Accuracy and variability of acoustic measures of voicing onset

Five commonly used methods for determining the onset of voicing of syllable-initial stop consonants were compared. The speech and glottal activity of 16 native speakers of Cantonese with normal voice quality were investigated during the production of consonant vowel (CV) syllables in Cantonese. Syllables consisted of the initial consonants /ph/, /th/, /kh/, /p/, /t/, and /k/ followed by the vowel /a/. All syllables had a high level tone, and were all real words in Cantonese. Measurements of voicing onset were made based on the onset of periodicity in the acoustic waveform, and on spectrographic measures of the onset of a voicing bar (f0), the onset of the first formant (F1), second formant (F2), and third formant (F3). These measurements were then compared against the onset of glottal opening as determined by electroglottography. Both accuracy and variability of each measure were calculated. Results suggest that the presence of aspiration in a syllable decreased the accuracy and increased the variability of spectrogram-based measurements, but did not strongly affect measurements made from the acoustic waveform. Overall, the acoustic waveform provided the most accurate estimate of voicing onset; measurements made from the amplitude waveform were also the least variable of the five measures. These results can be explained as a consequence of differences in spectral tilt of the voicing source in breathy versus modal phonation.     

On the perception of voicing in syllable-initial plosives in noise

Previous studies [Lisker, J. Acoust. Soc. Am. 57, 1547-1551 (1975); Summerfield and Haggard, J. Acoust. Soc. Am. 62, 435-448 (1977)] have shown that voice onset time (VOT) and the onset frequency of the first formant are important perceptual cues of voicing in syllable-initial plosives. Most prior work, however, has focused on speech perception in quiet environments. The present study seeks to determine which cues are important for the perception of voicing in syllable-initial plosives in the presence of noise. Perceptual experiments were conducted using stimuli consisting of naturally spoken consonant-vowel syllables by four talkers in various levels of additive white Gaussian noise. Plosives sharing the same place of articulation and vowel context (e.g., /pa,ba/) were presented to subjects in two alternate forced choice identification tasks, and a threshold signal-to-noise-ratio (SNR) value (corresponding to the 79% correct classification score) was estimated for each voiced/voiceless pair. The threshold SNR values were then correlated with several acoustic measurements of the speech tokens. Results indicate that the onset frequency of the first formant is critical in perceiving voicing in syllable-initial plosives in additive white Gaussian noise, while the VOT duration is not.     

Perceptual cues to the onset of voiced excitation in aspirated initial stops

Previous experiments on the perception of initial stops differing in voice-onset time have used sounds where the boundary between aspiration and voicing is clearly marked by a variety of acoustic events. In natural speech these events do not always occur at the same moment and there is disagreement among phoneticians as to which mark the onset of voicing. The three experiments described here examine how the phoneme boundary between syllable-initial, prestress /b/ and /p/ is influenced by the way in which voicing starts. In the first experiment the first 30 ms of buzz excitation is played at four different levels relative to the steady state of the vowel and with two different frequency distributions: In the F1-only conditions buzz is confined to the first formant, whereas in the F123 conditions all three formants are excited by buzz. The results reject the hypothesis that voicing is perceived to start when periodic excitation is present in the first formant. The results of the third experiment show also that buzz excitation confined to the fundamental frequency for 30 ms before the onset of full voicing (formant excitation) has little effect on the voicing boundary. The second experiment varies aspiration noise intensity and buzz onset intensity independently. Together with the first experiment it shows that: (1) at all buzz onset levels a change in aspiration intensity moves the boundary by about the 0.43 ms/dB found by Repp [Lang. Speech 27, 173-189 (1979)]; (2) when buzz onsets at levels greater than - 15 dB relative to the final vowel level, changes in buzz onset level again move the /b/-/p/ boundary by the same amount; (3) when buzz onsets at levels less than - 15 dB relative to the vowel, decreasing the buzz onset level gives more /p/- percepts than Repp's ratio predicts. This last result, taken with the results of the first experiment, may reflect a decision based on overall intensity about when voicing has started.     

The use of acoustic cues for phonetic identification: effects of spectral degradation and electric hearing

Although some cochlear implant (CI) listeners can show good word recognition accuracy, it is not clear how they perceive and use the various acoustic cues that contribute to phonetic perceptions. In this study, the use of acoustic cues was assessed for normal-hearing (NH) listeners in optimal and spectrally degraded conditions, and also for CI listeners. Two experiments tested the tense/lax vowel contrast (varying in formant structure, vowel-inherent spectral change, and vowel duration) and the word-final fricative voicing contrast (varying in F1 transition, vowel duration, consonant duration, and consonant voicing). Identification results were modeled using mixed-effects logistic regression. These experiments suggested that under spectrally-degraded conditions, NH listeners decrease their use of formant cues and increase their use of durational cues. Compared to NH listeners, CI listeners showed decreased use of spectral cues like formant structure and formant change and consonant voicing, and showed greater use of durational cues (especially for the fricative contrast). The results suggest that although NH and CI listeners may show similar accuracy on basic tests of word, phoneme or feature recognition, they may be using different perceptual strategies in the process.     

