From vibration to meaning
Hearing turns moving air into nerve signals. The ear is a chain of structures that catch vibrations, strengthen them, and convert them to electrical activity, which the brain then reads as speech, music or noise. The ear is conventionally divided into three parts: the outer ear, the middle ear and the inner ear.
What sound is
A sound begins with something vibrating, such as vocal cords, a guitar string or a speaker. The vibration pushes the surrounding air molecules, which creates a pattern of slightly higher and lower pressure that travels outward. The behavior of these moving pressure patterns is explained in the article on sound waves. Two features matter most to the ear: frequency, how many pressure cycles occur per second, which we hear as pitch; and amplitude, the size of the pressure variations, which we hear as loudness.
The outer ear
The visible part of the ear, the pinna, helps collect sound and gives subtle directional cues. Sound then travels down the ear canal to the eardrum (tympanic membrane), a thin, flexible membrane that vibrates in response. The ear canal also produces earwax, which helps protect it.
The middle ear
Behind the eardrum is an air-filled space containing the three smallest bones in the body, the ossicles: the malleus (hammer), incus (anvil) and stapes (stirrup). They form a linked chain that carries the eardrum’s vibrations across to a small opening in the inner ear called the oval window.
This chain matters because the inner ear is filled with fluid, and sound in air does not pass easily into fluid; much of it would simply bounce off. The ossicles, together with the fact that the eardrum is much larger than the oval window, concentrate the force and help transfer the energy into the fluid efficiently.
A narrow passage called the Eustachian tube connects the middle ear to the back of the throat. It lets air in and out so that pressure on both sides of the eardrum can equalize. That is the popping sensation you may notice during a flight or a drive up a mountain.
The inner ear
The cochlea is a fluid-filled, snail-shaped tube. When the stapes pushes on the oval window, it sets the fluid inside moving, and a flexible partition running along the cochlea, the basilar membrane, begins to ripple.
Different parts of this membrane respond best to different frequencies. The stiffer base near the oval window responds to higher pitches, and the wider, more flexible far end responds to lower ones. Sitting on the membrane are hair cells, each topped with microscopic bristle-like projections. When the membrane moves, these bristles bend, which opens tiny channels in the cell and changes its electrical state. The hair cell then releases a chemical messenger onto a nerve fiber, which fires a signal.
In this way vibration is transduced into the electrical impulses described in the article on neuron signals. Louder sounds generally produce stronger movement and more activity, and the location along the cochlea provides the code for pitch.
From the ear to the brain
The signals travel along the auditory nerve to the brainstem, where information from both ears is first compared, and onward through relay stations to the auditory cortex in the temporal lobe. Comparing the slight differences in timing and loudness between the two ears helps the brain work out where a sound is coming from.
The brain does much more than just receive. It picks one voice out of a noisy room, recognizes words and tunes, and links sounds to memory formation and feeling. Music or a familiar voice can bring on strong human emotions. Studies of the nervous system often include such cases among the more striking brain facts.
The range of human hearing
Young people with healthy hearing can typically detect frequencies from roughly 20 hertz up to about 20,000 hertz. Sensitivity is greatest in the range used by speech, and the upper limit typically declines with age. Many other animals hear well outside this range.
Balance in the same organ
The inner ear also contains the vestibular system, fluid-filled canals and small chambers that detect head rotation and the pull of gravity. It is not part of hearing, but it sits alongside the cochlea and is why ear problems can sometimes cause dizziness.
Hearing loss and protecting hearing
Hearing loss has several causes. Conductive hearing loss arises when sound cannot pass efficiently through the outer or middle ear, for instance from a blockage or fluid. Sensorineural loss involves damage to hair cells or the nerve. In humans, hair cells do not generally regenerate once damaged, which is why prolonged exposure to loud noise is a concern, and why many health authorities advise limiting loud sound and using ear protection around very loud noise. Aging also gradually affects hearing for most people.
Devices such as hearing aids and cochlear implants can help many people, and the right choice depends on the type of loss.
This article is general educational information, not medical advice. Individual cases differ, and anyone concerned about their hearing, ringing in the ears or sudden changes should consult a qualified health professional.