Like a telephone, the ear translates sound waves into electrical impulses. What we perceive as sound starts out as vibration, which spreads in waves, usually though the air. To get to the brain the waves must be transformed into electrical impulses, and this happens in the ear. The ear has three main parts, the outer, middle, and inner ear, and they work in sequence to interpret sound. The ear is also the organ of balance, and helps us orient ourseves in the physical world.
The outer ear collects sound. It is shaped to gather up sound waves and channel them into the ear canal. The pinna or auricle, the visible ear, is designed so that the noises coming from in front of a person are perceived as louder. Almost everyone’s ears stick out a little in back to scoop in sounds from in front. This naturally makes a listener turn toward what he is trying to hear, and makes someone in a conversation turn toward the people she is talking with. The folds in the outer ear are designed to bounce the sound around in such a way that the brain receives clues about the quality of the noise. Because vertebrates have two ears, they can tell which side the sound is coming from. The ear on the side towards the noise sends a louder signal to the brain, and from this difference, the brain can instantly calculate distance and direction.
From the pinna the sound wave is sent down the ear canal. It reaches the eardrum next, the entrance to the middle ear. The eardrum, also called the tympanum, is a diaphragm, a tightly stretched piece of specialized skin, and it vibrates in time to the sound waves that hit it. It vibrates faster for high sounds and slower for low sounds. The vibrations get bigger as the sound wave coming in is louder and slower for softer sounds. This changes the mechanical motion of the sound wave into a vibration.
The middle ear amplifies the sound. There’s a small bone that touches the inner side of the eardrum. It’s called the malleus, or hammer, because of its shape. It transfers the vibration from the eardrum on into the middle ear, by vibrating in time with the diaphragm. It transfers the sound to the anvil, the next bone along, and from there the vibration goes to the stirrup bone. The muscle surrounding these bones is generally slack, letting them vibrate, but can tighten up in a noisy situation, to slow he motion of the bones, damp the vibration and protect the ear. As the vibration goes through the small bones of the middle ear, it is amplified, in the same way that water pressure is amplified in a smaller diameter hose because the same push is applied over a smaller area. The hammer (malleus), anvil (incus) and stirrup (stapes), collectively called the ossicles, are the smallest bones in the body.
Next, the inner ear transforms the sound. The end of the stirrup bone connects to the inner ear, at an oval window in the cochlea. The spiral-shaped cochlea is called the organ of hearing, because it is here that the sound we perceive is actually created. Up to this point, sound has been a wave and then a vibration, and it traveled through air and bone. In the liquid-filled cochlea, it will be transformed into electricity. When the tiny stirrup bone presses rhythmically on the oval window of the cochlea, the liquid inside this coiled chamber moves. The cochlea is indeed curled in a spiral, but the tube forming the spiral is a tapered cone-shaped passage. If the cochlea were uncoiled, it would appear to be a long tapered cone, filled with fluid and divided by a membrane set with rows of hair cells. When the fluid in the cochlea moves, it moves the membrane. The membrane’s motion moves the hair cells set along it. The shape of the cochlea, its long taper, guides the vibration of the membrane so that it moves hair cells, or groups of hair cells, at a point and to a degree corresponding to the sound’s pitch, volume, and timbre. The activation of the cells is in a pattern analogous to the sound wave they receive. Their motion sends signals to bundles of nerves at the root of each cell. These nerves send electrical impulses (by trading ions) to the spiral ganglia, and from there to the auditory portion of the eighth cranial nerve, which goes to the brain. The brain interprets these impulses (that’s another story), and we hear.
The other major system of the inner ear is the vestibular system. The vestibular system uses the same method, hair cells detecting motion in fluid, to gather information about the body’s motion and it’s location in relation to gravity. The vestibular system shares space with the cochlea, and includes three semi-circular canals, at different orientations, where liquid moves. This system sends nerve messages to the body muscles, so the body can stay upright and oriented while we move, and also to muscles near the eyes, so they can compensate for motion of the head and still stay focused on what they are looking at. That’s why we can shake our head and still read.
The ears are well protected. Their delicate inner working are surrounded by hard bone. They have a system to damp down noise, as well as one to amplify it, but noisy modern life can overwhelm these protections. Probably everyone who lives in America has some hearing loss with age. Our ears are ill-defended against sudden onslaughts of sound, such as from car backfires, and our hearing is also degraded by long-term exposure to environments that are just too loud. Everyone knows not to play music on high, but we should also remember to avoid constant low level noise, and be proactive in creating a quiet space for ourselves. It’s good to live life at a more peaceful level, so that we can appreciate the subtleties of sound and preserve the gifts our ears bring us.
http://arts.ucsc.edu/EMS/Music/tech_background/TE-03/teces_03.html
http://thalamus.wustl.edu/course/audvest.html
