A whisper from across the room, music in your headphones, the cry of a baby, the rustle of leaves: our ears can detect an astonishing range of sounds, from the faintest pressure changes to the roar of a jet engine. They can pick out a single voice in a crowded room and tell us where a sound is coming from in a fraction of a second. Hearing depends on a chain of delicate structures that turn vibrations in the air into electrical signals in the brain. This explainer follows sound through the ear and into the brain. It explains how the body works and is not medical advice.
What is sound?
Sound is a vibration that travels through a medium, such as air, water or solids, as a wave of pressure. When an object vibrates, like a guitar string or vocal cords, it pushes air molecules together, creating areas of higher pressure, and pulls them apart, creating areas of lower pressure. These alternating compressions and rarefactions travel outward as sound waves.
Two properties of sound waves are especially important for hearing:
- Frequency is the number of vibrations per second, measured in hertz (Hz). We perceive frequency as pitch. Low-frequency sounds, like a bass drum, have a low pitch; high-frequency sounds, like a whistle, have a high pitch. Young humans can typically hear frequencies from about 20 Hz to 20,000 Hz, and the upper limit declines with age.
- Amplitude is the size of the pressure changes. We perceive it as loudness, measured in decibels (dB). The decibel scale is logarithmic: an increase of 10 dB represents ten times more sound intensity and is perceived as roughly twice as loud.
The three parts of the ear
The ear has three main sections: the outer ear, the middle ear and the inner ear.
The outer ear: collecting sound
The visible part of the ear, called the pinna or auricle, collects sound waves and funnels them into the ear canal. Its curved shape also slightly changes sounds depending on their direction, helping us tell whether a sound comes from above, below, in front or behind.
The ear canal is about an inch (2.5 centimeters) long and ends at the eardrum. Glands in the canal produce earwax, which traps dust and microorganisms and naturally moves outward. The ear usually cleans itself; inserting cotton swabs can push wax deeper and damage the eardrum.
The middle ear: amplifying sound
The eardrum, or tympanic membrane, is a thin membrane that vibrates when sound waves strike it. Behind it lies the middle ear, an air-filled space containing the three smallest bones in the body, called the ossicles:
- The malleus (hammer), attached to the eardrum.
- The incus (anvil).
- The stapes (stirrup), the smallest bone in the human body, about 0.1 inches (3 millimeters) long.
These bones form a chain that transmits vibrations from the eardrum to the inner ear. They also amplify the vibrations. The eardrum is much larger than the oval window, the membrane where the stapes connects to the inner ear. By concentrating the force from a large area onto a small one, and through the lever action of the bones, the middle ear increases the pressure by about 20 times. This amplification is necessary because the inner ear is filled with fluid, and sound transfers poorly from air to fluid.
The middle ear is connected to the back of the throat by the Eustachian tube, which equalizes air pressure on both sides of the eardrum. That is why your ears "pop" when you yawn or swallow during a flight or a drive through the mountains.
The inner ear: turning vibrations into signals
The inner ear contains the cochlea, a snail-shaped, fluid-filled structure about the size of a pea. When the stapes pushes on the oval window, it creates waves in the fluid inside the cochlea.
Running along the length of the cochlea is the basilar membrane, which supports the organ of Corti, containing thousands of specialized sensory cells called hair cells. Each hair cell has tiny projections on top called stereocilia.
As fluid waves travel through the cochlea, they make the basilar membrane vibrate. The stereocilia bend, opening tiny channels in the hair cells. Ions flow in, creating an electrical signal. The hair cells then release chemical messengers that activate nerve fibers of the auditory nerve, sending signals to the brain.
How the ear sorts pitch
The cochlea works like a frequency analyzer. The basilar membrane is narrow and stiff near the base of the cochlea and wider and more flexible near the tip. As a result:
- High-frequency sounds cause the most vibration near the base.
- Low-frequency sounds cause the most vibration near the tip.
Each hair cell responds best to a particular frequency, depending on its position. The brain interprets which hair cells are active to determine pitch. This arrangement is called tonotopic organization, like the keys of a piano laid out along a spiral.
