Effect of Mouth-Opening Levels on Sound Field Gain in the Ear Canal

The human ear canal can become deformed when the mouth opens and closes, changing the sound field in the external auditory canal. Thus, measuring the mandibular movement trajectory may provide a reference for identifying mouth-opening levels. The K7 system and Fonix 7000 sensor were used to perform several measurements of mouth-opening and -closing to obtain the mean maximal mouth opening level. The mandible was fixed at the 1/3, 2/3, and maximal mouth-opening levels. Subsequently, a 65-dB sweep tone, which is the sound pressure level of people talking at a normal level, was adopted as the sound stimulus to measure sound fields at 5, 10, 15, and 20 mm from the entrance of the external auditory canal. The stimulus frequencies of 200, 500, 1000, 2000, 4000, 6000, and 8000 Hz were noted.The differences in sound field gains at depths of 5, 10, 15, and 20 mm were analyzed for the various mouth-opening levels. The results show that except for the 1.70-dB decrease at a depth of 5 mm with an 8000-Hz stimulus frequency, the differences at the other depths and stimulus frequencies were within 1 dB. Thus, the mouth-opening levels will influence the gain in ear canal.