Hearing Damage

The Outer Ear

The external part of the ear is called the outer ear. It comprises of the pinna and the ear canal. The specific shape of the pinna helps to localize the sound source location in space and to concentrate the acoustic wave to the ear canal as a funnel. The ear canal transmits the acoustic wave collected to the middle ear through the tympanic membrane, which is also known as the eardrum.

The childhood belief that, like the tongue is crucial for speaking, the pinnae are crucial for hearing is not accurate. Someone without pinnae will still be able to hear, although sounds will be perceived softer and will be difficult to localize. Fortunately, prosthetic pinnae can be fitted to restore hearing to normal. The ear canal is a tube that may be obstructed by a foreign body or by an infection. Depending on the degree of the infection, it can usually be treated easily with ear drops or antibiotics.

The Middle Ear

The middle ear's function is to adapt the impedance of an acoustic wave traveling through the air to a wave traveling in a fluid. Without the middle ear, most of the acoustic energy will be reflected out of the cochlea.

If structural damage occurs to the ossicular chain (the three tiny bones known as the malleus, incus, and stapes), the patient will have a hearing loss and only perceive very high-level sounds. Hearing may be restored either through surgery or by using a bone-anchored hearing aid that transmits sound to the inner ear through bone conductions. As for the ear canal, the middle ear can be filled with fluid caused by an infection (otitis media). This type of infection is common in childhood and, if frequent, needs to be treated in order to avoid any developmental disorders (other problems as well as rupture to the eardrum). Treatments can be either through drug therapy or by inserting a small tube in the eardrum (called a “grommet”) that allows the fluid to drain in a controlled manner.

This hearing-damage cause is called conductive, which means that all the frequencies will be attenuated; however, high frequencies will be more affected than lower frequencies.

The Inner Ear

The inner ear is the most important part of the ear but also the most fragile and difficult to heal. It is the home of two types of hair cells: the outer hair cells and the inner hair cells. These cells are located on the basilar membrane that follows the spiral-shaped part of the inner ear, the cochlea, and has a resonance frequency that decreases with its length. Therefore, sounds vibrate different parts of the membrane depending on their frequency. The outer hair cells' main function is to amplify the energy around a specific place on the membrane to improve frequency discrimination. The inner hair cells transmit the acoustic vibration to the auditory nerves via electrochemical reaction. Both hair cells can be damaged by many causes, including loud noise/sound exposure, ototoxic drugs, or autoimmune diseases.

...