How Hearing Works

Sound reaches the eardrum through the outer ear. The middle ear transfers movement into cochlear fluid, hair cells create biological signals, and the brain performs essential processing.

Sound begins as pressure changes in air A sound wave is a pattern of pressure changes travelling through a material such as air. The outer ear collects part of this sound and guides it into the ear canal. The visible outer ear is called the pinna. Its shape also helps the brain judge whether a sound comes from above, below, in front or behind. Sound travelling along the ear canal reaches the eardrum, also called the tympanic membrane. The changing pressure makes this thin membrane move back and forth. The middle ear transfers sound into fluid Behind the eardrum is the air filled middle ear. It contains three linked bones called the ossicles. The ossicles are the malleus, incus and stapes. The malleus moves with the eardrum, and the stapes presses on the oval window of the inner ear. The eardrum and ossicles form an impedance matching system. They concentrate force onto the smaller oval window and reduce energy loss when movement enters cochlear fluid. This creates a pressure gain, but it does not create extra sound energy. Describing the bones only as a powerful amplifier can therefore be misleading. The Eustachian tube connects the middle ear with the back of the nose. It opens at times to help equalise pressure across the eardrum. The cochlea separates sound by pitch Movement of the stapes at the oval window creates pressure waves within the cochlea. The cochlea is a coiled, fluid filled part of the inner ear. These pressure waves move the basilar membrane. Different sound frequencies produce their strongest movement at different places along this membrane. Higher pitches usually produce their strongest response near the cochlear base. Lower pitches usually produce their strongest response nearer the apex. This place based arrangement helps preserve pitch information. A real sound usually contains several frequencies, so several cochlear regions may respond together. Hair cells convert movement into biological signals The organ of Corti sits on the basilar membrane and contains sensory hair cells. Each hair cell has tiny projections called stereocilia. Movement within the cochlea bends the stereocilia and changes the hair cell's electrical state. Inner hair cells then release a chemical messenger onto auditory nerve fibres. Those nerve fibres produce impulses that carry sound information towards the brain. The hair cells do not send a finished recording or recognisable words. Outer hair cells change the mechanical response of the cochlea. They improve sensitivity and help separate nearby frequencies more clearly. Human cochlear hair cells do not normally regenerate after substantial damage. Very loud or repeated noise can therefore cause lasting hearing loss. Hearing continues through the brain The auditory nerve carries cochlear information to the brainstem. Signals then pass through several brain relays before reaching the auditory cortex. Processing occurs at each stage. Brain networks analyse timing, pitch, loudness, speech patterns and other features needed for meaningful hearing. The brain also compares small timing and loudness differences between the two ears. These differences help locate a sound in space. Hearing is therefore an ear and brain process. A problem can affect sound conduction, cochlear conversion, nerve transmission or central processing. The inner ear also contributes to balance The vestibular system lies beside the cochlea within the inner ear. It serves a different sensory function. The semicircular canals detect rotational head movement. The otolith organs detect straight line acceleration and head position relative to gravity. Vestibular signals combine with vision and body position information. The semicircular canals alone do not report every aspect of balance or static head tilt. A later lesson develops this balance system in more detail. Hearing changes with vertigo can suggest inner ear involvement, but neurological causes can produce overlapping symptoms. Hearing examinations answer different questions Otoscopy uses a lighted instrument to inspect the ear canal and eardrum. It can show wax, a perforation or signs suggesting middle ear fluid. Otoscopy cannot show the cochlea, auditory nerve or auditory cortex. A normal looking eardrum does not prove that hearing is normal. Tympanometry measures how the eardrum and middle ear respond when ear canal pressure changes. It assesses middle ear movement rather than hearing sensitivity itself. Pure tone audiometry measures the quietest tones heard at different frequencies. Air conduction and bone conduction results help classify conductive hearing loss, sensorineural hearing loss or mixed hearing loss. The results are recorded on an audiogram. An audiogram describes hearing thresholds but cannot identify every cause or predict all real world speech understanding. Tuning fork tests are quick bedside screening tests. They can suggest a conductive or sensorineural pattern but may miss mild, bilateral or mixed hearing loss. Sudden hearing change needs prompt assessment Sudden hearing loss means hearing that develops over three days or less. It can affect one ear or both ears and may feel like blockage or fullness. Ask for an urgent GP appointment or contact NHS 111 immediately for sudden hearing loss. Do not wait to see whether it is only wax. Hearing that worsens over several days or weeks also needs urgent GP or NHS 111 advice. The same applies when hearing loss occurs with earache or ear discharge. Call 999 for sudden hearing change with facial weakness, arm weakness, speech difficulty or another possible stroke sign. Severe new difficulty walking or collapse also needs emergency help. Gradual hearing loss still deserves assessment. Age, noise exposure, wax, middle ear disease and several other causes can produce similar experiences.

Hearing requires efficient sound transfer, frequency separation, hair cell transduction, auditory nerve transmission and brain processing. No single ear structure creates recognisable sound by itself.