How We Hear
The Outer Ear
Hearing begins when sound waves are transmitted through the air. The outer ear – the part you see on a person’s head – acts like a funnel. It scoops sounds from the air and sends them into a narrow passage called the ear canal.
The sounds rush along the canal until they bump into the ear drum. The eardrum is a very delicate membrane which absorbs sound energy and moves in rhythm with it. The higher the sound, the faster the eardrum moves. The louder the sound, the bigger the size of each movement.
Sound is made up of tiny OSCILLATIONS or "waves" in air pressure. The more waves there are in a given unit of time, the higher the pitch of the sound will be. For example, a violin has many oscillations per second a fog horn has fewer. The number of waves per second is called the FREQUENCY of a sound and is measured in Hertz (cycles per second). A louder sound makes a bigger wave. The size of sound wave is known as the AMPLITUDE, with loud sounds called high amplitude and soft sounds, low amplitude.
The Middle Ear
The eardrum moves thousands of times each second and each of these movements is transferred to a "chain" of three bones, which are located in the middle ear. These bones are commonly known as the hammer, anvil and stirrup (their medical names are the malleus, incus and stapes). One end of the chain is attached to the eardrum and the other end lies against an opening called the "oval window". When the eardrum moves, it causes the bones to vibrate and they, in turn, transfer these vibrations to the oval window.
The Inner Ear
If the oval window were a real window and you were standing in front of it taking in the view, you would be staring into the COCHLEA, a snail-shaped organ which is located in the inner ear and which contains thousands of tiny hair cells suspended in fluid. The vibrations carried by the chain of bones are transmitted into the cochlea through the oval window.
When the sound first reaches the ear, the vibrations were carried by air. In the middle ear, they are conducted by solids: membranes and tiny bones. Now, upon entering the cochlea, the vibrations are carried by liquid: a fluid known as PERILYMPH.
The fluid carries the sound waves to a thin film known as REISSNER’S MEMBRANE, which passes them on to a second cochlear fluid known as ENDOLYMPH. From here, the vibrations swim along until they bump into something called the BASILAR MEMBRANE, which vibrates and passes this motion on to the hair cell.
The hair cells can be thought of as triggers because each one is connected to a nerve cell which fires off an electrochemical impulse whenever a hair cell moves. There are many individual hair cells connected to many individual nerves, but all the nerve cells join together to form what is called the AUDITORY NERVE. This nerve is like a highway along which impulses travel to reach the brain.
More complex and sophisticated than any man-made computer, the brain interprets the different signals coming to it from the auditory nerve, distinguishing between different sounds, telling us if they are loud or soft, high or low, pleasant or screechy, irrelevant or loaded with meaning, such as the sounds entering the left ear to those coming in through the right ear. Through this stereo or BINAURAL hearing, the brain can tell us the source of a sound, for example, whether it is far or near, behind or in front, to the left or right.
Sound may also reach the ear through other routes, for example, by bone conduction through the skull, but the pathway through the ear is the most important. The main organ of balance, the semi-circular canals, lies in the inner ear near the cochlea. Because of this, disorders of the inner ear may involve the organ of balance and the cochlea together. For example, in Meniere’s Syndrome the patient may encounter hearing loss, vertigo and balance problem which often worsens in the dark because vision helps balance.