Vocal Tract Area Functions Research Articles

Parametrization of the vocal tract area function using a subset selection approach (L).

This letter introduces a parametrization of the vocal tract area function based on the position of a few points along the vocal tract. A QR decomposition algorithm is applied to area function data in various vowel configurations in order to identify those points with the most independent position patterns across vowels. Each point defines the shape of an associated kinematic region, and the overall area function is determined by the combination of the kinematic regions' shapes. The results show that only four data points, located at the tongue body, lips, and two at the tongue back, are enough to obtain accurate reconstructions of the vowels' area functions.

Articulatory-acoustic relations in the production of alveolar and palatal lateral sounds in Brazilian Portuguese

Lateral approximant speech sounds are notoriously difficult to measure and describe due to their complex articulation and acoustics. This has prevented researchers from reaching a unifying description of the articulatory and acoustic characteristics of laterals. This paper examines articulatory and acoustic properties of Brazilian Portuguese alveolar and palatal lateral approximants (/l/ and /ʎ/) produced by six native speakers. The methodology for obtaining vocal tract area functions was based on three-dimensional/four-dimensional (3D/4D) ultrasound recordings and 3D digitized palatal impressions with simultaneously recorded audio signals. Area functions were used to calculate transfer function spectra, and predicted formant and anti-resonance frequencies were compared with the acoustic recordings. Mean absolute error in formant frequency prediction was 4% with a Pearson correlation of r = 0.987. Findings suggest anti-resonances from the interdental channels are less important than a prominent anti-resonance from the supralingual cavity but can become important in asymmetrical articulations. The use of 3D/4D ultrasound to study articulatory-acoustic relations is promising, but significant limitations remain and future work is needed to make better use of 3D/4D ultrasound data, e.g., by combining it with magnetic resonance imaging.

3D dynamic MRI of the vocal tract during natural speech.

To develop and evaluate a technique for 3D dynamic MRI of the full vocal tract at high temporal resolution during natural speech. We demonstrate 2.4 × 2.4 × 5.8 mm3 spatial resolution, 61-ms temporal resolution, and a 200 × 200 × 70 mm3 FOV. The proposed method uses 3D gradient-echo imaging with a custom upper-airway coil, a minimum-phase slab excitation, stack-of-spirals readout, pseudo golden-angle view order in kx -ky , linear Cartesian order along kz , and spatiotemporal finite difference constrained reconstruction, with 13-fold acceleration. This technique is evaluated using in vivo vocal tract airway data from 2 healthy subjects acquired at 1.5T scanner, 1 with synchronized audio, with 2 tasks during production of natural speech, and via comparison with interleaved multislice 2D dynamic MRI. This technique captured known dynamics of vocal tract articulators during natural speech tasks including tongue gestures during the production of consonants "s" and "l" and of consonant-vowel syllables, and was additionally consistent with 2D dynamic MRI. Coordination of lingual (tongue) movements for consonants is demonstrated via volume-of-interest analysis. Vocal tract area function dynamics revealed critical lingual constriction events along the length of the vocal tract for consonants and vowels. We demonstrate feasibility of 3D dynamic MRI of the full vocal tract, with spatiotemporal resolution adequate to visualize lingual movements for consonants and vocal tact shaping during natural productions of consonant-vowel syllables, without requiring multiple repetitions.

Construction and Evaluation of a Parametric One-Dimensional Vocal Tract Model

Articulatory speech synthesis based on aero-acoustic simulations of the vocal tract is computationally expensive and, therefore, requires simple yet precise models. Modeling the one-dimensional vocal tract area function directly instead of a higher dimensional vocal tract model is an efficient way to minimize the computational overhead of the simulations. In this paper, we propose a new parametric vocal tract model that is controlled by six points and capable of modeling a large variety of vocal tract shapes. We geometrically and perceptually evaluated the model on a set of 22 reference area functions corresponding to German vowels and consonants. The model was able to geometrically approximate the reference area functions with a minimum root-mean-square error of 0.302 cm$^2$, a maximum error of 1.142 cm$^2$, and a median error of 0.891 cm$^2$. After optimizations, a perceptual evaluation of the synthesis using our model in combination with a state-of-the-art aero-acoustic simulation achieved a vowel recognition rate of 90.7% and a consonant recognition rate of 73.2%.

An age-dependent vocal tract model for males and females based on anatomic measurements.

