Acoustics of ear canal measurement of eardrum SPL

This paper reports on a study to determine the effects of standing acoustic waves on the accuracy of ear canal determination of eardrum SPL. The study was conducted in three parts: (1) A theoretical analysis of standing waves of sound pressure in a lossless transmission line having the geometry of the average ear canal with terminations corresponding to the extremes in normal eardrum impedance, (2) actual probe measurements on ANSI S3.25-1979 ear simulators, modified so that ear canal length and eardrum impedance could be varied, (3) probe determination of eardrum SPL on real ears. Results from simulator and real ear measurements agree with the theoretical determinations obtained from the standing wave study. Theoretical and actual standing wave ratios agree with published real ear data [E.A.G. Shaw, “Transformation of sound pressure level from the free field to the eardrum in the horizontal plane,” J. Acoust. Soc. Am. 56, 1848–1861 (1974)].