Perceptual evaluations of spectral and temporal modifications of deaf speech.

The perceptual effect of modifying speech produced by deaf talkers was investigated to discover the changes necessary for disordered speech to be judged normal. Recordings of passages read by three deaf talkers were used as material. For the first two experiments, a three-syllable word was extracted from the deaf talkers' passages and from a similar passage recorded by a hearing talker. Each of the deaf speech samples was paired with the normal speech sample to generate various continua that differed in the spectral and temporal modifications applied to them. Within each continuum, the individual stimuli varied in the shape of the spectrum envelope and were produced by linear interpolation of LPC analysis parameters between the deaf and normal speech end points. Results suggest that correcting the temporal component of deaf speech alone is not enough to make it sound normal. Spectral corrections that approximate about 70% of normal appear to be necessary for the deaf speech samples to be judged normal. A third experiment made use of a 10-syllable segment of speech in which the relative contributions of spectral and temporal adjustments were investigated. The general conclusion of these three experiments is that spectral adjustments are more important to perceptual judgments of normality than temporal adjustments.

Phonetics of intervocalic consonant perception: retrospect and prospect.

This article reviews the production characteristics and perceptual cues of intervocalic consonants as a background for acoustic studies of consonant perception in fluent speech. Data show that in conversation intervocalic consonants occur much more commonly than consonants in initial or final position; all phonetic features are strongly represented. Production characteristics of intervocalic consonants are seen to depend on the tempo and rhythmic conditions of the syllables of which they are components. At a moderate tempo, consonants in syllable-final position combine with the onset consonant of the following syllable. This affects durational characteristics and may be explained by higher energy efficiency of CV units in production. Phonological phenomena are related to the shifts in syllable position and the temporal compensations of intervocalic consonant production. Studies of consonant perception in fluent context have dealt with tempo of utterance, position in word, and rhythmic pattern, as well as phonemic context. Major phenomena are effects of coarticulation, invariance in consonant perception, and cue interaction and masking. Much evidence suggests a dominance of the perceptual cues in the CV portion of VCCV and VCV sequences. We suggest that exploration of perception variables that affect consonants in fluent context would be expedited by reorienting experimental procedures to employ listener adjustment of stimuli, instead of the traditional phoneme identification and discrimination procedures with large sets of constant stimuli. Most of the relevant literature deals with stop consonants. Lateral, rhotic, and nasal consonants also deserve intensive study because of their very frequent occurrence. Theoretical issues of phoneme perceptual invariance and motor vs. auditory theory of perception are discussed in relation to proposed experiments which vary syllable tempo and stress pattern.

On enhancement of spectral contrast in speech for hearing-impaired listeners.

A digital processing method is described for altering spectral contrast (the difference in amplitude between spectral peaks and valleys) in natural utterances. Speech processed with programs implementing the contrast alteration procedure was presented to listeners with moderate to severe sensorineural hearing loss. The task was a three alternative (/b/,/d/, or /g/) stop consonant identification task for consonants at a fixed location in short nonsense utterances. Overall, tokens with enhanced contrast showed moderate gains in percentage correct stop consonant identification when compared to unaltered tokens. Conversely, reducing spectral contrast generally reduced percent correct stop consonant identification. Contrast alteration effects were inconsistent for utterances containing /d/. The observed contrast effects also interacted with token intelligibility.

Articulatory asymmetries.

Tongue contact patterns for /s/ and /l/ were investigated using dynamic palatography. Both spatial and temporal asymmetries were commonly found extending into the vocalic transitions for these consonants. Implications for the adequacy of tongue motion data taken in a single midsagittal plane are discussed, as well as for articulatory interpretation of speech signals and speaker recognition applications.

Perception of anticipatory coarticulation effects in vowel-stop consonant-bowel sequences.

Articulatory and acoustic studies of speech production have shown that the effects of anticipatory coarticulation may extend across several segments of an utterance. The present experiments show that such effects have perceptual significance. In two experiments, a talker produced consonant (C) and vowel (V) sequences in a sentence frame (e.g., "I say pookee") of the form "I say / C V1 C V2/" in which V1 was /u, ae/ and V2 was /i, a/. Each /i, a/ sentence pair was cross-spliced by exchanging the final syllable /C V2/ so that coarticulatory information prior to the crosspoint was inappropriate for te final vowel (V2) in crossed sentences. Recognition time (RT) for V2 in crossed and intact (as spoken) sentences was obtained from practiced listeners. In both experiments RT was slower in crossed sentences; crossed sentences also attracted more false alarms. The pattern of perceptual results was mirrored in the pattern of precross acoustic differences in experimental sentences (e.g., formants F2 and F3 were higher preceding /i/ than preceding /a/). Pretarget variation in the formants jointly predicted amount of RT interference in crossed sentences. A third experiment found interference (slower RT) and also facilitation (faster RT) from exchanges of pretarget coarticulatory information in sentences. Two final experiments showed that previous results were not dependent on the use of practiced listeners.

Perception of anticipatory coarticulation effects.

Articulatory and acoustic studies have shown that the effects of anticipatory coarticulation may extend across several segments in an utterance. But previous perceptual studies suggest that only the information carried by immediately adjacent segments is used in perception. To show that perception is not so limited, we persuaded ten talkers each to produce 12 sentences (e.g., "I say poozee") of the form "I say /C V1 z V2/" in which C was /p, t, k/, V1 was /u, ae/, and V2 was /i, a/. Each /i, a/ sentence pair was cross spliced by exchanging the final syllable /z V2/ so that coarticulatory information prior to the crosspoint was inappropriate for the final vowel V2 in crossed sentences. Recognition time (RT) for V2 in crossed and intact (as spoken) sentences was obtained from practiced listeners. The results were slower RT in crossed sentences, and amount of interference depended on both V1 and C context. Another experiment varied the location of crosspoints across /C V1/ and found that RT interference increased directly with amount and proximity to target of inappropriate precross acoustics. LPC analysis of the experimental sentences showed pretarget variations in F2 frequency which were jointly dependent on identity of C, V1, and V2. Pretarget F2 variations and C and V1 identity jointly predicted amount of RT interference in crossed sentences. Finally, experiments with pretarget F2 variations in synthetic speech repeated and extended the results with real speech. These studies lead to the conclusion that the perceptual significance of coarticulation is not limited to effects on immediately adjacent segments. Listeners appear to be sensitive to many acoustic effects of the mutual influence among the segments in a sequence.