How Normal Hearing Functions:
The outer ear is made up of the outside part of the ear that you can see: the pinna and the ear canal. The outer ear collects sound waves from the environment and sends them down the ear canal to the eardrum (#1).
The middle ear is made up of the eardrum and three tiny connected bones (ossicles). These bones are called the malleus, incus and stapes. When sound waves travel through the canal to the eardrum , they cause the eardrum to vibrate (#2). This vibration sets the three tiny bones into motion. (#3, #4, #5). The inner ear is made up of the snail-shaped cochlea and hearing (auditory) nerve (#10 & #11). The motion of the middle ear bones causes fluid in the cochlea to move, which in turn sets the tiny hair cells inside the cochlea into motion. These hair cells convert this movement into electrical impulses. The electrical impulses travel to the hearing nerve, where they are processed in the brain and interpreted as sound.
The Outer Ear:
The outside portion of the ear that is visible is called the pinna, or auricle. The pinna funnels sounds towards our ear canal and provides a natural volume increase where we perceive many consonant sounds of speech. Our ear canal, also known as the external auditory meatus, is lined with only a few layers of skin and fine hairs which allow for some protection against small airborne foreign objects. Wax, or cerumen, accumulates in the ear canal and serves as a protective barrier to the skin from bacteria and moisture. Ear wax is natural and serves a purpose, unless it completely blocks the ear canal. An over production of ear wax can be maintained by using a type of oil specifically designed to soften the cerumen and allow it to drain out of the ear canal on its own. Occasionally a visit to your physician is necessary to remove any wax that has caused complete blockage of the ear canal.
The Middle Ear:
The tympanic membrane, most commonly known as the eardrum, is the barrier of skin between the outer and middle ear. Although a very thin membrane, the eardrum is made up of three layers to increase its strength. There are three tiny bones, referred to as the ossicles, in the middle ear which is located directly behind the tympanic membrane. These three bones form a connected chain in the middle ear. One of the bones is embedded in the innermost layer of the tympanic membrane, and the third bone is connected to a membranous window of the inner ear. The ossicles take mechanical vibrations received at the tympanic membrane into the inner ear. The Eustachian tube is the middle ear’s air pressure equalizing system. The middle ear is encased in bone and does not associate with outside air except through the Eustachian tube. The Eustachian tube is normally closed, but can be involuntarily opened by swallowing, yawning or chewing. It can also be intentionally opened to equalize pressure in the ears, such as when flying in an airplane. When this happens, you might hear a soft popping sound.
The Inner Ear:
Our inner ear has two parts: semicircular canals and the cochlea. The semicircular canals do not contribute to hearing, but assist in maintaining balance as we move. The hearing organ of the inner ear is called the cochlea. The cochlea is a fluid-filled structure that looks like a snail. The cochlea changes the mechanical vibrations from the tympanic membrane and the middle ear bones into electrical impulses. Sensory cells, called hair cells, bend in the cochlea as the fluid is disrupted by the mechanical vibrations. This bending of the hair cells causes electrical signals to be sent from the cochlea to the brain by way of the the auditory nerve. The cochlea is arranged by frequency, much like a piano, and encodes sounds from 20Hz (low pitch) to 20,000Hz (high pitch) in humans.