Accuracy, recording interference, and articulatory quality of headsets for ultrasound recordings

In this paper we evaluate the accuracy, recording interference, and articulatory quality of two different ultrasound probe stabilization headsets: a metallic Ultrasound Stabilisation Headset (USH) and UltraFit, a recently developed headset that is 3D printed in Nylon. To evaluate accuracy, we recorded three native speakers of German with different head sizes using an optical marker tracking system that provides sub-millimeter tracking accuracy (NaturalPoint OptiTrack Expression). The speakers had to read C1V1C2V1/2 non-words (to diminish lexical influences) in three conditions: wearing the USH headset, wearing the UltraFit headset, and without a headset. To estimate the relative headset movement, we measured the movement between tracked points on the probe, headset, and speaker's nose. By also tracking visual marker points on the speaker's lip and chin, we compared the movement of the outer articulators with and without a headset and, thereby, measured how the headsets interfere with the articulatory space of the speaker. Additionally, we computed the differences in tongue profiles at the acoustic midpoint of V1 under the three conditions and evaluated the articulatory recording quality with a distance index and an area index. In the final evaluation, we also compared formant measurements of recordings with and without headsets. With this objective evaluation we provide a systematic analysis of different headsets for Ultrasound Tongue Imaging (UTI) and also contribute to the discussion of using UTI stabilization headsets for recording natural speech. We show that both headsets have a similar accuracy, with the USH performing slightly better overall but introducing the largest error for one speaker, and that the UltraFit headset shows more flexibility during recordings. Each headset influences the lip opening differently. Concerning the tongue movement, there are no significant differences between different sessions, showing the stability of both headsets during the recordings. Acoustic analysis of formant differences in vowels revealed that the USH headset has a larger influence on formant production than the UltraFit headset.