How does hearing work?
Our sense of hearing is incredibly versatile — it can detect the quietest of sounds, it can determine whether a noise came from far or near, and it can pick out a specific sound from the background noise of life.
How do we hear?
The ear can be split into three sections: the outer, middle, and inner ear. Each section plays a distinct role in hearing.
A diagram of the anatomy of the ear.
Also called the pinna or auricle, the outer ear is the part that is visible. Its primary job is to collect as much sound from the surrounding area as it can.
An external sound starts its journey here as it enters a thin passage called the ear canal.
The middle ear amplifies incoming sound. It does this with the help of the eardrum, which is a thin membrane also known as the tympanic membrane.
The eardrum separates the outer ear from the middle ear and helps to transmit sound vibrations to the inner ear.
The sound is amplified by three tiny bones called ossicles. The names of the ossicles are:
- The malleus (or hammer): This is attached to the eardrum.
- The incus (or anvil): This is attached to the malleus.
- The stapes (or stirrup): This, the smallest bone in the body, is attached to the incus.
When sound waves reach the eardrum, it vibrates. This vibration moves the ossicles, transmitting sound further into the ear.
The Eustachian tubes are thin, mucous-lined passages that help to maintain a stable pressure in the middle ear, so that sound waves are correctly transmitted. These tubes connect the middle ear to the back of the throat. When you "pop" your ears, the sound you hear is created by air being forced into the Eustachian tubes.
Once a sound has been amplified by the ossicles, the vibration enters the cochlea. This is a small, curled tube that looks like a snail's shell and is located in the inner ear. The cochlea is filled with liquid. It has an internal membrane, called the basilar membrane, which is covered in hair cells. Sound causes the fluid to rise and fall, moving the hair cells up and down as they "ride the wave."
Each hair cell has stereocilia — tiny, hair-like projections — along its top. As the hair cells move up and down, the stereocilia bump into structures above and are bent over. This opens up ion channels, creating a signal that is sent to the brain.
Different pitches — higher or lower — activate hairs in different parts of the cochlea. From their position, the brain can gather information about the pitch of the sound.
The information about the sound is sent from the cochlea along the auditory, or cochlear, nerve. It reaches the medulla, which is part of the brain stem. The brain stem is the part of the brain located closest to the back of the neck.
The auditory nerve also carries information from the brain back to the cochlea. The fibers of this nerve help us to suppress sounds that we are not interested in, allowing us to concentrate on just one sound among many. For instance, when having a conversation in a busy room, it helps us to focus on the voice of one person and ignore the others.
Pitch and intensity
It is useful to know the meaning of the words "pitch" and "intensity," because they are often used in relation to sound.
Pitch — how high or low a sound is — is also referred to as frequency and is measured in hertz (Hz). The higher the Hz, the more high-pitched the sound.
Intensity is another word for loudness, and it is measured in decibels (dB).
The human ear is commonly said to hear in the range of 20–20,000 Hz. However, in perfect lab conditions, some people can hear as low as 12 Hz and as high as 28,000 Hz. Hearing ability varies significantly from person to person. It tends to decline as we age, especially for higher frequencies.
Most sounds we hear on a day-to-day basis are in the range of 250–6,000 Hz, but our ears are most attuned to hear sounds around 2,000–5,000 Hz.
As for intensity, humans can detect sounds from 0–140 dB. To give the numbers a little perspective, a whisper is around 25–30 dB and conversations are usually 45–60 dB. A chainsaw is about 120 dB. The sound of a jet taking off 25 meters away is around 150 dB and would cause the eardrums to rupture.
The ear's role in balance
A diagram of the inner ear.
The ear is not only important for hearing — it is vital for our sense of balance. This is called our vestibular system.
Just above the cochlea are three small, fluid-filled loops called semicircular canals. One detects up-and-down movement, one detects side-to-side movement, and the other detects tilting.
The semicircular canals contain thousands of tiny, sensitive hairs. When we move our heads, the fluid in the semicircular canals also moves.
As the fluid moves past the hairs, they bend, relaying information to our brain about what type of movement is going on.
The movement of this fluid explains dizziness. When a person spins around, then stops suddenly, the fluid keeps moving for some time, continuing to push against the hairs. Because the hairs are still sending messages to the brain, the brain assumes that the person is still spinning.
The semicircular canals and the cochlea are joined together by the vestibule, which consists of two sacs, called the utricle and the saccule. These structures send the brain information about how the head is moving in relation to gravity and acceleration. For instance, the saccule helps us to detect whether we are traveling up or down in an elevator and whether we are lying down or standing up.
Because of the delicate complexity of the ear's anatomy, hearing can be affected by a number of diseases, lifestyle factors, and injuries.
Hearing loss is relatively common, affecting an estimated 2 or 3 children out of every 1,000 in the United States, as well as 15 percent of adults in the country.
Hearing loss can be split into two general types:
Conductive hearing loss: This is when sound is prevented from traveling through the outer and middle ear. Conductive hearing loss can be caused by fluid in the middle ear, an ear infection, benign tumors, or earwax. This type of hearing loss is often treatable.
Sensorineural hearing loss: This is caused by damage to the inner ear and is the most common form of permanent hearing loss. Causes include drugs that are toxic to hearing, called ototoxic drugs. Aging and some genetic diseases can also lead to this type of loss.
In some cases, an individual will have damage to the inner ear as well as a problem conducting sound. This is called mixed hearing loss. Hearing loss can also be referred to as bilateral, affecting both ears, or single-sided, affecting only one ear.
Below are a few more examples of ways in which hearing loss can occur:
Prolonged exposure to loud music can cause gradual hearing loss.
Loud noises: Being exposed to a single incredibly loud noise, such as an explosion, can reduce an individual's ability to hear.
Exposure to fairly loud noises over a long period can reduce hearing gradually. This may happen, for instance, in people who regularly use heavy machinery without ear protection.
Injury: Some injuries, such as traumatic brain injuries, can cause hearing loss. These injuries may result in a hole in the eardrum or damage to the middle ear.
Smoking: Tobacco smoking has been linked with an increased risk of sensorineural hearing loss.
Otosclerosis: This condition affects the small bones of the middle ear. The stapes slowly fuses with the other ossicles, preventing them from moving.
Ménière's disease: This causes dizziness, sensorineural hearing loss, and tinnitus — ringing in the ears.
Acoustic neuroma: This type of tumor can cause tinnitus and gives the sensation that the ear is full.
Cholesteatoma: This is an abnormal buildup of skin cells deep within the ear. Although rare, if it is not treated, it can damage the inner ear.
Presbycusis: This describes the hearing loss that happens as we age. Sounds may seem more muffled, and conversations become harder to follow. It is the most common cause of sensorineural hearing loss.
What is earwax?
Also known as cerumen, earwax is secreted in the ear canal. It helps to protect the skin from drying out and keeps the ear canal clean.
Earwax also offers some protection against bacteria, insects, fungi, and water. It is thought to be antibacterial due to its slight acidity and the presence of lysozyme, an enzyme which breaks down bacterial cell walls.
The largest component of earwax is shed layers of skin. It also contains hair and the secretions of two glands: the ceruminous and sebaceous glands of the ear canal. Other components of earwax are fatty acids, alcohols, and cholesterol.
In a nutshell
The ears are an intricate and delicate part of our sensory system. They work in harmony with the brain to help us understand the world of sound around us. They decode sounds so well, in fact, that many of us give the miracle of hearing little thought.