How the Ear Works
Normal Ear Function
- Sound is transmitted through the air as sound waves from the environment. The sound waves are gathered by the outer ear and sent down the ear canal to the eardrum.
- The sound waves cause the eardrum to vibrate, which sets the three tiny bones in the middle ear into motion.
- The vibration of the three bones is then transmitted to a special fluid in the inner ear.
- The movement of the fluid in the inner ear causes the nerve cells in the cochlea to bend and convert the impulses into electrical signals.
- These electrical signals are then transmitted along the hearing (auditory) nerve and up to the hearing centre of the brain where they are interpreted as sound.
The Outer Ear
The part of the outer ear that we see is called the pinna, or auricle. The pinna, with its grooves and ridges, provides a natural volume boost for sounds in the 2000 to 3000 Hz frequency range, where we perceive many consonant sounds of speech.
The ear canal, also called the external auditory meatus, is the other important outer ear landmark. The ear canal is lined with only a thin layer of skin and fine hairs, and is a highly vascularized area. This means that there is an abundant flow of blood to the ear canal. Wax (cerumen) is secreted by small glands in the outer ear canal and serves as a protective barrier to the skin from bacteria and moisture. Ear wax is normal but may accumulate to the extent that it may completely block the ear canal.
The Middle Ear
The eardrum, or tympanic membrane (TM), is the dividing structure between the outer and middle ear. Although it is an extremely thin membrane, the eardrum is made up of three layers to increase its strength.
The ossicles are the three tiny bones of the middle ear 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 transmit mechanical vibrations received at the tympanic membrane into the inner ear. When functioning normally they provide a certain degree of amplification of sound.
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 which opens at the back of the nose. This tubular structure is normally closed and opened by naturally when 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
The inner ear is an organ located deep within the temporal bone, which is the bone of the skull on both sides of the head above the outer ear. The inner ear has two main structures: the balance organs known as the Utricle, Saccule and semicircular canals, and the hearing organ known as the cochlea.
- Balance organ - These structures do not contribute to hearing, but assist in maintaining balance as we move.
- Cochlea - This is the hearing organ of the inner ear, which is a fluid-filled structure that looks like a snail. The cochlea changes the mechanical vibrations from the tympanic membrane and the ossicles into a sequence of electrical impulses. Sensory nerve 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 to the brain by way of 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.