Acoustic and perceptual correlates of the non-nasal–nasal distinction for vowels

Abstract

For each of 5 vowels [i, e, a, o and u] following .**GRAPHIC**. a continuum from non-nasal to nasal were synthesized. Nasalization was introduced by inserting a pole-zero pair in the vicinity of the first formant in all all-pole transfer function. The frequencies and spacing of the pole and zero were systematically varied to change the degree of nasalization. The selection of stimulus parameters was determined from acoustic theory and the results of pilot experiments. The stimuli were presented for identification and discrimination to listeners whose language included a non-nasal-nasal vowel opposition (Gujarati, Hindi, and Bengali) and to American listeners. There were no significant differences between language groups in the 50% crossover points to the identification functions. Some vowels were more influenced by range and context effects than were others. The language groups showed some differences in the shape of the discrimination functions for some vowels. There is apparently a basic acoustic property of nasality, independent of the vowel, to which the auditory system responds in a distinctive way regardles of language background. There may be one or more additional acoustic properties that may be used to various degrees in different languages to enhance the contrast between a nasal vowel and its non-nasal congener. A proposed candidate for the basic acoustic property is a measure of the degree of prominence of the spectral peak in the vicinity of the first formant. Additional secondary properties include shifts in the center of gravity of the low-frequency spectral prominence, leading to a change in perceived vowel height, and changes in overall spectral balance.