Principles of Acoustic Immittance


Written by Travis M. Moore
Last edited 31-Jan-2020


Quantifying the Middle Ear System

Simply put, the main job of the middle ear (ME) is to transfer sound entering the ear canal to the cochlea. That's an important role, so we need to be able to determine whether the ME is functioning properly. There are two handy measures we could use, and when we say "immittance" we are referring to both: impedance and admittance.

One way to assess the ME would be to present sound into the ear canal, and measure how much sound is reflected back from the tympanic membrane. The more sound reflected back, the worse the ME is functioning. You could even say the ME is impeding the sound it reflects back from traveling though it. In fact, the technical term for this phenomenon is impedance.

The other half of the story is admittance. Admittance is just the reciprocal of impedance. In other words, admittance describes how easily sound travels through the ME. This is the measure most clinical equipment actually measures. A good question would be "does it matter whether we use impedance or admittance? They're reciprocal, right?" It turns out that due to the physical and mechanical properties of the outer and ME, we get better measurements when we deal in admittance. Hunter and Shahnaz (2014) give two specific benefits of using admittance: (1) individual differences in the volume of the ear canal doesn't matter as much, and (2) admittance is more sensitive to changes in ME function than impedance.

To summarize, when we refer to "immittance testing," we are usually referring to measuring the admittance of the ME; how easily does sound travel through the ME system? The higher the admittance value, the better sound is making it from the ear canal to the cochlea via the ME.


Test Your Understanding

Impedance describes how poorly the ME is conducting sound, while admittance describes how well the ME is conducting sound.
A lot of sound is being reflected back out of the canal because the ME isn't letting it through very easily.

Next Topic: Tympanometry

REFERENCES

1Hunter, L. & Shahnaz, N. (2014). Acoustic immittance measures: Plural Publishing.
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