Terrance M. Nearey

Modeling the role of inherent spectral change in vowel identification

Statistical analysis of F1 and F2 measurements from nucleus and offglide sections of isolated Canadian English vowels shows significant formant frequency change not only for the ‘‘phonetic diphthongs’’ /e/ and /o/, but also for the ‘‘monophthongs’’ /ι/, /q/, and /1/. In a perceptual experiment, brief sections were extracted from ‘‘nucleus’’ and ‘‘offglide’’ portions of naturally produced vowels. Two sections from each vowel were presented to listeners in each of three conditions: (1) natural order (nucleus followed by offglide); (2) repeated nucleus (nucleus followed by itself); and (3) reverse (offglide followed by nucleus). Listeners’ error rates for the natural order condition were comparable to those for unmodified full vowels (averaging 14% and 13%, respectively). Significantly higher error rates were found for the repeated nucleus (32%) and reverse (38%) conditions. Observed confusion matrices were strongly correlated with predictions from a pattern recognition model incorporating the formant measur...

Effects of consonant environment on vowel formant patterns

A significant body of evidence has accumulated indicating that vowel identification is influenced by spectral change patterns. For example, a large-scale study of vowel formant patterns showed substantial improvements in category separability when a pattern classifier was trained on multiple samples of the formant pattern rather than a single sample at steady state [J. Hillenbrand et al., J. Acoust. Soc. Am. 97, 3099-3111 (1995)]. However, in the earlier study all utterances were recorded in a constant /hVd/ environment. The main purpose of the present study was to determine whether a close relationship between vowel identity and spectral change patterns is maintained when the consonant environment is allowed to vary. Recordings were made of six men and six women producing eight vowels (see text) in isolation and in CVC syllables. The CVC utterances consisted of all combinations of seven initial consonants (/h,b,d,g,p,t,k/) and six final consonants (/b,d,g,p,t,k/). Formant frequencies for F1-F3 were measured every 5 ms during the vowel using an interactive editing tool. Results showed highly significant effects of phonetic environment. As with an earlier study of this type, particularly large shifts in formant patterns were seen for rounded vowels in alveolar environments [K. Stevens and A. House, J. Speech Hear. Res. 6, 111-128 (1963)]. Despite these context effects, substantial improvements in category separability were observed when a pattern classifier incorporated spectral change information. Modeling work showed that many aspects of listener behavior could be accounted for by a fairly simple pattern classifier incorporating F0, duration, and two discrete samples of the formant pattern.

Speech perception as pattern recognition.

This work provides theoretical and empirical arguments in favor of an approach to phonetics that is called double-weak. It is so called because it assumes relatively weak constraints both on the articulatory gestures and on the auditory patterns that map phonological elements. This approach views speech production and perception as distinct but cooperative systems. Like the motor theory of speech perception, double-weak theory accepts that phonological units are modified by context in ways that are important to perception. It further agrees that many aspects of such context dependency have their origin in natural articulatory processes. However, double-weak theory sides with proponents of auditory theories of phonetics by accepting that the real-time objects of perception are well-defined auditory patterns. Because speakers find ways to obey “orderly output conditions” (Sussman et al., 1995), listeners are able to successfully decode speech using relatively simple pattern-recognition mechanisms. It is suggested that this situation has arisen through a stylization of gestural patterns to accommodate real-time limits of the perceptual system. Results from a new perceptual experiment, involving a four-dimensional stimulus continuum and a 10-category /hVC/ response set, are shown to be largely compatible with this framework.

Effects of Place of Articulation and Vowel Context on VOT Production and Perception for French and English Stops

The focus of this study is the nature of the secondary effects of place of articulation and vowel quality on VOT (and other acoustic properties) in the production of stops in French and English and the consequences of these factors on the perception of VOT continua by speakers of those languages. Although the largest source of differences in VOT is that associated with the voicing contrast, a number of previous studies indicate that both French and English stops show an effect of place of articulation, with velars showing longer VOT than labials or coronals in accord with the general tendencies noted by Lisker and Abramson (1967; see also Volaitis and Miller 1992). However, some modulation of VOT by vowel context has also been reported. Thus, Fischer-Jorgensen (1972) finds that, in French voiceless stops, VOT is longer before a high vowel than before a low vowel. She further reports an interaction between the place of articulation of the consonant and the identity of the vowel. Specifically, for /p/, the longest voicing lags occur before high rounded vowels (/u/ and /y/); but for /t/ and /k/, they are longest before /i/. Yeni-Komshian, Caramazza and Preston (1977: 43) also report longer voicing lags before /i/ than before /a/ or /o/ for unilingual French speakers.

Evaluation of a strategy for automatic formant tracking

Variations on an automatic formant tracking strategy developed at Alberta will be compared to manual formant measurements from two databases of vowels spoken by men, women, and children (in Texas or Michigan). ‘‘Correct’’ vowel formant candidates for F1, F2, and F3 may be found roughly 85–90 percent of the time for adult male speakers using autocorrelation LPC with the following settings: F3 maximum at 3000 Hz, LPC order of 14, sampling rate of 10 kHz [J. Markel and A. Gray, Linear Prediction of Speech (Springer, New York, 1975)]. Experience shows good results are also often found with females’ and children’s speech, provided the sampling rate and F3 maximum are scaled appropriately for each speaker. Our new basic strategy involves analyzing each utterance at several distinct sampling rates and coordinated F3 cutoff frequencies with a fixed LPC order. Each scaling choice provides an independent set of candidates that is post‐processed by a simple tracking algorithm. A correlation measure between a spectro...

