Effects of External Ear Canal Pressure on the Middle-Ear Muscle Reflex Threshold

Abstract
Twenty normal-hearing individuals served as subjects in an experiment designed to determine the relationships between positive and negative air pressure in the external auditory canal and the intensity required to elicit the acoustic reflex. Pressure was varied from +240 to −240 mm H 2 O. Changes in the magnitude of acoustic impedance were measured on an acoustic impedance meter and displayed graphically on a Y-T recorder. As air pressure was varied in the canal and the tympanic membrane was displaced from its position of greatest compliance, systematic increases in the intensity required to elicit the reflexes were noted. The magnitude of the differences was smaller than might have been anticipated, not exceeding a mean of 5.1 dB at −240 mm H 2 O.

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