How Hearing Works

We saw in the last section that sound travels through the air as vibrations in air pressure. To hear sound, your ear has to do three basic things:

• Direct the sound waves into the hearing part of the ear
• Sense the fluctuations in air pressure
• Translate these fluctuations into an electrical signal that your brain can understand

The pinna, the outer part of the ear, serves to "catch" the sound waves. Your outer ear is pointed forward and it has a number of curves. This structure helps you determine the direction of a sound. If a sound is coming from behind you or above you, it will bounce off the pinna in a different way than if it is coming from in front of you or below you. This sound reflection alters the pattern of the sound wave. Your brain recognizes distinctive patterns and determines whether the sound is in front of you, behind you, above you or below you.

Ear diagram courtesy NASA

Your brain determines the horizontal position of a sound by comparing the information coming from your two ears. If the sound is to your left, it will arrive at your left ear a little bit sooner than it arrives at your right ear. It will also be a little bit louder in your left ear than your right ear.

­Since the pinnae face forward, you can hear sounds in front of you better than you can hear sounds behind you. Many mammals, such as dogs, have large, movable pinnae that let them focus on sounds from a particular direction. Human pinnae are not so adept at focusing on sound. They lay fairly flat against the head and don't have the necessary muscles for significant movement. But you can easily supplement your natural pinnae by cupping your hands behind your ears. By doing this, you create a larger surface area that can capture sound waves better. In the next section, we'll see what happens as a sound wave travels down the ear canal and interacts with the eardrum.

Once the sound waves travel into the ear canal, they vibrate the tympanic membrane, commonly called the eardrum. The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4 inches) wide. It is positioned between the ear canal and the middle ear. The middle ear is connected to the throat via the eustachian tube. Since air from the atmosphere flows in from your outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. This pressure balance lets your eardrum move freely back and forth.

The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound wave.

Ear illustration courtesy NIDCD Normal ear anatomy

This tiny flap of skin acts just like the diaphragm in a microphone. The compressions and rarefactions of sound waves push the drum back and forth. Higher-pitch sound waves move the drum more rapidly, and louder sound moves the drum a greater distance.

The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch noises. When the brain receives a signal that indicates this sort of noise, a reflex occurs at the eardrum. The tensor tympani muscle and the stapedius muscle suddenly contract. This pulls the eardrum and the connected bones in two different directions, so the drum becomes more rigid. When this happens, the ear does not pick up as much noise at the low end of the audible spectrum, so the loud noise is dampened.

In addition to protecting the ear, this reflex helps you concentrate your hearing. It masks loud, low-pitch background noise so you can focus on higher-pitch sounds. Among other things, this helps you carry on a conversation when you're in a very noisy environment, like a rock concert. The reflex also kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a lot of the other sounds around you.

The eardrum is the entire sensory element in your ear. As we'll see in the coming sections, the rest of the ear serves only to pass along the information gathered at the eardrum.

We saw in the last section that the compressions and rarefactions in sound waves move your eardrum back and forth. For the most part, these changes in air pressure are extremely small. They don't apply much force on the eardrum, but the eardrum is so sensitive that this minimal force moves it a good distance.

As we'll see in the next section, the cochlea in the inner ear conducts sound through a fluid, instead of through air. This fluid has a much higher inertia than air -- that is, it is harder to move (think of pushing air versus pushing water). The small force felt at the eardrum is not strong enough to move this fluid. Before the sound passes on to the inner ear, the total pressure (force per unit of area) must be amplified.

This is the job of the ossicles, a group of tiny bones in the middle ear. The ossicles are actually the smallest bones in your body. They include:

• The malleus, commonly called the hammer
• The incus, commonly called the anvil
• The stapes, commonly called the stirrup

Sound waves vibrate the eardrum, which moves the malleus, incus and stapes.

The malleus is connected to the center of the eardrum, on the inner side. When the eardrum vibrates, it moves the malleus from side to side like a lever. The other end of the malleus is connected to the incus, which is attached to the stapes. The other end of the stapes -- its faceplate -- rests against the cochlea, through the oval window.

When air-pressure compression pushes in on the eardrum, the ossicles move so that the faceplate of the stapes pushes in on the cochlear fluid. When air-pressure rarefaction pulls out on the eardrum, the ossicles move so that the faceplate of the stapes pulls in on the fluid. Essentially, the stapes acts as a piston, creating waves in the inner-ear fluid to represent the air-pressure fluctuations of the sound wave.

