Vocal Tract Impedance Research Articles

A method for measurement of the vocal tract impedance at the mouth

In this contribution a method is presented for the measurement of vocal tract resonances. The technique uses a non-invasive acoustic excitation of the vocal tract and a fast and robust detection. The method is an alternative to the linear predictive coding (LPC) analysis for patients with voice and speech disorders. Sweep signals are emitted and recorded simultaneously from the small end of a tube placed in front of the mouth opening. The use of a pressure sensor and a velocity sensor provides a direct measurement of the vocal tract impedance at the mouth (VTMI). For selected sustained German vowels, and some consonants, a comparison of results from LPC analysis and VTMI measurements is given. The results indicate a good agreement in the frequency range from 500 to 5000 Hz. The feasibility of the VTMI method for diagnostic and therapeutic applications is subject to current research.

Coarticulation in the temporal domain: evidence from Telugu-speaking prelingually deaf children

This paper examines certain aspects of segmental and temporal coarticulation in the speech of six school-going deaf children whose native language is Telegu. Drawing on notions such as inter-gestural coordination, articulatory kinematics, phonetic sonority and vocal-tract impedance, it was hypothesised that the extent of segmental and temporal coarticultory effects in this language would depend on (l) the rhythmic status of a syllable within a word and (2) the degree of residual hearing and lip-reading skills of the subjects. The results, demonstrating that deletion of an intervening consonant in the medial position of words with geminate consonants (i.e. CVCCV words) and clusters (i.e. CVC1C2V words) contribute to prolongation of the vowel preceding the deleted consonant (resulting in CV:CV structures) and that this is particularly true in the speech of children with poor residual hearing and poor lip-reading ability, essentially supported the hypothesis. The implications of the results of this study are discussed in the light of existing theories of co articulation and suggestions are made for future research.

The measured vocal tract impedance for clarinet performance and its role in sound production.

The role of the vocal tract in clarinet performance has been investigated by examining the vocal tract impedance, Zu. Theory predicts that Zu peaks aligned with harmonics of the pitch frequency stabilize the oscillation. The vocal tract impedance was measured directly with a one-microphone technique and indirectly from a calculation using the continuity of flow equation pu/Zu+pd/Zd=0, where pu and pd are the mouth and mouthpiece pressures and Zd is the instrument impedance. The continuity equation was verified for single tones and then used to examine the role of the vocal tract in a variety of musical situations. In orchestral excerpts, the performer tends to align resonances with the first and/or second harmonics, and there is some dependence on the musical context. For multiphonics, the performer creates a resonance that supports an oscillation at a linear combination of the audible pitch frequencies. For tones played with pitchbend, induced by slacking the jaw muscles, the playing frequency is controlled by the frequency of a large-amplitude vocal tract resonance, for which ‖Zu‖≊100–400 CGS ohms. Zu changes only slightly when clarion tones are played without the register key, suggesting that other variables are involved.

Phonation threshold pressure: A missing link in glottal aerodynamics.

Phonation threshold pressure has previously been defined as the minimum lung pressure required to initiate phonation. By modeling the dependence of this pressure on fundamental frequency, it is shown that relatively simple aerodynamic relations for time-varying flow in the glottis are obtained. Lung pressure and peak glottal flow are nearly linearly related, but not proportional. For this reason, typical power-law relations that have previously been proposed do not hold. Glottal impedance for time-varying flow must be defined differentially rather than as a simple ratio between pressure and flow. It is shown that the peak flow, the peak flow derivative, the open quotient, and the speed quotient of inverse-filtered glottal flow waveforms all depend explicitly on phonation threshold pressure. Data from singers are compared with those from nonsingers. The primary difference is that singers obtain two to three times greater peak flow for a given lung pressure, suggesting that they adjust their glottal or vocal tract impedance for optimal flow transfer between the source and the resonator.

Vocal closure patterns and source‐tract acoustic interaction

For a strong or well‐carrying voice, glottal volume velocity waveforms obtained by inverse filtering are generally consistent with the hypothesis that an interaction between the time‐varying glottal impedance and an inertive component in the vocal tract impedance is causing an increase in the relative strength of higher harmonics and a decrease in average air flow. However, the required ratio of vocal tract inertance to glottal impedance is about a factor of 2 less than the greatest estimate that can be made from the glottal and vocal tract geometries using standard acoustic principles. The hypothesis that the unexplained difference is related to the shape of the vocal folds during the closing phase of their vibratory cycle was investigated by comparing glottal air flow and vocal fold contact area waveforms, with the latter obtained using an electroglottograph. To obtain an adequately accurate glottal airflow pulse and adequate synchronization of the recorded waveforms, the glottal flow waveshape was obtained by inverse filtering pharyngeal air pressure instead of the more commonly used radiated pressure or oral air flow. The transfer function of the required inverse filter is described, as well as problems and advantages of this procedure.

The effect of the player's vocal tract on woodwind instrument tone.

Many players of woodwind instruments believe that the tone of the instrument is influenced by the player's vocal tract. If this is so, it would be expected that the vibrating reed in the player's mouth would generate an appreciable sound pressure in his vocal tract. However, measurements of the vocal tract impedance (looking into the mouth) give values an order of magnitude less than the impedances of the clarinet air column resonances. The sound pressure built up in the vocal tract would thus be expected to be much smaller than that in the clarinet. A clarinetlike system was arranged to sound by drawing air through it from the normally open end, the reed being out in the open for easy accessibility. When the mouth was placed around the mouthpiece with the system sounding, the changes in the waveform and harmonic structure of the tone were minimal. When a system with a more sharply tuned resonance was substituted for the vocal tract, the waveform changes were observable but not large. It appears that the player's vocal tract has a negligible influence on the instrument tone.

Vocal tract area functions from autoregressive analysis of two‐point acoustic data

It is well known that the frequencies of the low‐order driving point impedance poles and zeros of the vocal tract can be used to determine a smoothed representation of the vocal tract area function. The impedance poles are congruent with the vocal tract formants; the pole frequencies are specified by the location of peaks in the speech spectrogram. The impedance zero frequencies are indirectly related to formant bandwidths. The bandwidths, however, are difficult to accurately measure from speech. A method is presented for estimating vocal tract impedance zero frequencies directly as the peaks of the transfer function relating the acoustic speech signal to the acceleration signal from a transducer taped to the skin of the neck at the thyroid notch. This transfer function is estimated from pressure and acceleration signals using an autoregressive technique. Area function reconstructions derived from estimated pole and zero frequencies from human speech are presented. [Work supported by NIH.]

Glottal Wave Periods in VCV Environments

Current interest has centered on the effects of vocal tract impedance changes upon glottal function. The present study was designed to determine fundamental frequency changes between vowels and consonants in simple VCV syllables. Thirteen vocally normal male subjects were asked to phonate the syllables /ava/and /ivi/ at two fundamental frequencies. Their phonations were analyzed by visicorder-caliper methods. The results demonstrated that the consonant in each series typically and significantly lowered the fundamental frequency from 4% to 15%. Although the results were confounded in some subjects by inflectional and erratic behavior beyond our calibrated 0.2% measurement error, the results substantiated the reports of other investigators regarding the importance of mechanical coupling between glottis and vocal cavities.