Superpowers of the Ear: The Middle Ear

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In a previous article we learned about the superpowers of the outer ear and promised we would tell you more about the middle ear, as well as that little drumhead at the end of your ear canal—the tympanic membrane.

The middle ear is a pocket of air bounded by the ear drum on the outer side and the cochlea on the inner side. The middle ear is home to the three smallest named bones in the human body: the malleus, the incus, and the stapes. These three bones—more popularly known by their nicknames hammer, anvil, and stirrup—are in charge of overcoming impedance mismatch in the ear (more on that later).

The cochlea (which we will talk about more in the next post) is filled with a water-based fluid. In order to get vibrations from the air into the cochlea, something drastic must happen to the vibrations. They must be amplified.

Why do the vibrations need to be amplified? Think of it this way: is it harder to move your hand back and forth through air or through water? Water is much denser and requires more effort to move. You could say that the water impedes your motion more than the air. This is what is known as an impedance mismatch. In order for airborne sound waves to continue into the cochlea they need a boost. This is where the bones come in.

The motion of the ear drum is transferred to the motion of the ossicle chain (the three bones), effectively multiplying the force of the vibrations 30 times. This amplification comes from the difference in area between the ear drum (which is comparatively large) at one end of the ossicle chain, and the oval window (which is comparatively small) on other.

Mathematically, a force (F) acting on an area (A) is a pressure (P):

P = \frac{F}{A}

We know from our discussion on moving hand through air and water that we want to have enough force to overcome the impedance mismatch between the air and the water. Pressure fluctuations in the air (sound waves) are amplified 30 times by the decrease in area so the ossicles can act like a piston to transfer vibrations into the fluid of the cochlea. Without this transfer, you wouldn’t be able to hear.

P_{at\ cochlea} = \frac{30F}{A}

In addition to the impedance matching powers of the middle ear, the middle ear is also important for popping your ears. You know when you go up in a plane or when you change altitude on a drive and everything starts to sound muffled? Then you yawn or swallow or chew gum (whatever you do to adjust your ears) and there is a popping sound? That popping noise is the sound of air entering the middle ear through the Eustachian tube to balance the air pressure on both sides of your ear drum so it can vibrate freely.

So, in summary, the middle ear has two main superpowers. It performs the delicate work of transferring vibrations from the air to the fluid of the inner ear and it lets you rebalance the pressure on your ear drum.

Next, we’ll learn about what is (arguably) the most exciting part of the ear—the inner ear—where things start to get other-worldly.


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