Amplifying the sound increases the energy, which is necessary for the vibrations to travel through the fluid of the inner ear. Fluid offers more resistance than air, so requires a greater force to push through.
If a sound is too loud, muscles in the middle ear constrict to reduce the effects of the sound as an attempt to protect the inner ear. This is known as acoustic reflex. However, a sudden, loud noise can cause immediate, permanent damage to the ear because of a delay when the auditory nerve must first respond to the sound before the muscles in the middle ear can contract.
The vibration of the stirrup against the oval window transmits the pattern of the sound wave to the inner ear and the fluid in the upper and lower galleries of the cochlea.
Sound waves start the motion of the hair cells on the basilar membrane. Each frequency of the sound wave affects a specific section of the basilar membrane, thereby stimulating a response in the hair cells exactly at that location. If the sound has a very high frequency, the basilar membrane resonates with cells near the base of the cochlea; but, if the sound wave has a low frequency, the basilar membrane resonates closer to the tip of the cochlea.
Hair cells are set in motion by the sound wave and vibrate against the tectorial membrane, displacing the cilia on the hair cells. This results in a chemical reaction within the hair cells which, in turn, triggers electrical responses in the auditory nerve. The louder or more intense the sound, the more impulses are set off.
Electrical impulses travel through the auditory nerve to various information-processing stations in the brain, which begin to process sounds to determine their origin. The circuits end in the auditory cortex, located in the temporal lobes on each side of the brain.
The cortex, commonly called gray matter because of its grayish, wrinkled appearance, is a thin layer of tissue where most of the sorting, processing, and filing of information is carried out.
At the same time, there is also plenty of communication between the right and left temporal lobes so that signals can be compared. The comparisons and analyses in the stations and the auditory cortex also play a role in suppressing background noise and in allowing a person to focus on the sounds he wants to hear.
Scientists are still trying to understand how the brain interprets messages from the cochlea and identifies them as distinct sounds. At low frequencies, the electrical impulses follow the same pattern as the sound waves; but, at high frequencies, the pattern varies.
The brain gives meaning to sound through speech and language and is closely associated with the ability to hear – an ability that begins at birth. For example, at 3 months of age, babies are able to tell the difference between its parents’ voices and those of others.
A good and simple diagram of the ear can be seen on the Enchanted Learning site.
Peripheral hearing mechanism consists of both the conductive and sensorineural mechanisms.
- Conductive mechanism (involves outer and middle ear)
- Sensorineural mechanism (involves inner ear and the auditory nerve)
Hearing loss involving both can result in mixed hearing loss.
Central auditory mechanism involves the central auditory pathway which consists of the ascending pathway (corpus callosum, auditory cortex, primary auditory cortex, auditory radiation, medial geniculate body, medial geniculate body, inferior colliculi, commisure of IC, lateral lemniscus, superior olivary complex, trapezoid body, cochlear nuclei, dorsal cochlear nuclei, ventral cochlear nuclei).
Hearing loss involvement produces central auditory processing disorders.