The influence of spectral prominence on perceived vowel quality

Abstract

Research indicates that, when the first and second formants of a vowel are separated by less than about 3.5 Bark, perception of its height and some other aspects of its quality is determined by some weighted average of the low-frequency spectrum, rather than by particular harmonic or hypothetical formant frequencies (as is the case with more widely spaced formants). This spectral averaging has been called the center of gravity (COG) effect. Although the existence of the effect is generally accepted, the factors that govern it are poorly understood. One possibility is that the influence of the spectral envelope on perceived vowel quality increases as low-frequency spectral prominences become less well defined. A series of three experiments examined this possibility in: (1) nasal vowels, where the lowest spectral prominence is broader and flatter than that of oral vowels; (2) one- versus two-formant vowels with bandwidths appropriate for oral vowels; and (3) two-formant vowels with very narrow or very wide bandwidths. The results of these experiments show that, when two or more spectral peaks lie within 3.5 Bark of one another, F1 and the centroid (an amplitude-weighted average frequency that estimates the COG in the low-frequency spectrum) roughly determine the boundaries within which the perceptual COG lies; the frequencies of spectral peaks dominate responses when formant bandwidths are narrow, whereas overall spectral shape exerts more influence when spectral prominences are wide. Assuming that all vowels undergo the same processing, it is suggested that vowel quality, particularly height, is determined both by the frequency of the most prominent harmonics in the low-frequency region and by the slopes of the skirts in the vicinity of these harmonics. These two effects are most clearly separable in vowels with poorly defined spectral prominences whose shape cannot be adequately described by specifying the frequencies and degree of prominence of just one or two harmonics, or hypothetical formant peaks.

Minimal spectral contrast of formant peaks for vowel recognition as a function of spectral slope

“…Excitation patterns, computed in the way described in this paper, have considerable power to explain phenomena in speech perception (e.g., Assmann & Summerfield, 1989;Beddor & Hawkins, 1990) and psychoacoustics (e.g., Moore & Glasberg, 1986). However, it does not follow that they are directly equivalent to representations of signals generated within the human auditory system.…”

Spectral Integration of Dynamic Cues in the Perception of Syllable-Initial Stops

“…However, especially in matching tasks, great variability in the responses posed a problem for interpretation of the results (see Rosner and Pickering, [1994], for a detailed review and criticism of formant integration experiments). Although some of the shortcomings have been addressed subsequently [Beddor and Hawkins, 1990;Assmann, 1991;Hoemeke and Diehl, 1994;Fahey et al, 1996], the skepticism remained, leading to a lack of interest in further exploration of the effect. However, Rosner and Pickering's [1994, p. 157] conclusion that 'the determination of an indicator corresponding to F2′ seems unnecessary for vowel identification' is not surprising, given that the role of such an indicator has not been clearly specified or else has been overestimated in the past.…”

Nasalization of Vowels in Nasal Environments in Babbling: Evidence for Frame Dominance

“…In addition Hawkins and Stevens [1985] report that in languages that have fewer nasal than oral vowels, it is most commonly the mid vowels that are missing which is also consistent with the fact that nasal resonance effects tend to be centered in the midrange of oral F 1 values, thereby minimizing the distinction between nasal and non-nasal mid vowels. Beddor and Hawkins [1990] have also addressed the question of the bases for vowel height perception in nasalized vowels. They concluded that 'the frequency of the peak labeled FN, the nasal formant, was higher than the peak labeled F 1 in the high and mid vowels, but less than F 1 in the low vowels.…”