The vocal tract is the cavity in human beings and in animals where sound that is produced at the sound source (larynx in mammals; syrinx in birds) is filtered.

Quantitative association of vocal-tract and facial behavior
A parametric approach to vocal tract length normalization
A parametric approach to vocal tract length normalization
Compensatory Articulation During Speech: Evidence from the Analysis and Synthesis of Vocal-Tract Shapes Using an Articulatory Model
Compensatory Articulation During Speech: Evidence from the Analysis and Synthesis of Vocal-Tract Shapes Using an Articulatory Model
The Origin of Sound Patterns in Vocal Tract Constraints
Vocal tract area functions from magnetic resonance imaging
Analysis of vocal tract shape and dimensions using magnetic resonance imaging: Vowels
Vocal tract resonances in oscine bird sound production: evidence from birdsongs in a helium atmosphere
Techniques for estimating vocal-tract shapes from the speech signal