The purpose of this study was to take a first step toward constructing a developmental and sex-specific version of a parametric vocal tract area function model representative of male and female vocal tracts ranging in age from infancy to 12 yrs, as well as adults. Anatomic measurements collected from a large imaging database of male and female children and adults provided the dataset from which length warping and cross-dimension scaling functions were derived, and applied to the adult-based vocal tract model to project it backward along an age continuum. The resulting model was assessed qualitatively by projecting hypothetical vocal tract shapes onto midsagittal images from the cohort of children, and quantitatively by comparison of formant frequencies produced by the model to those reported in the literature. An additional validation of modeled vocal tract shapes was made possible by comparison to cross-sectional area measurements obtained for children and adults using acoustic pharyngometry. This initial attempt to generate a sex-specific developmental vocal tract model paves a path to study the relation of vocal tract dimensions to documented prepubertal acoustic differences.

Development of Visual Support Way for Pronunciation Practice Using Reflection Coefficients of Burg's Method

In pronunciation training system, it is effective to instruct the manner of articulation to the trainee. We have developed a pronunciation training system, where the lip posture and tongue shape that are estimated by the formants extracted by AR model using Burg's method are shown to the trainee. However, formants estimation requires long computation time. In this report, we attempt to reconstruct these visual instructions from the reflection coefficients of Burg's method. As a results of principal component analysis for the reflection coefficients, it was shown that the discrimination of the vowels only by the reflection coefficients is difficult. On the other hand, the tongue shape, the front and back position of the tongue, was effectively visualized using vocal tract area function estimated by the reflection coefficients.

An acoustically-driven vocal tract model for stop consonant production

The purpose of this study was to further develop a multi-tier model of the vocal tract area function in which the modulations of shape to produce speech are generated by the product of a vowel substrate and a consonant superposition function. The new approach consists of specifying input parameters for a target consonant as a set of directional changes in the resonance frequencies of the vowel substrate. Using calculations of acoustic sensitivity functions, these “resonance deflection patterns” are transformed into time-varying deformations of the vocal tract shape without any direct specification of location or extent of the consonant constriction along the vocal tract. The configuration of the constrictions and expansions that are generated by this process were shown to be physiologically-realistic and produce speech sounds that are easily identifiable as the target consonants. This model is a useful enhancement for area function-based synthesis and can serve as a tool for understanding how the vocal tract is shaped by a talker during speech production.

Robust phoneme classification for automatic speech recognition using hybrid features and an amalgamated learning model

Phoneme recognition is an important aspect of speech processing and recognition. Research on phoneme recognition is several years old and numerous algorithms have been developed over the years to improve its accuracy. In this paper, a quantitative analysis of phoneme recognition using supervised learning is investigated. Most approaches to phoneme recognition rely on using mel frequency cepstrum based features for identification of the phoneme class. In our approach, we take into consideration the vocal tract area function along with mel frequency cepstrum coefficients and analyze the change in accuracy obtained by its introduction in the feature set. Support Vector Machines have been an attractive approach to pattern recognition and its usage as a supervised learning model has been popular in the speech processing community. We compare Support Vector Machines to other supervised learning models like the Naive Bayes, the k-Nearest Neighbors and the linear discriminant analysis classifiers, for our feature set. We impose a soft voting rule between the three best classifiers to produce our variation of a voting classifier. We enhance the accuracy of our classifier by using a priority based approach to estimate the three most likely phonemes, after the predicted phoneme. Through a figurative and quantitative approach, we show that our modified algorithm outperforms other traditional methods. Experiments were conducted on the WSJCAM0 corpus, a British English corpus.

Analysis of children’s high front vowel area function using three-dimensional ultrasound imaging

Understanding children’s speech production can follow from an analysis of children’s vocal tract area functions. While some studies have used imaging techniques on children to measure tongue contours, to our knowledge, no vocal tract area functions of children have been examined. We attempt to measure the oral cavity vocal tract area functions of children’s vowels using three-dimensional ultrasound imaging recordings of the tongue and palate. In this investigation, we model the oral cavity area function of the vowel [i] of children and compare it to that of an adult oral cavity area function measured using the same ultrasound technique. We show evidence that the tongue positioning and articulation of the vowel [i] is dramatically different for children and adults. Specifically, we focus on how the different tongue positioning of children and adults affects the vocal tract transfer function of [i]. Some theories of children’s speech, either mechanically or perceptually based, can be inferred from these findings and will be compared to previous studies. [Work supported in part by NSF Grant 1551113, NSF Grant 1551131, NSF Grant 1514544, and NIH Grant R01DC007124.]

Towards real-time two-dimensional wave propagation for articulatory speech synthesis

The precise simulation of voice production is a challenging task, especially when real-time performances are sought. To fulfill real-time constraints, most articulatory vocal synthesizers have to rely on highly simplified acoustic and anatomical models, based on 1D wave propagation and on the usage of vocal tract area functions. In this work, we present a 2D propagation model, designed to simulate the air flow traveling through the midsagittal contour of the vocal tract. Building on the work by Allen et al. [Andrew Allen and Nikunj Raghuvanshi, “Aerophones in flatland: Interactive wave simulation of wind instruments,” ACM Trans. Graph. 34, Article 134 (2015)], we leverage OpenGL and GPU parallelism for a real-time precise 2D airwave simulation. The domain is divided into cells according to a Finite-Difference Time-Domain scheme and coupled with a self-oscillating two-mass vocal fold model. To investigate the system’s ability to simulate the physiology of the vocal tract and its aerodynamics, two studies are presented. First, we compare the performances in vowel production of our 2D approach with other 1D wave propagation systems in literature, using area functions. Subsequently, this case is extended by replacing area functions with 2D vocal tract contours derived from 3D MRI data.

