How Hearing Works
Normal Ear Function
- Sound is transmitted through the air as sound/pressure 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 (ossicles) in the middle ear into motion.
- The motion of the three bones causes the fluid in the inner ear, or the cochlea, to move.
- The movement of the fluid in the inner ear causes the hair cells in the cochlea to move and bend. These hair cells change this fluid movement into electrical impulses.
- These electrical impulses are transmitted to the hearing (auditory) nerve and up to the brain, where they are interpreted as sound.
How our brain interprets sound is complex and varies tremendously from individual to individual. It is important to realize that just because your ears were able to pick up a sound and transmit it to your brain, it does not mean your brain will interpret it properly. Hearing and understanding/comprehending are two completely different things.
The Outer Ear
The part of the outer ear that we see is called the pinna, or auricle (fun fact: it is only found in mammals). The pinna, with its grooves and ridges, provides a means to collect, and funnel sound down the ear canal. If you have ever cupped your hands over your ears, you have added to this funnelling effect. The pinna also helps with localizing sound (i.e. telling you where a sound came from).
The ear canal, also called the external auditory meatus, is the other important outer ear landmark. The ear canal is lined with only a few layers of skin and fine hair, and is a highly vascularized area. This means that there is an abundant flow of blood to the ear canal. Wax (cerumen) accumulates in the ear canal and serves as a protective barrier to the skin from bacteria and moisture. Ear wax is normal, unless it completely blocks the ear canal.
The outer ear and ear canal together provide us with 15-20 decibels (dB) of amplification around 3000 Hz.
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 tympanic membrane is approximately 7.5 - 9mm in diameter and is 0.1mm thick and it serves to collect sound that is funnelled down the ear canal.
The ossicles are the three tiny bones of the middle ear located directly behind the tympanic membrane. These three bones (malleus, incus, stapes) form a connected chain in the middle ear. One of the bones is embedded in the innermost layer of the tympanic membrane (malleus), and the third bone is connected to a membranous window of the inner ear (stapes). 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 the temporal bone and does not associate with outside air except through the Eustachian tube. This tubular structure is normally closed, but it 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 by plugging your noise and blowing (Valsalva maneuver). When this happens, you might hear a soft popping sound.
The entire middle ear system provides 25 decibels (dB) of amplification. When combined with the 15-20 dB from the outer ear, this totals 30-35 dB of amplification.
The Inner Ear
The inner ear is an organ located deep within the petrous portion of the temporal bone, which is the bone of the skull on both sides of the head above the outer ear. This is one of the densest bones in the human body. The inner ear has two main structures: the cochlea and vestibule.
- 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 cells, called hair cells, bend in the cochlea as the fluid is disrupted by the mechanical vibrations. Outer hair cells (OHCs) serve to amplify and fine tune the sound (making it distinct and crisp), while Inner Hair Cells (IHCs) serve to pick up the amplified sound, converting the sound into an electrical impulse that is sent down the auditory nerve to the brain. The cochlea is arranged by frequency, much like a piano, and encodes sounds from 20Hz (low pitch) to 20,000Hz (high pitch) in humans.
- Vestibule - This is the part of the inner ear that helps us with our balance. Like the cochlea, the vestibule is filled with fluid. However, unlike the cochlea, the vestibule has 5 distinct organs, each of which has a specific purpose. Two of these organs (utricle and saccule) help us with our sense of linear motion and helps tell our body which way is up and down (i.e. responds to gravity). The other 3 organs are called the semicircular canals and they are involved with rotational and angular movement.