For accurate REAG measurements it is necessary to position the tip of the probe tube close to the tympanic membrane so that the effect of standing waves is reduced. Previous research (Sullivan, 1988) has indicated that it is possible to estimate the location of a probe tip relative to the tympanic membrane by means of standing waves, but more recent measurements by Voss & Allen (1994) indicate that there may be some phase shift which, if large enough, would invalidate the procedure. A series of measurements using a 6 kHz tone were performed on 6 adult subjects to determine the location of the standing wave notch. The mean distance of the notch from the eardrum was found to be 14.1 mm. It was concluded that this is a reliable method of estimating the distance from the tympanic membrane to the standing wave notch and therefore it can be used with confidence to position the probe tip close to the tympanic membrane. Microphone location effects for hearing aids will also be discussed briefly with particular reference to measurement results for CIC hearing aids which indicate that, although CICs give a high frequency boost, the boost additional to that available from an ITC is less than generally believed.