Estimating the location of probe microphones relative to the tympanic membrane

This experiment investigated the accuracy with which the location of a probe tip relative to the tympanic membrane can be estimated by means of standing waves. The ear canal length of each ear of six adult subjects was measured with a probe tube using a tactile method. A 6-kHz warble tone was then generated, and the position of the standing wave notch in the ear canal was determined using a probe microphone by noting the place where the sound pressure level was at a minimum. The distance of the notch from the tympanic membrane was then calculated. The mean distance of the notch from the tympanic membrane was found to be 14.1 mm. It was concluded that this technique is reliable and suitable for clinical use when it is important that the probe tube be placed within a known distance of the eardrum for accurate measurement of real-ear aided gain.