Equivalent Ear Canal Volume

The purpose of tympanometry is to measure the middle-ear admittance under varying ear canal air pressure

However, admittance measured at the probe tip reflects the COMBINED admittance of the external auditory canal and and the admittance of the ME.

Accuracy of the ME admittance relies on obtaining an accurate estimate of the "equivalent" ear canal volume. If the admittance of the ear canal can be measured independently of the admittance of the ME, then just the admittance of the tympanic membrane can be measured.

Ear canal volume is compensated using a baseline method by subtracting out the admittance at either the positive or negative "tail" of the tympanogram

Most clinical immittance units provide a baseline correction feature when measuring tympanometry. It subtracts the equivalent ear canal volume so that the tympanogram is plotted with admittance starting at 0 mmho.

Baseline compensation may be done either at an extreme positive value, usually +200 daPa, or an extreme negative value, such as -400 daPa. At these values, it is assumed that the middle ear is stiff enough to cause its admittance to be 0, so the admittance measured at the probe tip is just from the ear canal. So at these pressures, 1 cubic centimeter of trapped air = the acoustic admittance of 1 mmho.

-Basically, the admittance at an extreme positive value (like +200 daPa) or an extreme negative value (-400 daPa) is just the admittance of the ear canal and so that value is the ECV and is then subtracted out to give you just the admittance of the ME

Clinical measurements of ear canal admittance (volume) are most commonly taken from the admittance positive tail with a 226-Hz probe tone, which overestimates ECV by 40%

	Created by Sara Wagner almost 10 years ago

Notes from our powerpoint slides to supplement the notes in my mind map