THE SPEECH SPECTRUM AND ITS RELATIONSHIP TO INTELLIGIBILITY OF SPEECH (ARTICULATION INDEX, FORMANT REGIONS, FRENCH AND STEINBERG)

The present experiment was designed to investigate and understand the causes of failures of the Articulation Index as a predictive tool. An electroacoustic system was used in which: (1) The frequency response was optimally flattened at the listener's ear. (2) An ear-insert earphone was designed to give close electroacoustic control. (3) An infinite-impulse-response digital filter was used to filter the speech signal from a pre-recorded nonsense syllable test. (4) Four formant regions were filtered in fourteen different ways.; It was found that the results agreed with past experiments in that: (1) The Articulation Index fails as a predictive tool when using band-pass filters. (2) Low frequencies seem to mask higher frequencies causing a decrease in intelligibility.; It was concluded that: (1) It is inappropriate to relate the total fraction of the speech spectrum to a specific intelligibility score since the fraction remaining after filtering may be in the low-, mid-, or high-frequency range. (2) The relationship between intelligibility and the total area under the spectral curve is not monotonic. (3) The fourth formant region (2925Hz to 4200Hz) enhanced intelligibility when included with other formant regions. Methods for relating spectral regions and intelligibility were discussed.