Abstract: A probe tube is inserted into the ear canal of a patient to a first position in which the probe tube tip is spaced well away from the ear drum. A broad band sound signal is provided to the patient's ear and a frequency response of sound detected at the probe tube tip is taken. The probe tube tip is then moved a known distance to a second position within the ear canal, the broad band sound stimulus signal is again applied, and the frequency response of sound detected by the probe tube tip is taken. The distance of the probe tube tip from the ear drum can be determined from the two frequency responses, particularly by subtracting the frequency response in dB at one position from the frequency response in dB at the other position, and the difference frequency response will show a distinct peak at a quarter wavelength null frequency, which allows the distance of the probe tube tip from the ear drum to be estimated based on the relationship.