The examinations of the CROS (Contralateral Routing of Signals) type hearing aid were performed under a number of listening conditions, both advantageous and disadvantageous, in order to determine whether the relative merits of the instrument could be demonstrated in the laboratory. The listening conditions were designed by various combinations of the following three parameters: i) the origination of test sound to the bad ear. ii) the presentation of a background with voice babble. iii) the use of hearing aids. The acuity of the sound localization was measured by the speaker system.
The subjects for the investigation were 38 listeners with unilateral hearing impairments, which were classified into two groups by their hearing acuities of the better ear.
1) The unaided scores in case of the indirect loudspeaker were reduced by 6% in quiet and by about 20% in voice babble, due to the shadow effect of the head.
2) Under aided conditions, this shadow effect was covered by the hearing aid, but the discrimination score was poorer than under unaided condition in case of the direct loudspeaker.
3) The aided discrimination scores to the speech delivered from the side of the poorer ear were about 5% better in quiet, and about 10% in voice babble than unaided scores. These advantages were mostly found in the group, which had the mild deafness in the better ear.
4) The localization of the sound source under aided condition was deviated to the side of the better ear, as same under unaided, however, a few improvement was shown by means of the difference of tonal timbre.
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