The effect of formant trajectories and phoneme durations on vowel intelligibility

We examined how much listeners can benefit from listening to ldquoclearrdquo (CLR) speech compared to ldquoconversationalrdquo (CNV) speech, both spoken at different speaking rates. Vowel intelligibilities of four front vowels (/i:/, /I/, /E/, and /ei/) in background noise were measured with four speaking styles (CNV/SLOW, CNV, CLR, and CLR/FAST). Results showed only tense vowels of CLR speech had a significant difference between CNV and CLR speaking styles, after energy and F0 contour were normalized. We synthesized hybrid (HYB) speech whose formant features were equal to those of CLR speech, while all other features were taken from CNV speech. Primary conclusions from this study are (1) naturally-spoken fast CLR speech was not as intelligible as CLR speech, (2) enhancing formant frequencies to resemble those of CLR speech was effective at improving vowel intelligibility, and (3) spectral tilt and formant bandwidths were not contributing factors to the CLR speech benefit.