Why is the Y-axis on Tympanograms Measured in Terms of Compliance?

Question

Answer

The fundamental purpose of tympanometry is to describe physical characteristics of the tympanic membrane. The measured dB SPL during tympanometry is dependent on the initial level of the probe tone (irrelevant), the remaining volume of the ear canal (can be relevant) and the acoustic characteristics at the lateral surface of the tympanic membrane (directly relevant). In a group of normal individuals the measured dB SPL value would be all over the place depending on the initial probe tone level (eg. some manufacturers use 65 dB SPL, others use 70 dB SPL), how far the probe was positioned in the ear canal and the physical characteristics at the tympanic membrane. To make the dB SPL value interpretable, it would be necessary to standardize the probe tone level (has not been done) and account for the ear canal volume (we do do this). Even after doing these two things the result would be in sound units rather than in units that describe acoustic characteristics of the surface of the the tympanic membrane (relates more to the physical condition of the ear). It is more intuitive to convert the measured dB SPL value to represent the acoustic characteristics at the tympanic membrane (hence units like mmho). The same result will occur regardless of the different probe tone levels and regardless of the remaining volume of the external ear canal.

Gerald R Popelka, Ph.D.
Professor and Chief of Audiology
Otolaryngology-Head and Neck Surgery
Stanford University
Stanford, CA