Validating diffeomorphic 3-D vocal tract transitions constrained by real-time magnetic resonance imaging using acoustic formants

The shape of the human vocal tract plays a critical role in shaping the acoustic output of speech, yet capturing and modeling its complex 3-D dynamics remains a challenge. We present a novel acoustic evaluation framework for vocal tract morphing based on both volumetric and real-time Magnetic Resonance Imaging (MRI) data. Our approach uses Large Deformation Diffeomorphic Metric Mapping (LDDMM) applied to volumetric mesh data, constrained by dynamic midsagittal motion captured via real-time MRI. Building on previous work that separately used volumetric MRI for static postures and real-time MRI for dynamic imaging, we now relate 3-D shape deformation directly to speech output. Data were collected from an adult female speaker of Australian English, including real-time midsagittal MRI videos with synchronized audio, and high-resolution 3-D scans of sustained articulatory postures. For each intermediate mesh representing vocal tract deformation, we compute acoustic transfer functions and synthesize speech signals. These are temporally aligned with the recorded speech and compared via extracted formant frequencies (F1–F3). By quantifying formant deviations over time, we assess the acoustic plausibility of deformations guided by real-time MRI. To our knowledge, this is the first study to directly validate a diffeomorphic vocal tract morphing pipeline using measured speech acoustics.