HEARING THRESHOLD AND EAR-CANAL PRESSURE LEVELS WITH VARYING ACOUSTIC FIELD

A well damped circumaural enclosure is equipped with five independent acoustic driver units located at cardinal positions. The acoustic field near the external ear may thus be varied in a manner corresponding to angle of incidence. The acoustic pressure at a well defined position at the ear canal entrance is measured with a probe tube passing under the circumaural cushion. With each subject and frequency, the driver voltage level at hearing threshold LC and the ear canal response REC are determined for each driver unit in turn. A double series of measurements has been made with six subjects from 1 12 kHz and three to six subjects from 13 to 16 kHz at 1 kHz intervals. The ear canal pressure level at hearing threshold LE is found to be virtually independent of the acoustic field geometry up to 12 kHz. Changes in LC and REC observed after a one week interval can be related to three independent mechanisms: (1) changes in subject hearing sensitivity, (2) changes in coupling between probe tube orifice and ear canal wave pattern, and (3) changes in coupling between the various driver units and the ear canal. Changes in subject sensitivity are approximately 2 dB, independent of frequency, up to at least 13 kHz. Probe coupling changes are small below 9 kHz. At the higher frequencies, driver coupling changes are occasionally very large (more than 20 dB in extreme cases), generally much greater than changes in subject sensitivity, and dependent on frequency in a distinctive manner related to the geometry of the sound field. © 1969, Acoustical Society of America. All rights reserved.