Schlieren study of external airflow during the production of nasal and oral vowels in French

Abstract

Speech airflow has long been studied using a variety of tools. The present paper discusses the potential value of a novel application of existing video technology, schlieren imaging, for the study of external airflow during speech production. Schlieren imaging enables real-time visualization of airflow through refraction of light and has been predominately used to study airflow in fields such as aerospace engineering. Although the potential of schlieren imaging in the study of speech production was suggested nearly 40 years ago (Davies, 1979), to date, this potential has remained largely unexplored. The aim of the present study was thus to conduct a preliminary investigation of such potential. Specifically, we examined the contrast between nasal and oral vowels in French, a contrast where there are expected differences in airflow from the nose versus the mouth in the space just in front of the face. Audio-visual recordings were made of nine French speakers producing monosyllabic CV words with nasal versus oral vowels, e.g. “pas” vs. “paon”. To analyse the data quantitatively, optical flow analysis was used; this method analyses motion by comparing differences in location of individual pixels between subsequent frames in video recordings. Results revealed significantly different patterns in external nasal airflow between nasal and oral vowels, e.g. the amount of the airflow was larger with nasal vowels than oral vowels. Between-speaker variation was also noted in the patterns of the airflow. Certain methodological challenges were encountered concerning the recording environment and optical flow analysis. Nevertheless, the findings show promise for schlieren imaging or similar technologies in the future study of speech production. Reference: Davies, T. P. (1979) Schlieren photography- a tool for speech research. Acoustics Letters 3(3),73-75.