Human Hearing How the ear works Notes from How Hearing Works by Tom Harris.

Human HearingHuman HearingHow the ear worksHow the ear worksNotes from How Hearing Works by Tom HarrisNotes from How Hearing Works by Tom Harris

Ear Structure and Ear Structure and HearingHearing

Unlike other senses, the ear uses a mechanical Unlike other senses, the ear uses a mechanical process, not chemical.process, not chemical.

Uses physical movement.Uses physical movement.

Much like a microphone, components of our Much like a microphone, components of our ears vibrate along with changes in air pressure.ears vibrate along with changes in air pressure.

Compression and rarefactionCompression and rarefaction

Three Necessary Functions of the Three Necessary Functions of the EarEar

DirectDirect the sound waves into the hearing part the sound waves into the hearing part of the earof the ear

SenseSense the fluctuations in air pressure the fluctuations in air pressure

TranslateTranslate these fluctuations into an electrical these fluctuations into an electrical signal that your brain can understand signal that your brain can understand

DirectionalityDirectionality

The The pinnapinna, the outer part of the ear, serves to "catch" , the outer part of the ear, serves to "catch" the sound waves. Your outer ear is pointed forward and the sound waves. Your outer ear is pointed forward and it has a number of curves. it has a number of curves.

This structure helps you determine the direction of a This structure helps you determine the direction of a sound. If a sound is coming from behind you or above sound. If a sound is coming from behind you or above you, it will bounce off the pinna in a different way than if you, it will bounce off the pinna in a different way than if it is coming from in front of you or below you. it is coming from in front of you or below you.

This sound reflection alters the pattern of the sound This sound reflection alters the pattern of the sound wave. Your brain recognizes distinctive patterns and wave. Your brain recognizes distinctive patterns and determines whether the sound is in front of you, behind determines whether the sound is in front of you, behind you, above you or below you.you, above you or below you.

Ear diagram courtesy NASAEar diagram courtesy NASA

Your brain determines the horizontal position of Your brain determines the horizontal position of a sound by comparing the information coming a sound by comparing the information coming from your two ears. from your two ears.

If the sound is to your left, it will arrive at your If the sound is to your left, it will arrive at your left ear a little bit sooner than it arrives 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 right ear. It will also be a little bit louder in your left ear than your right ear.left ear than your right ear.

The EardrumThe Eardrum

Once the sound waves travel into the ear canal, they vibrate Once the sound waves travel into the ear canal, they vibrate the the tympanic membranetympanic membrane, commonly called the , commonly called the eardrumeardrum. .

The eardrum is a thin, cone-shaped piece of skin, about 10 The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4 inches) wide. It is positioned between the millimeters (0.4 inches) wide. It is positioned between the ear canal and the middle ear. ear canal and the middle ear.

The middle ear is connected to the throat via the The middle ear is connected to the throat via the eustachian tubeeustachian tube. Since air from the atmosphere flows in . Since air from the atmosphere flows in from your outer ear as well as your mouth, the air pressure from your outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. on both sides of the eardrum remains equal.

This pressure balance lets your eardrum move freely back This pressure balance lets your eardrum move freely back and forth and forth

The eardrum is rigid, and very sensitive. Even The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move the slightest air-pressure fluctuations will move it back and forth. It is attached to theit back and forth. It is attached to the tensor tensor tympani muscletympani muscle, which constantly pulls it , which constantly pulls it inward. inward.

This keeps the entire membrane taut so it will This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a vibrate no matter which part of it is hit by a sound wave. sound wave.

This tiny flap of skin acts just like the This tiny flap of skin acts just like the diaphragm in a microphone. The compressions diaphragm in a microphone. The compressions and rarefactions of sound waves push the and rarefactions of sound waves push the drum back and forth. drum back and forth.

Higher-pitch sound waves move the drum more Higher-pitch sound waves move the drum more rapidly, and louder sound moves the drum a rapidly, and louder sound moves the drum a greater distance. greater distance.

Ear illustration courtesy NIDCDEar illustration courtesy NIDCD

The eardrum can also serve to protect the inner ear The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch noises. from prolonged exposure to loud, low-pitch noises. When the brain receives a signal that indicates this When the brain receives a signal that indicates this sort of noise, a reflex occurs at the eardrum. sort of noise, a reflex occurs at the eardrum.

The tensor tympani muscle and the The tensor tympani muscle and the stapedius stapedius musclemuscle suddenly contract. This pulls the eardrum suddenly contract. This pulls the eardrum and the connected bones in two different directions, and the connected bones in two different directions, so the drum becomes more rigid. so the drum becomes more rigid.

