Written by Travis M. Moore
Last edited 31-Jan-2020
The admittance of the middle ear (ME) system is measured using a technique called tympanometry. Tympano refers to the tympanic membrane (TM), better known as the eardrum. The suffix "-metry" simply refers to the process of measuring. As is so often the case, the secret is revealed in the name! We measure admittance of the ME by making measurements involving the eardrum. It makes sense that this is the case, seeing as the TM makes up a large part of the lateral wall of the ME.
Picture the ME as a coffee mug with a balloon stretched across the top of it, held in place with a rubber band. Imagine that the mug is empty and you poke the balloon. You push on the balloon with your finger until the tension of the balloon won't allow you to move your finger any further. You make a measurement and find that your fingertip has made it 2 inches into the mug.
Still with me? Now let's fill the mug to the brim with sand. You push on the balloon with your finger again, and now you can only reach 0.5 inches into the mug. Is the difference due to the balloon? Of course not: the difference is due to the sand behind the balloon. So even without looking inside the mug you can easily tell between empty and filled with sand based on how far your finger protruded into the mug. This scenario is more or less what we are doing with immittance measures: we're getting an idea of the status of the ME (e.g., is it filled with fluid?) without having to see the ME. For example, if our admittance value is extremely low, that means almost all the sound presented to the ear canal has been reflected off the TM. A likely culprit would be a ME filled with fluid (like the mug filled with sand).
Tympanometry replaces the mug/balloon with the ME, and the depth of the finger with sound pressure level. The actual measurement of admittance is accomplished with a tiny speaker and microphone sealed in the ear canal. The more sound the microphone picks up, the lower the admittance value. The less sound the microphone picks up, the better sound is moving through the ME, and the higher the admittance value.
An interesting thing happens as the TM is stretched tightly: its impedance value rises. With enough tension on the TM the acoustic impedance becomes infinite, and no sound is transmitted through it; the ME is essentially cut off from sound in the ear canal. If we add an air pump to the speaker and microphone sealed in the ear canal, we can increase the air pressure and manipulate the tension on the TM. If we increase the air pressure a little, the TM tightens a little, and a little more sound is reflected back to the microphone. If we increase the ear canal pressure until the TM is stretched tight we can prevent sound from entering the ME altogether.
Changing the impedance of the TM is a neat party trick, but why would we ever want to do that? There are actually two reasons.