Sibilant Fricative Merging in Taiwan Mandarin: An Investigation of Tongue Postures using Ultrasound Imaging.

In Taiwan Mandarin, retroflex [ʂ] is allegedly merging with dental [s], reducing the traditional three-way contrast between sibilant fricatives (i.e., dental [s]-retroflex [ʂ]-alveopalatal [ɕ]) to a two-way contrast. Most of the literature on the observed merging focuses on the acoustic properties and perceptual identification of the sibilants, whereas much less attention has been drawn to the articulatory evidence accounting for the aforementioned sibilant merging. The current study employed ultrasound imaging techniques to uncover the tongue postures for the three sibilant fricatives [s, ʂ, ɕ] in Taiwan Mandarin occurring before vowels [a], [ɨ], and [o]. Results revealed varying classes of the [s-ʂ] merger: complete merging (overlap), no merging (non-overlap), and context-dependent merging (context-dependent overlap, which only occurred before [a]). The observed [s-ʂ] merger was also confirmed by the perceptual identification by trained phoneticians. Center of gravity (CoG), a reliable spectral moment of identifying different sibilant fricatives, was also measured to reflect the articulatory-acoustic correspondence. Results showed that the [s-ʂ] merger varies across speakers and may also be conditioned by vowel contexts and that articulatory mergers may not be entirely reflected in CoG values, suggesting that auxiliary articulatory gestures may be employed to maintain the acoustic contrast.

A biomechanical case study on the optimal orthodontic force on the maxillary canine tooth based on finite element analysis.

Excessive forces may cause root resorption and insufficient forces would introduce no effect in orthodontics. The objective of this study was to investigate the optimal orthodontic forces on a maxillary canine, using hydrostatic stress and logarithmic strain of the periodontal ligament (PDL) as indicators. Finite element models of a maxillary canine and surrounding tissues were developed. Distal translation/tipping forces, labial translation/tipping forces, and extrusion forces ranging from 0 to 300 g (100 g=0.98 N) were applied to the canine, as well as the force moment around the canine long axis ranging from 0 to 300 g·mm. The stress/strain of the PDL was quantified by nonlinear finite element analysis, and an absolute stress range between 0.47 kPa (capillary pressure) and 12.8 kPa (80% of human systolic blood pressure) was considered to be optimal, whereas an absolute strain exceeding 0.24% (80% of peak strain during canine maximal moving velocity) was considered optimal strain. The stress/strain distributions within the PDL were acquired for various canine movements, and the optimal orthodontic forces were calculated. As a result the optimal tipping forces (40-44 g for distal-direction and 28-32 g for labial-direction) were smaller than the translation forces (130-137 g for distal-direction and 110-124 g for labial-direction). In addition, the optimal forces for labial-direction motion (110-124 g for translation and 28-32 g for tipping) were smaller than those for distal-direction motion (130-137 g for translation and 40-44 g for tipping). Compared with previous results, the force interval was smaller than before and was therefore more conducive to the guidance of clinical treatment. The finite element analysis results provide new insights into orthodontic biomechanics and could help to optimize orthodontic treatment plans.