A method was designed to determine the mandibular positions during speech; in which three phenomena, namely, (1) the path of the mandibular movements, (2) the spectrogram of the voice, and (3) the voice sound-were simultaneously recorded on the same frame of 16 mm motion picture films. The /s/, /∫/ and /m/ positions of an edentulous subject were measured under the conditions of various vertical dimensions of the complete dentures. The results may be summarized as follows : 1) Not only the /s/ position but also the /m/ position are variable instantaneously according to the changes in the intraoral condition. 2) The interocclusal distances in the /s/ positions were almost constant regardless of the difference in the occlusal vertical dimension to some extent. But in the case of closed occlusal vertical dimension, it became larger; it became smaller in the case of open occlusal vertical dimension. 3) The /m/ positions varied according to the vowels which preceded and followed them. The interocclusal distance in the /m/ position decreased gradually according to the increase of the occlusal vertical dimension. 4) The changes in the /m/ position were gentler than those in the /s/ position. It seems that the proper range of the occlusal vertical dimension can be determined by observing the mandibular movements during /s/,/∫/ and /m/ pronunciation
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