The ears are delicate and sensitive organs. Sounds in the environment produce tiny changes in air pressure. The ears detect these changes and send the information to the brain for processing. They are also important for maintaining balance.
A person’s sense of hearing is incredibly versatile. It can detect extremely quiet sounds, determine whether a noise came from far or near, and isolate a specific sound within dense background noise.
In the United States, 1 in 8 people aged 12 years and older develops hearing loss in both ears.
In this article, we explore the anatomy of the ear, describe how hearing works, and investigate common causes of hearing loss.
The ear has three main sections: the outer, middle, and inner ear. Each section serves a distinct purpose in hearing.
The outer ear is the visible part, also known as the pinna. Its primary job is to collect as much sound from the surrounding area as it can.
External sounds then enter a thin passage called the ear canal.
The middle ear amplifies incoming sound with the help of a thin membrane called the eardrum, or tympanic membrane.
The eardrum separates the outer ear from the middle ear and helps to transmit sound vibrations to the inner ear.
Three tiny bones, called ossicles, amplify the sound. The names of the ossicles are:
- The malleus, or hammer: This connects to the eardrum.
- The incus, or anvil: This connects to the malleus.
- The stapes, or stirrup: This is the smallest bone in the body and links to the incus.
The eardrum vibrates when sound waves reach it. This vibration moves the ossicles, transmitting sound further into the ear.
Meanwhile, the Eustachian tubes are thin, mucus-lined passages that help maintain stable pressure in the middle ear. Stable pressure allows sound waves to transmit correctly.
These tubes connect the middle ear to the back of the throat. A person can “pop” their ears by forcing air into the Eustachian tubes.
After the ossicles amplify the sound waves, the vibrations enter the cochlea.
The cochlea is a small, curled tube full of liquid that sits in the inner ear. 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 the structures above them. The bumping causes them to bend, and this opens up ion channels, creating a signal that the ear delivers to the brain.
Higher and lower pitches of sound activate hairs in different parts of the cochlea. The brain gathers information about pitch from the position of the activated hairs.
The cochlea sends this information along the auditory, or cochlear, nerve. The signal reaches the medulla, which is part of the brain stem. The brain stem is the area of the brain closest to the back of the neck.
The auditory nerve also carries information from the brain to the cochlea. The fibers of this nerve help suppress distracting sounds, allowing us to concentrate on just one sound among many.
For instance, when we are having a conversation in a busy room, the fibers of the auditory nerve help us focus on hearing one voice while ignoring other sounds.
People refer to pitch as frequency and measure it in hertz. The higher the hertz, the higher the pitch of the sound.
Intensity is another word for loudness, and people measure it in decibels (db).
The human ear usually hears sounds that are 20–20,000 hertz. However, in perfect lab conditions, some people can hear sounds as low as 12 hertz or as high as 28,000 hertz.
Hearing ability varies significantly from person to person. It tends to decline with age, especially the hearing of higher frequencies.
Most everyday sounds are 250–6,000 hertz. However, the ears are most attuned to sounds of 2,000–5,000 hertz.
As for intensity: Humans can detect sounds of 0–140 db. 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 ears are also vital for maintaining balance. The inner ear contains the vestibular system, a part of the body that is largely responsible for spatial orientation and the coordination of movement as they relate to balance.
Three small, fluid-filled loops, called semicircular canals, sit just above the cochlea. One detects up-and-down movement, the next detects side-to-side movement, and the third detects tilting.
The fluid in the semicircular canals shifts when a person moves their head. These canals also contain thousands of tiny, sensitive hairs, which bend as the fluid flows past them. This bending relays information to the brain about the type of movement.
When a person spins around and stops suddenly, the fluid keeps moving for some time, continuing to push against the hairs. The hairs continue to send messages to the brain, so the brain assumes that the person is still spinning. This is dizziness.
A vestibule joins the semicircular canals and the cochlea. It contains two sacs, called the utricle and the saccule, which send the brain information about how the head is moving in relation to gravity and acceleration.
For instance, the saccule helps a person tell whether they are traveling up or down in an elevator and, more importantly, whether they are lying down or standing up.
Various health conditions, lifestyle factors, and injuries can cause hearing loss.
There are two general types. Conductive hearing lossoccurs when sound cannot travel through the outer and middle ear.
Fluid in the middle ear, an ear infection, a tumor, damage to an ossicle, and a buildup of earwax can each cause conductive hearing loss. This type is often treatable.
Meanwhile, damage to the inner ear leads to the most common form of permanent hearing loss: sensorineural hearing loss. Causes include aging, genetic diseases, and drugs that are toxic to hearing, called ototoxic drugs.
Some people have inner ear damage alongside problems with the conducting of sound. This results in what doctors call “mixed hearing loss.”
A doctor may also refer to hearing loss as bilateral, affecting both ears, or single-sided, affecting one ear.
Below are several possible causes of hearing loss:
- Loud noises in the short term: Exposure to one extremely loud noise, from an explosion, for example, can reduce the ability to hear.
- Loud noises in the long term: Exposure to loud noises over a long period can gradually reduce hearing. This may occur, for instance, in people who regularly use heavy machinery without ear protection.
- Injury: Some injuries, such as traumatic brain injuries, can cause hearing loss. An injury may puncture the eardrum or otherwise damage the middle ear.
- Smoking: A 2019 study linked smoking tobacco with an increased risk of sensorineural hearing loss.
- Otosclerosis: This condition affects the small bones of the middle ear, preventing the ossicles from moving.
- Ménière’s disease: This causes dizziness, sensorineural hearing loss, and tinnitus, or ringing in the ears.
- Acoustic neuroma: An acoustic neuroma is a type of tumor that can cause tinnitus and a feeling of a blockage in the ear.
- Cholesteatoma: This is a rare, atypical buildup of skin cells deep within the ear. Without treatment, it can damage the inner ear.
- Presbycusis: This refers to natural hearing loss due to aging, and it is the most common cause of sensorineural hearing loss. Sounds may become more muffled and conversations harder to follow.
The ear canal secretes earwax, or cerumen. It helps protect the skin from drying out and keeps the ear canal clean.
Earwax also offers some protection against bacteria, insects, fungi, and water. Its antibacterial properties may stem from its slight acidity and the presence of lysozyme — an enzyme that breaks down bacterial cell walls.
The largest component of earwax is dead skin. It also contains hair and secretions from glands within the ear canal. Other components of earwax include fatty acids, alcohols, and cholesterol.
The ears are an intricate, delicate part of the sensory system. They send signals to the brain to help the person hear and understand their physical position.
The ears transmit information so effectively that many people give the complex process of hearing little thought. However, prolonged or sudden exposure to loud sounds, aging, and smoking tobacco can each cause hearing loss.