showed wide intra-subject coupler-related real ear gain variation (30 dB). Most of the aids showed a distinct correlation between coupler-related overall gain, and mould compliance. Some of the aids gave more uniform real ear gain, i.e. low interaction between subject and frequency-dependent gain. These results are elucidated by measuring the aid's acoustic output impedance.