The information provided in this answer is derived from the British Society of Audiology (BSA) Recommended Procedure for Tympanometry (page 15).
The value of ear canal volume should be disregarded when high-frequency probe tones are used because it will not be precise.
Both ear canal volume and middle ear pressure should be disregarded when using a 1-kHz probe tone. At 226 Hz, mmho and cm3 are interchangeable; however, this is not the case at higher frequencies where it is not possible to accurately calculate cavity volumes. Without an accurate measure of volume it is not possible to calculate pressure, since pressure = density/volume.
The exception is for use as an indicator of a possible blockage (i.e. very small volume given), although this should be verified (e.g. otoscopy or checking the probe). It is recommended that the traces recorded are classified as normal or abnormal using a classification system reported by Baldwin (2006; adapted from Marchant et al 1986).