When this happens, the ear does not pick up as When this happens, the ear does not pick up as much noise at the low end of the audible spectrum, much noise at the low end of the audible spectrum, so the loud noise is dampened (so the loud noise is dampened (Threshold ShiftThreshold Shift).).

In addition to protecting the ear, this reflex In addition to protecting the ear, this reflex helps you concentrate your hearing. It masks helps you concentrate your hearing. It masks loud, low-pitch background noise so you can loud, low-pitch background noise so you can focus on higher-pitch sounds. focus on higher-pitch sounds.

Among other things, this helps you carry on a Among other things, this helps you carry on a conversation when you're in a very noisy conversation when you're in a very noisy environment, like a rock concert. The reflex also environment, like a rock concert. The reflex also kicks in whenever you start talking -- otherwise, kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a the sound of your own voice would drown out a lot of the other sounds around you. lot of the other sounds around you.

Amplifying SoundAmplifying Sound

Uses Uses ossicles -ossicles - Human preamp Human preamp

The cochlea in the inner ear conducts sound The cochlea in the inner ear conducts sound through a fluid, instead of through air. through a fluid, instead of through air.

This fluid has a much higher inertia than air -- that This fluid has a much higher inertia than air -- that is, it is harder to move (think of pushing air versus is, it is harder to move (think of pushing air versus pushing water). The small force felt at the eardrum pushing water). The small force felt at the eardrum is not strong enough to move this fluid. is not strong enough to move this fluid.

Before the sound passes on to the inner ear, the Before the sound passes on to the inner ear, the total pressure (force per unit of area) must be total pressure (force per unit of area) must be amplified. amplified.

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

The The malleusmalleus, commonly called the hammer, commonly called the hammer

The The incusincus, commonly called the anvil, commonly called the anvil

The The stapesstapes, commonly called the stirrup , commonly called the stirrup

When air-pressure compression pushes in on the eardrum, When air-pressure compression pushes in on the eardrum, the ossicles move so that the faceplate of the stapes the ossicles move so that the faceplate of the stapes pushes in on the cochlear fluid. pushes in on the cochlear fluid.

When air-pressure rarefaction pulls out on the eardrum, When air-pressure rarefaction pulls out on the eardrum, the ossicles move so that the faceplate of the stapes pulls the ossicles move so that the faceplate of the stapes pulls in on the fluid. in on the fluid.

Essentially, the stapes acts as a piston, creating waves in Essentially, the stapes acts as a piston, creating waves in the inner-ear fluid to represent the air-pressure the inner-ear fluid to represent the air-pressure fluctuations of the sound wave.fluctuations of the sound wave.

This is hydraulic forceThis is hydraulic force

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

Fluid WaveFluid Wave

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

The cochlea structure consists of three adjacent tubes The cochlea structure consists of three adjacent tubes separated from each other by sensitive membranes. separated from each other by sensitive membranes.

In reality, these tubes are coiled in the shape of a snail shell, 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 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 them stretched out. It's also clearer if we treat two of the tubes, the tubes, the scala vestibuliscala vestibuli and the and the scala mediascala media, as one , as one chamber. chamber.

The membrane between these tubes is so thin that sound The membrane between these tubes is so thin that sound waves travel as if the tubes weren't separated at all. waves travel as if the tubes weren't separated at all.

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

Hair CellsHair CellsThe The organ of cortiorgan of corti is a structure containing is a structure containing thousands of tiny thousands of tiny hair cellshair cells. It lies on the surface . It lies on the surface of the of the basilarbasilar membrane and extends across the membrane and extends across the length of the cochlea.length of the cochlea.

When these hair cells are moved, they send an When these hair cells are moved, they send an electrical impulse through the cochlear nerve. The electrical impulse through the cochlear nerve. The cochlear nerve sends these impulses on to the cochlear nerve sends these impulses on to the cerebral cortex, where the brain interprets them. cerebral cortex, where the brain interprets them.

The brain determines the pitch of the sound based The brain determines the pitch of the sound based on the position of the cells sending electrical on the position of the cells sending electrical impulses. Louder sounds release more energy at impulses. Louder sounds release more energy at the resonant point along the membrane and so the resonant point along the membrane and so move a greater number of hair cells in that area. move a greater number of hair cells in that area.

Ear diagram courtesy NASAEar diagram courtesy NASA

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

Cochlear Implants