Use of formant centralization ratio for vowel impairment detection in normal hearing and different degrees of hearing impairment.

Hearing-impaired (HI) speakers show changes in vowel production and formant frequencies, as well as more cases of overlapping between vowels and more restricted formant space, than hearing speakers. This study was intended to explore whether the use of different acoustic parameters (Formant Centralization Ratio (FCR), Vowel Space Area (VSA), F2i/F2u ratio (second formant of/i,u/)) was suitable or not for characterizing impairments in the articulation of vowels in the speech of HL speakers. In fact, correlated acoustic parameters are used to determine the limits of tongue movements in vowel production in different severity degrees of hearing impairment. Speech recordings of 40 speakers with HL and 40 healthy controls were acoustically analyzed. The vowels (/a/,/i/,/u/) were extracted from the word context and, then, the first and second formants were calculated. The same vowel-formant elements were used to construct the FCR, expressed as (F2u + F2a + F1i + F1u)/(F2i + F1a), the F2i/F2u ratio, and the vowel space area (VSA), expressed as ABS((F1i*(F2a-F2u)+F1a*(F2u-F2i)+F1u*(F2i-F2a))/2). The FCR differentiated HL groups from the control group and the discrimination was not gender-sensitive. All parameters were found to be strongly correlated with each other. The findings of this study showed that FCR was a more sensitive acoustic parameter than F2i/F2u ratio and VSA to distinguish speech of the HL groups from that of the normal group. Thus, FCR is considered to be applicable as an early objective measure of impaired vowel articulation in HL speakers.