The Ear

See also loudness perception and dBA/dB scales

The above image was taken from http://www.harford.cc.md.us/Faculty/wrounds/ear.gif

The ear has three main parts:

The outer ear (the part you can see) opens into the ear canal. The eardrum separates the ear canal from the middle ear. Small bones in the middle ear help transfer sound to the inner ear. The inner ear contains the auditory (hearing) nerve, which leads to the brain.

Any source of sound sends vibrations or sound waves into the air. These funnel through the ear opening, down the ear, canal, and strike your eardrum, causing it to vibrate. The vibrations are passed to the small bones of the middle ear, which transmit them to the hearing nerve in the inner ear. Here, the vibrations become nerve impulses and go directly to the brain, which interprets the impulses as sound.

Click here for a page to take you through revision of how sound is propagated.

A collective name for the bones in the ear is the ossicles

A collective name for the vestibular and cochlear nerves is the auditory nerve

Check your knowledge of this by trying the crossword

The pinna, the outer part of the ear, serves to "catch" the sound waves. It also helps you determine the direction of a sound. Your brain determines the horizontal position of a sound by comparing the information coming from your two ears.

Once the sound waves travel into the ear canal, they vibrate the tympanic membrane or eardrum. This 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. Air from the atmosphere flows in from your outer ear onto one side and from your mouth on the other (the middle ear is connected to the throat via the eustachian tube) so the air pressure on both sides of the eardrum remains equal. This pressure balance lets your eardrum move freely back and forth with even the slightest air-pressure fluctuations. Higher-pitch sound waves move the drum more rapidly (higher frequency), and louder sound moves the drum a greater distance (bigger amplitude). The eardrum is the entire sensory element in your ear. The rest of the ear serves only to pass along the information gathered at the eardrum.

For the most part, the changes in air pressure due to sound waves we hear 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. However, the cochlea in the inner ear conducts sound through a fluid, instead of through air. This fluid has a much higher inertia than air so the small force felt at the eardrum is not strong enough to move this fluid. Therefore before the sound passes on to the inner ear, the total pressure (force per unit of volume) must be amplified.

This amplification is caried out by the ossicles:

- The malleus - the hammer

- The incus - the anvil

- The stapes - the stirrup

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 rests against the cochlea, through the oval window.

When air-pressure compression pushes in on the eardrum, the ossicles move so that the stapes pushes in on the cochlear fluid. When air-pressure rarefaction pulls out on the eardrum, the ossicles move so that 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.

- 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 by the cochlea.

The cochlea structure consists of three adjacent tubes separated from each other by sensitive membranes. These tubes are coiled in the shape of a snail shell. The membrane between these tubes is so thin that sound waves travel as if the tubes weren't separated at all. 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.

Click here for a diagram of how this works (it 'opens out' the snail shell arrangement for clarification!)

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 less rigid. 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 with a big amplitude. 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. 

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.

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.