Vocal tract area functions from magnetic resonance imaging

Abstract

There have been considerable research efforts in the area of vocal tract modeling but there is still a small body of information regarding direct 3-D measurements of the vocal tract shape. The purpose of this study was to acquire, using magnetic resonance imaging (MRI), an inventory of speaker-specific, three-dimensional, vocal tract air space shapes that correspond to a particular set of vowels and consonants. A set of 18 shapes was obtained for one male subject who vocalized while being scanned for 12 vowels, 3 nasals, and 3 plosives. The 3-D shapes were analyzed to find the cross-sectional areas evaluated within planes always chosen to be perpendicular to the centerline extending from the glottis to the mouth to produce an "area function." This paper provides a speaker-specific catalogue of area functions for 18 vocal tract shapes. Comparisons of formant locations extracted from the natural (recorded) speech of the imaged subject and from simulations using the newly acquired area functions show reasonable similarity but suggest that the imaged vocal tract shapes may be somewhat centralized. Additionally, comparisons of the area functions reported in this study are compared with those from four previous studies and demonstrate general similarities in shape but also obvious differences that can be attributed to differences in imaging techniques, image processing methods, and anatomical differences of the imaged subjects.

An overview of the physiology, physics and modeling of the sound source for vowels.

“…The specific implementation of the vocal tract model [24] is based on the wave-reflection approach [25,26] and includes losses from yielding walls, viscosity, acoustic radiation, and heat conduction. The vocal tract shape used in this case was an /i/ vowel measured from an adult male speaker [27]. The first two formant frequencies were calculated to be F 1 ¼ 290 Hz and F 2 ¼ 2; 360 Hz.…”

“…The supraglottal vocal tract in this case was configured with an area function for the vowel /A/ measured from the same adult male speaker [27] as in the previous case of /i/. Its two lowest formant frequencies were calculated to be F 1 ¼ 800 Hz and F 2 ¼ 1; 163 Hz.…”

Determining the Relevant Criteria for Three-dimensional Vocal Tract Characterization

“…All measurements were taken in mm. The start (glottis) and end (mouth) of the vocal tract were manually defined by the researcher, labelled as (1) on Figure 3, and then following an algorithm originally developed to analyse upper airway geometry and volume with regard to sleep disorders [36], and adapted to generate a 2D area function from a mid-sagittal slice [37], the area function was calculated using an iterative bisection algorithm: Firstly the line joining the start and end of the vocal tract was calculated (2), and then a plane was defined at the midpoint of this line, normal to it (3). The intersection of this plane with the vocal tract was found, and its area and centre were then calculated (4), and the centre stored as a point on the midline of the vocal tract.…”

Evolution of vowel production studies and observation techniques.

“…In order to establish the relationship between vocal tract shape and speech sounds, the lateral view of the vocal tract is insufficient, and the 3D vocal tract configuration must be obtained. Since magnetic resonance imaging (MRI) is capable of the 3D measurement of the body in multi-slice images, this technique has been applied to direct measurement of the vocal tract shape and area function [38][39][40][41][42]. Although the MRI provides a powerful visualization technique for speech research, it has a few problems; low signals from the teeth make toothless images, supine position of the subjects can affect natural articulation, and intense machine noise inhibits highquality audio recording.…”