Changes In Formant Frequencies Research Articles

Improving Naturalness and Intelligibility of Helium-Oxygen Speech, Using Vocoder Techniques

Previous analysis of helium speech has shown that the peculiar characteristic of this speech is due primarily to changes in formant frequencies of the speaker. Changes in the fundamental pitch frequency of the speaker are small and usually can be neglected. Computation of the resonant frequencies of the vocal tract, as a function of the gas mixture occupying the tract, gives a reasonable estimate of the changes that can be expected in the formant frequencies. A modified channel vocoder was designed to restore approximately the normal values of the talker's formant frequencies, while preserving his fundamental pitch frequency. This “formant-restoring vocoder” (FRV) separates the spectral energy of the helium speech into a number of narrow bands, which then amplitude-modulate lower-frequency pitch harmonics derived directly from the helium speech. Helium speech from Sealab II was processed by an FRV simulated on a digital computer. Results of several simulations indicated that considerable improvement in naturalness and intelligibility of helium speech can be achieved.

Improving Naturalness and Intelligibility of Helium Speech, Using Vocoder Techniques

Previous analysis of helium speech has shown that the peculiar characteristic of this speech is due primarily to changes in formant frequencies of the speaker. Even though some change does take place in the fundamental pitch frequency of the speaker, this effect is small and usually can be neglected. Computation of the resonant frequencies of the vocal tract as a function of the gas mixture occupying the tract gives a reasonable estimate of the changes that take place in the formant frequencies. A modified self-excited channel vocoder was designed to restore approximately the normal values of the talker's formant frequencies while preserving his fundamental pitch frequency. This “formant-restoring-vocoder” (FRV) separates the spectral energy of the helium speech into a number of narrow bands that then amplitude-modulate lower-frequency pitch harmonics derived directly from the hedium speech. The lower-frequency harmonics are separated by bandpass filters that are proportionately narrower and lower in frequency than the spectral-energy separating filters. Helium speech from SEALAB II was processed by an FRV simulated on a digital computer. Results of several simulations indicated that considerable improvement in naturalness and intelligibility of helium speech can be achieved.

Perception of Damped Sine Waves

This paper discusses the perception of resonant frequency shifts in damped sine waves as a function of the time and magnitude of the shifts. The relation between the perception of the changes in frequency and damping of the wave form is also investigated. The interest in this study stems from the fact that changes in formant frequency (transitions) constitute an important class of cues in speech. The equipment producing the proper stimuli has been built and perceptual tests have been run testing a number of possible hypotheses.

Preliminary Investigation of the Acoustical Characteristics of Inter-Phonemic Transitions

The study employed selected vowels, each combined with prevocalic or post-vocalic consonants in consonant-vowel or vowel-consonant syllables. Each combination was recorded non-contextually by several speakers and then tested by listeners for recognizability of phonemes. Oscillograms and sound spectrograms were made of all syllables correctly identified. These were used to obtain several measures of syllabic component duration and formant frequency change during transition. Measures of direction and extent of frequency change during transitions appeared to offer most promise of relationship to known physiological features of articulation and were subjected to more complete analysis. Results of the study indicate the following: (1) for the first two formants, the transitions reflect aspects of tongue placement for the vowel, and place and manner of articulation and voicing characteristics of the consonant; (2) lack of speaker agreement resulted in variations obscuring most aspects of patterning for the third formant. Specific hypotheses were tested within the limits of the experimental procedure and results are reported. Future research trends are suggested.

Noise Duration as a Cue for Distinguishing among Fricative, Affricate, and Stop Consonants

An earlier experiment [Gerstman, Liberman, Delattre, and Cooper, J. Acoust. Soc. Am. 26, 952 (1954)] demonstrated that variations in the duration of a change in formant frequency can serve as an effective cue for distinguishing one class of speech sounds from another. The duration cue was found to operate independently of place cues; i.e., it served to distinguish the stop b from the semivowel w, both of which are articulated at a speaker's lips. Exploratory work now suggests that another temporal cue, noise duration, operates in much the same fashion, distinguishing among classes of sounds produced at the same place in the mouth. A tape recording of the spoken syllable ∫a (as in shop) was progressively shortened by cutting off increasing amounts of the noise portion with which the syllable begins. When the noise portion was approximately half its original length the syllable was heard as ∫a (as in chop), and when most of the noise had been removed the syllable was heard by some listeners as ka and by others as ta. (Inasmuch as ∫a and t∫a are both produced at a point in the mouth that lies be|tween the places where k and t are formed, the listeners' uncertainty as to whether the stop is k or t is understandable.) Thus the consonant, originally a fricative, became an affricate and then a stop. The experiment has been repeated with more highly controlled synthetic speech patterns. Here it was found that a second cue of some importance was the time required by the friction to rise to full intensity. [This work was supported in part by the Carnegie Corporation of New York, and in part by the Department of Defense in connection with Contract DA49-170-sc-1642.]

Rate and Duration of Change in Formant Frequency as Cues for the Identification of Speech Sounds

Earlier experiments have shown that the direction and extent of the frequency changes in the formants of synthetic speech sounds can serve as effective cues for the identification of certain phones according to place of production. Examination of spectrograms and exploratory work with synthetic speech has suggested that the temporal course of these changes may provide a basis for distinguishing among some classes of speech sounds that differ in manner of production. It was found in the present experiment that the tempo of the change between two fixed formant frequencies is, in fact, sufficient to enable naive listeners to distinguish rather reliably among stop-consonant-plus-vowel (e.g., ba), semi-vowel-plus-vowel (e.g., wa), and a sound of shifting vowel color (e.g., ua). Further experiments suggest that the duration of the frequency changes are more important than the rate at which the changes occur.