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Abstract
The main advantages of wearing 2 ear level hearing aids are the possibilities of directional hearing and of avoiding the acoustical head shadow. These effects are rather easy to measure. There exists another effect which would seem, at first sight, to be of major importance: The fact that in an environment of hampering sounds one understands better with 2 ears than with only 1: the cocktail party effect (CPE).
As in already described [Kuyper and de Boer, 1969] there are many ways to measure and even to define the CPE. The most natural way (and the best if one investigates hearing aids) is to use loudspeakers. Carhart [1965] describes a method which avoids the effect of head shadow. Unfortunately it was impossible for us to obtain in double prothetized subjects an effect which was greater than the variance.
In order to avoid this variance we improved a method developed by Feldmann in 1963, and in this way succeeded in obtaining significant results for each single subject.
Applying this method on prothetized subjects, forced us to use the telephone coil instead of the microphone of the post-aural hearing-aids.