Intensity-importance functions for bandlimited monosyllabic words.

A study was carried out to determine the relative importance to speech intelligibility of different intensities within the speech dynamic range. The functions that were derived are analogous to previous descriptions of the relative importance of different frequencies and are referred to here as intensity-importance functions (IIFs). They were obtained as follows. Sharply filtered bands of speech (NU6 monosyllabic words) were mixed with filtered noise and presented alone or in pairs at 19 signal-to-noise ratios (-25 to 41 dB). When paired bands were tested, the level and signal-to-noise ratio (SNR) of one band were held constant while the level and SNR of the other band were varied. The listeners were 100 normal hearers, organized into five 20-person groups. Each group provided speech recognition data for one of five frequency regions (141-562, 562-1122, 1122-1778, 1778-2818, and 2818-8913 Hz). Comparisons of the results for each group indicated that IIFs vary with frequency and SNR. Current methods for predicting intelligibility from physical measurements of speech audibility would need to be revised in order to take such findings into consideration.