Abstract
(1) It is well known that unilaterally hearing-impaired patients in noisy surroundings reveal particular communication disabilities, which can to a large extent be eliminated by utilizing a classical CROS arrangement, i.e. contralaterlal routing of signals from the poorer ear to the better. During the period 1972–74 this fact has been experienced empirically by 203 adult patients at the Department of Audiology, Södersjukhuset. (2) 30 patients have been further investigated in noisy surroundings as to their discrimination abilities, using P.B. lists and various azimuth positions under “anechoic” conditions in a specially arranged set-up. (3) It has been clinically proven that under the everyday acoustic conditions that have been used, i.e. S/N = + 5 dB and with the sound source at the poorer ear, the discrimination loss compared with the sound source at the better ear is ca 40 %, a loss which is almost eliminated when a classical CROS arrangement is employed. The reduction in listening ability in the unaided situation is constituted by the disappearance of the high frequency speech elements because of the head shadow, as well as the masking effect of the same elements by noise. Both these factors are compensated for in the CROS situation. It must be stressed that during the fitting procedure the CROS-aid has been adjusted for threshold assessment (not for the most comfortable level) with the primary aim of compensating for the head's shadow effect and with minimum masking interference.