Simulator investigation of the effects of occluded ear characteristics on remote measurement of eardrum SPL

Abstract

Because of developments in remote measurement of eardrum sound pressure level (SPL) in hearing aid occluded ears, an investigation was made of the effects of external ear characteristics on such measurements. Two systems were investigated: a probe microphone contained within the earmold, and a miniature microphone in the ear canal located as close as possible to the medial end of the earmold. Ear stimulators (ANSI S3.25-1979) were modified to have a range of eardrum impedances approximating 96% of the population with normal middle ears [Zwislocki and Feldman, ASHA Monogr. No. 15 (1970)] in three steps and to have a range of ear canal lengths of 9.7 to 17.7 mm in four steps. SPL measurements were made over a frequency range of 0.2 to 6 kHz for each combination of length and impedance. As predicted by transmission line calculations, both ear canal length and eardrum impedance affected the measured SPL. Although individual measurements made by each system differed somewhat because of differences in measurement location, the total range of differences between measured and actual eardrum SPL was almost identical for both systems. With optimum calibration in an ear simulator, the maximum error produced by either system was within ±5 dB of the true eardrum SPL.