Hearing is Moving

Hearing is Moving

Chris Bresee

Picture yourself listening to Yo-yo Ma playing the cello. You sit relatively still, head pointed in the performer's direction, while his arm guides his bow across the strings, setting off vibrations that are amplified by the large hollow resonating chamber of the instrument. His other hand traces intricate patters as he changes the length of the strings to allow them to vibrate at different frequencies. He is using many muscles and joints simultaneously to produce very complex movements. Compared to this, the act of listening seems relatively inactive. But in order to allow you to hear sound, your ears are doing something that is arguably just as complex and active as Yo-yo Ma's cello playing, all without even the need for your awareness, let alone conscious direction.

In this post and the next,I will explain the process that lets you hear. The process of hearing can be divided roughly into four parts: capture and amplification of the sound waves, separation of the sound waves by pitch, conversion of the mechanical energy of sound waves into the electrical energy of nerve impulses (transduction), and processing of those impulses by the brain. Each of these four parts is very complex, and we don't actually know how it all happens yet, so here I'll focus on the organizing principles of the system instead of the details.

Amplification:

Sound waves hit the outer ear (the part of the ear you can see) and are funneled into the earcanal (the little tube into the side of your head that you've been told not to poke pencils into). At the end of the canal is your eardrum, a very thin membrane completely covering the end of the tube. Sound (as pressure pulses in the air) vibrates this membrane like the head of a drum, and this causes a chain of tiny bones touching the other side of your eardrum to vibrate as well. The other side of the eardrum is the middle ear, an air-filled space in your skull that contains the three middle ear bones (ossicles), the smallest bones in your body. These bones are linked together like levers in a way that exaggerates the vibrations, and they are attached to muscles (also the smallest ones in your body) that control how much these bones amplify the sound by varying the stiffness of the joints. If the muscles are tense, making the joints stiffer, the bones move less, and the sound is amplified less.

Separation:

The last bone in this chain (the stirrup) is connected like a piston to a fluid-filled space in your innerear, specifically to a part that looks like a snail shell, called the cochlea (which actually means snail shell in Latin). Another membrane runs down the middle of your cochlea (the basilar membrane). The very top of the cochlea has a membrane

that is narrow but thick, and the very bottom has a membrane that is wide and thin. As the stirrup bone pushes and pulls on the inner ear like a piston, pressure waves travel up the length of the cochlea. High-pitched sound isn't strong enough to vibrate the stiff membrane at the top of the cochlea, but it is strong enough to vibrate the floppier membrane at the bottom, while low pitches can vibrate the stiff membrane at the top. This phenomenon is called resonance.

So if Yo-yo Ma plays the A note above middle C, causing vibrations to go through the air and hit your ear 440 times per second, then your middle ear bones will move up and down 440 times per second, causing a particular part of the membrane in your inner ear to move up and down 440 times per second. In the next post I'll discuss how these vibrations are translated into electrical energy in your brain which allows you to hear the A note.