Perception of front vowels: The role of harmonics in the first formant region

Vowel matching and identification experiments were carried out to investigate the perceptual contribution of harmonics in the first formant region of synthetic front vowels. In the first experiment, listeners selected the best phonetic match from an F1 continuum, for reference stimuli in which a band of two to five adjacent harmonics of equal intensity replaced the F1 peak; F1 values of best matches were near the frequency of the highest frequency harmonic in the band. Attenuation of the highest harmonic in the band resulted in lower F1 matches. Attenuation of the lowest harmonic had no significant effects, except in the case of a 2-harmonic band, where higher F1 matches were selected. A second experiment investigated the shifts in matched F1 resulting from an intensity increment to either one of a pair of harmonics in the F1 region. These shifts were relatively invariant over different harmonic frequencies and proportional to the fundamental frequency. A third experiment used a vowel identification task to determine phoneme boundaries on an F1 continuum. These boundaries were not substantially altered when the stimuli comprised only the two most prominent harmonics in the F1 region, or these plus either the higher or lower frequency subset of the remaining F1 harmonics. The results are consistent with an estimation procedure for the F1 peak which assigns greatest weight to the two most prominent harmonics in the first formant region.

Identification of frequency-shifted vowels

Within certain limits, speech intelligibility is preserved with upward or downward scaling of the spectral envelope. To study these limits and assess their interaction with fundamental frequency (F0), vowels in /hVd/ syllables were processed using the STRAIGHT vocoder and presented to listeners for identification. Identification accuracy showed a gradual decline when the spectral envelope was scaled up or down in vowels spoken by men, women, and children. Upward spectral envelope shifts led to poorer identification of childrens vowels compared to adults, while downward shifts had a greater impact on mens vowels compared to women and children. Coordinated shifts (F0 and spectral envelope shifted in the same direction) generally produced higher accuracy than conditions with F0 and spectral envelope shifted in opposite directions. Vowel identification was poorest in conditions with very high F0, consistent with suggestions from the literature that sparse sampling of the spectral envelope may be a factor in vowel identification. However, the gradual decline in accuracy as a function of both upward and downward spectral envelope shifts and the interaction between spectral envelope shifts and F0 suggests the additional operation of perceptual mechanisms sensitive to the statistical covariation of F0 and formant frequencies in natural speech.

Testing theories of vowel inherent spectral change.

Three competing accounts of vowel inherent spectral change in English all agree on the importance of initial formant frequencies; however, they disagree about the nature of the perceptually relevant aspects of formant change. The onset+offset hypothesis claims that the final formant values themselves matter. The onset+slope hypothesis claims that only the rate of change counts. The onset+direction hypothesis claims that only the general direction of change in formant frequencies is important. A synthetic-vowel perception experiment was designed to differentiate among the three. Results provide support for the superiority of the onset+offset hypothesis.

Vowel Inherent Spectral Change in the Vowels of North American English

Nearey and Assmann (1986) coined the term ‘vowel inherent spectral change’ (VISC) to refer to change in spectral properties inherent to the phonetic specification of vowels. Although such change includes the relatively large formant changes associated with acknowledged diphthongs, the term was explicitly intended to include reliable (but possibly more subtle) spectral change associated with vowel categories of North American English typically regarded as monophthongs. This chapter reviews statistical and graphical evidence of dynamic formant patterns in vowels of several CV and CVC syllable types in three regional dialects of English: Dallas, Texas (Assmann and Katz, 2000), Western Michigan (Hillenbrand et al., 1995) and Northern Alberta (Thomson 2007). Evidence is reviewed for the importance of VISC in vowel perception. While certain apparent VISC patterns show up across dialects, both dialect differences and differences in context make it clear that more sophisticated methods will be required to fully separate several factors affecting formant change in vowels. Promising preliminary results are presented using a new non-linear regression method that extends compositional models of Broad and Clermont (1987, 2002, 2010) to include dual vowel targets.

The direct and indirect roles of fundamental frequency in vowel perception.

Several experiments have found that changing the intrinsic f0 of a vowel can have an effect on perceived vowel quality. It has been suggested that these shifts may occur because f0 is involved in the specification of vowel quality in the same way as the formant frequencies. Another possibility is that f0 affects vowel quality indirectly, by changing a listeners assumptions about characteristics of a speaker who is likely to have uttered the vowel. In the experiment outlined here, participants were asked to listen to vowels differing in terms of f0 and their formant frequencies and report vowel quality and the apparent speakers gender and size on a trial-by-trial basis. The results presented here suggest that f0 affects vowel quality mainly indirectly via its effects on the apparent-speaker characteristics; however, f0 may also have some residual direct effects on vowel quality. Furthermore, the formant frequencies were also found to have significant indirect effects on vowel quality by way of their strong influence on the apparent speaker.