Relative contributions of passband and filter skirts to the intelligibility of bandpass speech: Some effects of context and amplitude.

Warren et al. (1995) reported over 90% intelligibility for everyday sentences reduced to a 1/3-octave band (center frequency 1,500 Hz, slopes 100 dB/octave, slow-rms peak levels 75 dB). To investigate the basis of this high intelligibility, Warren and Bashford (1999) partitioned the sentences. Surprisingly, the rectangular 1/3-octave passband had only 24% intelligibility, whereas the filter skirts separated by a 1/3-octave notch had an intelligibility of 83%, despite their severe spectral tilts. Experiment 1 of the present study substituted monosyllabic words for sentences. Wholeband intelligibility was 26%, the passband 4%, and the filter skirts 16%. Experiment 2 measured intelligibility for 1/3-octave sentences having peak levels ranging from 85 down to 35 dB. Whole band intelligibility ranged from 90% to 68%, and the filter skirt pairs had from two to four times the passband's intelligibility (which did not vary significantly with level). Hence, steep (100 dB/octave) filter skirts make the dominant contribution to intelligibility of nominally 1/3-octave speech across a wide range of presentation levels.