Recognition of telegraphy in hearing-impaired telegraph operators

Recognition of Morse signs masked by noise was measured in five hearing-impaired telegraph operators with a high-frequency hearing loss. They all described problems typical for subjects with high-frequency hearing loss, i.e. problems with recognition of speech in noisy environments. Telegraphy speed, listening level, bandwidth of the masking-noise, frequency and interaural phase were systematically varied. At narrow-band masking with a bandwidth of 230 Hz (1/3 octave 891-1121 Hz) centred at 1000 Hz no statistical significant difference in recognition of telegraphy signs was found between the hearing-impaired subjects and the normal-hearing subjects investigated in our earlier studies at any telegraphy speed or at any S/N ratio. All hearing-impaired subjects improved in recognition when the telegraphy speed was lowered. When a broad-band noise (100-2500 Hz) was used as a masker all the hearing-impaired subjects improved in recognition when the frequency was lowered from 2000 to 500 Hz. At 40 signs/min telegraphy speed the hearing-impaired subjects showed a significantly lower recognition (p < 0.05) at low S/N ratios both at 2000 and 500 Hz frequency compared to the normal-hearing subjects investigated in our earlier studies. At monaural listening the normal-hearing subjects were significantly better (p < 0.01) in recognition. When the telegraphy signs were presented 180 degrees out of phase between the two ears, all subjects improved in recognition compared to when the telegraphy signs were presented in phase. This improvement was equal in hearing-impaired subjects and in the normal-hearing subjects investigated in our earlier studies.