There are two types of hair cells. Inner hair cells, about 3,500 in each ear, send most of the information to the brain. Outer hair cells, about 12,000, act like tiny amplifiers, changing their length in response to sound and sharpening the ear's sensitivity and frequency selectivity.
From the ear to the brain
The auditory nerve carries signals from the cochlea to the brainstem. From there, information travels through several processing stations to the auditory cortex in the temporal lobes of the brain. Along the way, the brain analyzes the timing, pitch and loudness of sounds.
Locating sounds
Having two ears helps us locate sounds. The brain compares:
- Timing differences: A sound coming from the right reaches the right ear slightly before the left ear, by up to about 0.6 milliseconds.
- Loudness differences: The head blocks some sound, so the ear closer to the source hears it slightly louder, especially for high frequencies.
- Shape cues: The pinna alters sounds slightly depending on their direction, helping judge elevation and front-back position.
Understanding sounds
The brain does much more than detect sounds. It recognizes voices, understands speech, appreciates music and filters out background noise. The ability to focus on one conversation in a noisy room is known as the cocktail party effect.
Hearing and balance
The inner ear also contains the vestibular system, which helps maintain balance. It includes three fluid-filled semicircular canals, arranged at right angles to each other, which detect rotation of the head, and two organs, the utricle and saccule, which detect linear movement and gravity. Hair cells in these structures, similar to those in the cochlea, send information about movement and position to the brain. When signals from the inner ear, eyes and body do not match, as on a boat, people can feel dizzy or motion sick.
Hearing loss
Hearing loss is common, particularly with age. There are two main types:
- Conductive hearing loss happens when sound cannot pass efficiently through the outer or middle ear, for example because of earwax buildup, fluid from an ear infection or problems with the ossicles. It is often treatable.
- Sensorineural hearing loss results from damage to the hair cells in the cochlea or to the auditory nerve. In humans, hair cells do not regenerate, so damage is usually permanent. Causes include aging, loud noise exposure, some medicines, infections and genetics.
Hearing aids amplify sounds, and cochlear implants can bypass damaged hair cells by directly stimulating the auditory nerve with electrical signals, helping many people with severe hearing loss.
Protecting your hearing
Exposure to loud sounds is a major preventable cause of hearing loss. Health organizations generally advise:
| Sound level | Example | Note |
|---|---|---|
| about 30 dB | whisper | quiet |
| about 60 dB | normal conversation | safe |
| about 85 dB | heavy traffic | prolonged exposure can cause damage |
| about 100 dB | concert, power tools | damage possible in a short time |
| about 120 dB and above | sirens, fireworks up close | pain threshold, immediate risk |
To protect hearing, keep headphone volume moderate, take breaks from noisy environments, and use earplugs or earmuffs at concerts, sports events and when using loud tools. Ringing in the ears after loud noise, called tinnitus, is a warning sign of possible damage.
Questions readers ask
What is the smallest bone in the body? The stapes, or stirrup, in the middle ear, about 3 millimeters long.
Why do my ears pop on airplanes? Air pressure changes, and the Eustachian tube opens to equalize pressure on both sides of the eardrum.
How do we tell high and low sounds apart? Different parts of the cochlea vibrate most strongly at different frequencies, activating specific hair cells.
Can damaged hair cells grow back? In humans, hair cells in the cochlea do not naturally regenerate, so noise damage is usually permanent.
How does the ear help with balance? The vestibular system in the inner ear detects head rotation, movement and gravity.
In short
Hearing begins when sound waves enter the ear canal and make the eardrum vibrate. Three tiny bones in the middle ear amplify these vibrations and pass them to the fluid-filled cochlea, where hair cells sorted by frequency convert them into electrical signals. The auditory nerve carries these signals to the brain, which interprets pitch, loudness, direction and meaning. The inner ear also helps us keep our balance. Because hair cells cannot regrow, protecting our ears from loud noise is one of the best ways to keep this remarkable sense working for life.