Acoustic cues of ‘‘retroflex’’ and ‘‘bunched’’ American English rhotic sound

Abstract

The North American rhotic liquid has two well‐known and maximally distinct articulatory variants, the classic retroflex tongue posture and the classic bunched tongue posture. The evidence for acoustic difference between them is reexamined using magnetic resonance images of the vocal tracts from two similar‐sized subjects with different tongue postures of sustained /r/. Three‐dimensional finite element analysis is performed to investigate the acoustic wave propagation property inside the vocal tract, the acoustic response, and the area function extraction based on pressure isosurfaces. Sensitivity functions are studied for formant‐cavity affiliation. It is revealed that these two variants have similar patterns of F1–F3 and zero frequency. However, the retroflex variant is predicted to have a larger difference between F4 and F5 than the bunched one (1400 Hz versus 700 Hz). This difference can be explained by the geometric differences between them, in particular, the shorter, narrower, and more forward palatal constriction of the retroflex variant. In addition, the predicted formant pattern is confirmed by measurement from acoustic data of sustained /r/ and /r/ in nonsense words from several additional subjects. These results may prove to be useful to researchers in speech motor control, speech pathology, speaker verification, speech recognition, and speech synthesis. [Research supported by NIH.]