Development of a parametric basis for vocal tract area function representation from a large speech production database

Parametric representations of the vocal tract area function have long been of interest in speech production research because of their close relationship with speech acoustics. Many representations are possible depending on which qualities are considered desirable. For instance, several representations have been developed which trade articulatory interpretability for low-dimensionality and other useful mathematical properties, such as orthogonality. It has not been well-established whether these abstract representations are related to inherent constraints on vocal tract deformation (e.g., articulatory subspaces), or whether these representations can achieve high accuracy with a small number of parameters when applied to area functions from large amounts of continuous speech. The present study approaches these issues by extracting a data-driven basis representation for vocal tract area functions, and subsequently demonstrating the relationship between that basis and previously proposed spatial Fourier bases. Analysis was performed on more than a quarter-million individual area functions extracted from the USC-TIMIT database using region-of-interest analysis. Results suggest that a spatial Fourier basis with only four harmonics provides over 90% accuracy, and that one additional basis function corresponding to labial aperture increases representation accuracy to near 100%.

Muffled and Brisk Speech Evaluation with Criterion Based on Temporal Differentiation of Vocal Tract Area Function

Muffled and Brisk Speech Evaluation with Criterion Based on Temporal Differentiation of Vocal Tract Area Function

Interspeaker variability in relative tongue size and vowel production

The tongue varies across speakers in terms of the proportion of the overall speech production apparatus that it occupies. Differences in tongue size have the potential to result in speaker-specific articulatory strategies for shaping the vocal tract area function and, in turn, individual patterns of vowel acoustics. The present study examines the interplay between relative tongue size and vowel production using real-time magnetic resonance imaging with synchronous audio. Two populations of native American English subjects are considered, one containing healthy adult speakers with no relevant pathologies, and another containing speakers who had undergone glossectomy as treatment for tongue cancer. All subjects were imaged in the midsagittal plane while reading phonetically balanced English sentences. The size of the tongue and the speech production apparatus were quantified from an overall average posture, and their ratio was correlated with the shape of the vowel space in terms of acoustics (e.g., formant frequencies), constrictions (i.e., location and degree of minimum constriction), and parameterized vocal tract cross-distance functions. Results indicate that relative tongue size can be used to explain and predict observable interspeaker differences in vowel production.

Estimation of vocal tract area functions in children based on measurement of lip termination area and inverse acoustic mapping

Although vocal tract area functions for adult talkers can be acquired with medical imaging techniques such as magnetic resonance imaging (MRI), similar information concerning children’s vocal tracts during speech production is difficult to obtain. This is largely because the demanding nature of the data collection tasks is not suitable for children. The purpose of this study was to determine the feasibility of mapping formant frequencies measured from the [i, ae, a, u] vowels produced by three children (age range 4 to 6 years), to estimated vocal tract area functions. Formants were measured with a pitch-synchronous LPC approach, and the inverse mapping was based on calculations of acoustic sensitivity functions [Story, J. Acoust. Soc. Am. 119, 715–718]. In addition, the mapping was constrained by measuring the lip termination area from digital video frames collected simultaneously with the audio sample. Experimental results were augmented with speech simulations to provide some validation of the technique. [Research supported by NIH R01-DC011275.]

A methodological and preliminary study on the acoustic effect of a trumpet player’s vocal tract

A methodological study is presented to examine the acoustic role of the vocal tract in playing the trumpet. Preliminary results obtained for one professional player are also shown to demonstrate the effectiveness of the method. Images of the vocal tract with a resolution of 0.5 mm (2 mm in thickness) were recorded with magnetic resonance imaging to observe the tongue posture and estimate the vocal-tract area function during actual performance. The input impedance was then calculated for the player's air column including both the supra- and subglottal tracts using an acoustic tube model including the effect of wall losses. Finally, a time-domain blowing simulation by Adachi and Sato [J. Acoust. Soc. Am. 99, 1200-1209 (1996)] was performed with a model of the lips. In this simulation, the oscillating frequency of the lips was slightly affected by using different shapes of the vocal tract measured for the player. In particular, when the natural frequency of the lips was gradually increased, the transition to the higher mode occurred at different frequencies for different vocal-tract shapes. Furthermore, simulation results showed that the minimum blowing pressure required to attain the lip oscillation can be reduced by adjusting the vocal-tract shape properly.

