Frequency Importance Functions for Words, Sentences, and Continuous Discourse

This research determined frequency importance functions (FIFs) for words, sentences, and continuous discourse under comparable conditions so that contextual effects of speech could be isolated. A male talker recorded 616 monosyllabic words, 176 meaningful sentences, and 44 continuous discourse (CD) passages. Twenty-four participants with normal hearing made intelligibility estimates of the CD passages and sentences and identified words in each of 44 low- and high-pass filtering and signal-to-noise ratio conditions. Plots of frequency versus percent of contributed intelligibility, or the FIFs, revealed that the frequency band that contributes the most to intelligibility is centered near 2000 Hz for all three types of speech. The results show a single peak in the importance function and a statistical analysis of the shape of the FIF (with kurtosis the pertinent measure) shows that there is a significant difference in the shape of the FIF based upon speech type. The data were also calculated into near-octave bands with similar results. The statistical analysis presented provides the basis for a test of the hypothesis: The degree of context or message redundancy is related to the relative importance of the frequency bands. The findings potentially have clinical as well as predictive implications.