The ossicles amplify the force from the eardrum in two ways. The main amplification comes from the size difference between the eardrum and the stirrup. The eardrum has a surface area of approximately 55 square millimeters, while the faceplate of the stapes has a surface area of about 3.2 square millimeters. Sound waves apply force to every square inch of the eardrum, and the eardrum transfers all this energy to the stapes. When you concentrate this energy over a smaller surface area, the pressure (force per unit of volume) is much greater. To learn more about this hydraulic multiplication, check out How Hydraulic Machines Work.

The configuration of ossicles provides additional amplification. The malleus is longer than the incus, forming a basic lever between the eardrum and the stapes. The malleus moves a greater distance, and the incus moves with greater force (energy = force x distance).

This amplification system is extremely effective. The pressure applied to the cochlear fluid is about 22 times the pressure felt at the eardrum. This pressure amplification is enough to pass the sound information on to the inner ear, where it is translated into nerve impulses the brain can understand.

The cochlea is by far the most complex part of the ear. Its job is to take the physical vibrations caused by the sound wave and translate them into electrical information the brain can recognize as distinct sound.

The cochlea structure consists of three adjacent tubes separated from each other by sensitive membranes. In reality, these tubes are coiled in the shape of a snail shell, but it's easier to understand what's going on if you imagine them stretched out. It's also clearer if we treat two of the tubes, the scala vestibuli and the scala media, as one chamber. The membrane between these tubes is so thin that sound waves travel as if the tubes weren't separated at all.

The piston action of the stapes moves the fluid in the cochlea. This causes a vibration wave to travel down the basilar membrane.

The stapes moves back and forth, creating pressure waves in the entire cochlea. The round window membrane separating the cochlea from the middle ear gives the fluid somewhere to go. It moves out when the stapes pushes in and moves in when the stapes pulls out.

The middle membrane, the basilar membrane, is a rigid surface that extends across the length of the cochlea. When the stapes moves in and out, it pushes and pulls on the part of the basilar membrane just below the oval window. This force starts a wave moving along the surface of the membrane. The wave travels something like ripples along the surface of a pond, moving from the oval window down to the other end of the cochlea.

The basilar membrane has a peculiar structure. It's made of 20,000 to 30,000 reed-like fibers that extend across the width of the cochlea. Near the oval window, the fibers are short and stiff. As you move toward the other end of the tubes, the fibers get longer and more limber.

This gives the fibers different resonant frequencies. A specific wave frequency will resonate perfectly with the fibers at a certain point, causing them to vibrate rapidly. This is the same principle that makes tuning forks and kazoos work -- a specific pitch will start a tuning fork ringing, and humming in a certain way will cause a kazoo reed to vibrate.

As the wave moves along most of the membrane, it can't release much energy -- the membrane is too tense. But when the wave reaches the fibers with the same resonant frequency, the wave's energy is suddenly released. Because of the increasing length and decreasing rigidity of the fibers, higher-frequency waves vibrate the fibers closer to the oval window, and lower frequency waves vibrate the fibers at the other end of the membrane. In the next section, we'll look at how tiny hairs help us hear sound.

In the last section, we saw that higher pitches vibrate the basilar membrane most intensely near the oval window, and lower pitches vibrate the basilar membrane most intensely at a point farther down the cochlea. But how does the brain know where these vibrations occur?

This is the organ of corti's job. The organ of corti is a structure containing thousands of tiny hair cells. It lies on the surface of the basilar membrane and extends across the length of the cochlea.

Until a wave reaches the fibers with a resonant frequency, it doesn't move the basilar membrane a whole lot. But when the wave finally does reach the resonant point, the membrane suddenly releases a burst of energy in that area. This energy is strong enough to move the organ of corti hair cells at that point.

When these hair cells are moved, they send an electrical impulse through the cochlear nerve. The cochlear nerve sends these impulses on to the cerebral cortex, where the brain interprets them. The brain determines the pitch of the sound based on the position of the cells sending electrical impulses. Louder sounds release more energy at the resonant point along the membrane and so move a greater number of hair cells in that area. The brain knows a sound is louder because more hair cells are activated in an area.

The cochlea only sends raw data -- complex patterns of electrical impulses. The brain is like a central computer, taking this input and making some sense of it all. This is an extraordinarily complex operation, and scientists are still a long way from understanding everything about it.

In fact, hearing in general is still very mysterious to us. The basic concepts at work in human and animal ears are fairly simple, but the specific structures are extremely complex. Scientists are making rapid advancements, however, and they discover new hearing elements every year. It's astonishing how much is involved in the hearing process, and it's even more amazing that all these processes take place in such a small area of the body.