Vocal tract area functions and formant frequencies in opera tenors’ modal and falsetto registers

According to recent model investigations, vocal tract resonance is relevant to vocal registers. However, no experimental corroboration of this claim has been published so far. In the present investigation, ten professional tenors' vocal tract configurations were analyzed using MRI volumetry. All subjects produced a sustained tone on the pitch F4 (349 Hz) on the vowel /a/ (1) in modal and (2) in falsetto register. The area functions were estimated from the MRI data and their associated formant frequencies were calculated. In a second condition the same subjects repeated the same tasks in a sound treated room and their formant frequencies were estimated by means of inverse filtering. In both recordings similar formant frequencies were observed. Vocal tract shapes differed between modal and falsetto register. In modal as compared to falsetto the lip opening and the oral cavity were wider and the first formant frequency was higher. In this sense the presented results are in agreement with the claim that the formant frequencies differ between registers.

A test of formant frequency analyzes with simulated child-like vowels.

Speech production by children is typically characterized by a fairly high fundamental frequency of phonation and a short vocal tract length that produces high formant frequencies. Together these two characteristics contribute to the difficulty of making accurate measurements of the formants because the vocal tract transfer function may be undersampled by the voice source harmonics. In addition, the close proximity of the low-numbered harmonics (including the fundamental) to a formant may lead to strong nonlinear interaction of the acoustic pressures in the vocal tract and the glottal airflow. The purpose of this study was to use standard spectrographic and LPC techniques, as well as new pitch-synchronous method, to measure formant frequencies of child-like vowels that have been simulated with a speech production model. Each vowel will be simulated with two different representations of the voice source: a glottal area model that allows for nonlinear source-tract interaction and a glottal flow model in which vocal tract characteristics cannot affect the source. The results of the analyzes will be compared to the actual formant frequencies calculated directly from the known vocal tract area functions used to generate the vowels. [Research supported by NIH R01-DC04789.]

The effect of oral articulation on the acoustic characteristics of nasalized vowels

To study the acoustic characteristics of nasalized vowels, the effects of velopharyngeal opening and oral articulation are considered. Based on vocal tract area functions for one American English speaker, spectral evolutions for the nasalization of three English vowels /a/, /i/, and /u/ were studied by simulating transfer functions for vowels with only velar movement, and for different nasal consonant-vowel utterances, which include both velar and oral movements. Simulations indicate extra nasal spectral poles and zeros and oral formant shifts as a result of the velopharyngeal opening and oral movements, respectively. In this sense, if oral articulation is coordinated with velar movement in such a way that nasal acoustic features are prominently attenuated, corresponding compensatory articulation can be developed to reduce hypernasality. This may be realized by (1) adjusting the articulatory placement for isolated nasalized vowels or by (2) changing the relative timing of coarticulatory movements for dynamic speech. The results demonstrate the effect of oral articulation on the acoustics of nasalized vowels. This effect allows oral articulation to compensate for velopharyngeal dysfunction, which may involve a constellation of speech production disorders resulting from anomalous velopharyngeal closure and which is usually accompanied by hypernasality and nasal emission of air.

Identification of synthetic vowels based on a time-varying model of the vocal tract area function

The purpose of this study was to conduct an identification experiment with synthetic vowels based on the same sets of speaker-dependent area functions as in Bunton and Story [(2009) J. Acoust. Soc. Am. 125, 19-22], but with additional time-varying characteristics that are more representative of natural speech. The results indicated that vowels synthesized using an area function model that allows for time variation of the vocal tract shape and includes natural vowel durations were more accurately identified for 7 of 11 English vowels than those based on static area functions.

Normalization for vocal tract differences using long term average spectrum.

Differences in vocal tract size and anatomy lead to substantial differences in acoustic realization of vowels. Much of the variability in vocal tract area functions is captured in the area function of the “neutral” (non-articulating) vocal tract of each talker. A listener could perceptually normalize talker variability substantially by extracting the neutral vocal tract and using it as a referent. Alternatively, the listener could extract the auditory representation of the output of the neutral tract and use it as a referent for the auditory representation of the vowel space. We will present such a model of perceptual talker normalization for vowels and demonstrate its effectiveness for a variety of vocal tract shapes and lengths. Additionally, we will provide evidence that this normalization may be implemented in an auditory process that (i) extracts the long term average spectrum from running speech (providing an estimate of the neutral vowel), (ii) represents new input relative to this spectral average (providing normalization), and (iii) is not specific to speech. Additional analyses and perceptual results will demonstrate how this model can extend to normalization of consonant perception. [Work supported by NIH/NIDCD to A.J.L. and B.H.S. and Diversity Supplement to A.D.V.]