Throaty Voice Quality: Subglottal Pressure, Voice Source, and Formant Characteristics

"Throaty" voice quality has been regarded by voice pedagogues as undesired and even harmful. This study attempts to identify acoustic and physiological correlates of this quality. One male and one female subject read a text habitually and with a throaty voice quality. Oral pressure during p-occlusion was measured as an estimate of subglottal pressure. Long-term average spectrum analysis described the average spectrum characteristics. Sixteen syllables, perceptually evaluated with regard to throaty quality by five experts, were selected for analysis. Formant frequencies and voice source characteristics were measured by means of inverse filtering, and the vocal tract shape of the throaty and normal versions of the vowels [a,u,i,ae] of the male subject were recorded by magnetic resonance imaging. From this material, area functions were derived and their resonance frequencies were determined. The throaty versions of these four vowels all showed a pharynx that was narrower than in the habitually produced versions. To test the relevance of formant frequencies to perceived throaty quality, experts rated degree of throatiness in synthetic vowel samples, in which the measured formant frequency values of the subject were used. The main acoustic correlates of throatiness seemed to be an increase of F1, a decrease of F4, and in front vowels a decrease of F2, which presumably results from a narrowing of the pharynx. In the male subject, voice source parameters suggested a more hyperfunctional voice in throaty samples.     

Average discharge rate representation of voice onset time in the chinchilla auditory nerve

Responses of chinchilla auditory-nerve fibers to synthesized stop consonants differing in voice onset time (VOT) were obtained. The syllables, heard as /ga/-/ka/ or /da/-/ta/, were similar to those previously used by others in psychophysical experiments with human and with chinchilla subjects. Average discharge rates of neurons tuned to the frequency region near the first formant generally increased at the onset of voicing, for VOTs longer than 20 ms. These rate increases were closely related to spectral amplitude changes associated with the onset of voicing and with the activation of the first formant; as a result, they provided accurate information about VOT. Neurons tuned to frequency regions near the second and third formants did not encode VOT in their average discharge rates. Modulations in the average rates of these neurons reflected spectral variations that were independent of VOT. The results are compared to other measurements of the peripheral encoding of speech sounds and to psychophysical observations suggesting that syllables with large variations in VOT are heard as belonging to one of only two phonemic categories.     

Some difference limens for the perception of breathiness

Perception of breathy voice quality appears to be cued by changes in the vowel spectrum. These changes are related to alterations in the intensity of aspiration noise and spectral slope of the harmonic energy [Shrivastav and Sapienza, J. Acoust. Soc. Am., 114 (4), 2217-2224 (2003)]. Ten young-adult listeners with normal hearing were tested using an adaptive listening task to determine the smallest change in signal-to-noise ratio that resulted in a change in breathiness. Six vowel continua, three female and three male, were generated using a Klatt synthesizer and served as stimuli. Results showed that listeners needed as much as 20-dB increase in aspiration noise to perceive a change in breathiness against a relatively normal voice. In contrast, listeners needed approximately an 11-dB increase in aspiration noise to discriminate breathiness against a severely breathy voice. The difference limens for breathiness were observed to vary across the six talkers. Voices having aspiration noise that was predominantly in the high frequencies had smaller difference limens. No significant differences for male and female voice were observed.     

Speech processing studies using an acoustic model of a multiple-channel cochlear implant

The speech perception of two multiple-channel cochlear implant patients was compared with that of three normally hearing listeners using an acoustic model of the implant for 22 different speech tests. The tests used included a minimal auditory capabilities battery, both closed-set and open-set word and sentence tests, speech tracking and a 12-consonant confusion study using nonsense syllables. The acoustic model represented electrical current pulses by bursts of noise and the effects of different electrodes were represented by using bandpass filters with different center frequencies. All subjects used a speech processor that coded the fundamental voicing frequency of speech as a pulse rate and the second formant frequency of speech as the electrode position in the cochlea, or the center frequency of the bandpass filter. Very good agreement was found for the two groups of subjects, indicating that the acoustic model is a useful tool for the development and evaluation of alternative cochlear implant speech processing strategies.     

A temporal analysis of auditory-nerve fiber responses to spoken stop consonant-vowel syllables

Auditory-nerve fiber spike trains were recorded in response to spoken English stop consonant-vowel syllables, both voiced (/b,d,g/) and unvoiced (/p,t,k/), in the initial position of syllables with the vowels /i,a,u/. Temporal properties of the neural responses and stimulus spectra are displayed in a spectrographic format. The responses were categorized in terms of the fibers' characteristic frequencies (CF) and spontaneous rates (SR). High-CF, high-SR fibers generally synchronize to formants throughout the syllables. High-CF, low/medium-SR fibers may also synchronize to formants; however, during the voicing, there may be sufficient low-frequency energy present to suppress a fiber's synchronized response to a formant near its CF. Low-CF fibers, from both SR groups, synchronize to energy associated with voicing. Several proposed acoustic correlates to perceptual features of stop consonant-vowel syllables, including the initial spectrum, formant transitions, and voice-onset time, are represented in the temporal properties of auditory-nerve fiber responses. Nonlinear suppression affects the temporal features of the responses, particularly those of low/medium-spontaneous-rate fibers.