In an adaptive listening test, the bandwidth of speech in complementary notched noise was varied. The bandwidth (center frequency 1 kHz) required for 50% speech intelligibility is called Speech Reception Bandwidth Threshold (SRBT). The SRBT was measured for 10 normal-hearing and 30 hearing-impaired listeners. The average SRBT of the normal-hearing listeners is 1.4 octave. The performance of seven hearing-impaired listeners is considered normal, whereas 23 hearing-impaired listeners have a wider-than-normal SRBT. The SRBT of a hearing-impaired listener may be wider than normal, due to inaudibility of a part of the speech band, or to an impairment in the processing of speech. The Speech Intelligibility Index (SII) is used to separate these effects. The SII may be regarded as the proportion of the total speech information that is available to the listener. Each individual SRBT is converted to an SII value. For the normal-hearing listeners, the SII is about 0.3. For 21 hearing-impaired listeners, the SII is higher. This points to a speech-processing impairment in the 1-kHz frequency region. The deviation of an individual SII value from 0.3 can be used to “quantify” the degree of processing impairment.
