Miniature microphone probe tube measurements in the external auditory canal.

The use of a miniature microphone probe tube for sound-pressure level measurements in the outer earcanal was studied in four experiments. The main reason was to evaluate this method for clinical applications and for measurements of hearing protector attenuation. All measurements were registered in 1/3-oct bands. The dynamic range, frequency response of the microphone system, the insertion loss of the probe tube walls, and the influence of the probe tube in the ear canal on the sound-pressure level were examined. The first experiment attempted to determine an optimal placement of the probe tube in the earcanal. Sound-pressure levels resulting from a known free-field sound stimulus were measured at 12 different positions in the earcanal of human subjects. The results indicated that the position which is least sensitive for small movements of the probe tube on the measure was within 1 to 3 mm of the tympanic membrane. In a second experiment the changes in sound transfer function to the tympanic membrane caused by the microphone itself, when positioned just outside the ear canal entrance, were studied for 72 angles of sound incidences in ten subjects. A few minor but significant changes were observed with the microphone in this position. In a third experiment, sound transfer functions were measured from free sound field to the tympanic membrane in 19 subjects. The measurements were performed in 1/3-oct frequency bands (0.2 to 20 kHz) with sound incidences from 24 azimuthal positions and for each of these, three different elevations. The inter- and intrasubject-variability are analyzed and discussed. Finally, the reliability of probe tube microphone measurements was evaluated by repeated measurements of the sound transfer function, five times in ten subjects with the sound incidence from one position. The results indicated very small variations between repeated measurements.