Physiology of Hearing: How Sound Waves Become Perception
The physiology of hearing explains how sound energy travels from the environment, passes through the external and middle ear, moves fluid inside the cochlea, and becomes electrical activity interpreted by the brain. Hearing therefore depends on a continuous chain: sound collection → mechanical conduction → cochlear transduction → neural transmission → perception.
This note follows that chain from the sound wave to the auditory cortex, highlights how pitch and loudness are coded, and connects the normal process to common patterns of hearing loss.
Learning objectives
- Explain basic sound properties relevant to hearing.
- Trace air-conducted sound through the outer and middle ear into the cochlea.
- Describe cochlear travelling waves, hair-cell transduction and the roles of inner and outer hair cells.
- Outline the central auditory pathway and how the brain localises sound.
- Relate interruption at different points to conductive and sensorineural hearing loss.
What is sound?
Sound is a mechanical disturbance that travels through an elastic medium as alternating regions of compression and rarefaction. In air, it reaches the ear as a pattern of changing pressure. The ear and brain analyse several properties of this pattern.
The number of cycles per second, measured in hertz (Hz). It is the main physical correlate of pitch: faster cycles are generally heard as higher-pitched sounds.
The size of pressure variation relates to sound intensity. Intensity is commonly expressed on a logarithmic decibel scale. Perceived loudness also depends on frequency and the listener’s auditory system.
Most everyday sounds combine frequencies and change over time. Their spectral and temporal patterns contribute to sound quality, speech recognition, and the ability to distinguish one source from another.
Human hearing has a broad frequency range that varies with age and health. The ear is especially important for communication because it can detect small changes in sound timing, frequency and intensity that help distinguish speech and locate a sound source.
The stages of hearing
The auricle gathers sound and the external auditory canal directs it toward the tympanic membrane.
The tympanic membrane and ossicles carry and couple vibration to the oval window of the cochlea.
Cochlear fluid movement displaces the basilar membrane and bends the stereocilia of hair cells in the organ of Corti.
Hair cells change mechanical movement into electrical receptor signals; spiral ganglion neurons carry coded activity in the cochlear nerve.
Brainstem, midbrain, thalamic and cortical networks analyse the signal and allow the person to recognise and understand sound.
1. Sound collection and the external ear
The auricle collects sound and its folds create direction-dependent acoustic changes. These cues help the brain estimate whether a sound comes from in front, behind, above or below. The external auditory canal carries sound to the tympanic membrane and contributes resonance, especially in speech-relevant frequencies.
The external ear also offers protection. Hair and cerumen trap particles, and the narrow, curved canal reduces direct exposure of the tympanic membrane. Sound pressure arriving at the membrane causes it to move back and forth in time with the incoming waveform.
2. Middle-ear conduction and impedance matching
The middle ear transmits vibration from the air-filled external canal to the fluid-filled cochlea. Air and cochlear fluid have different acoustic impedances. Without an efficient transfer mechanism, much of the sound energy would be reflected at the air-fluid boundary. The tympanic membrane and ossicles improve transfer by increasing pressure at the stapes footplate.
Vibrates in response to changes in canal pressure. Its large surface moves the attached malleus and starts the ossicular chain.
The malleus and incus act as a linked lever system, transferring movement to the stapes while changing the force applied at the oval window.
The effective vibrating area of the tympanic membrane is greater than the stapes footplate. Concentrating force onto the smaller footplate helps increase pressure in the cochlea.
The stapes footplate moves at the oval window, creating pressure waves in cochlear fluid. The round window bulges in the opposite direction so that cochlear fluid can move within a rigid bony enclosure. The ossicular system is therefore a mechanical transformer that helps air-conducted sound enter the inner ear efficiently.
Air conduction and bone conduction
- Air conduction: sound travels through the external canal, tympanic membrane and ossicles before stimulating the cochlea.
- Bone conduction: vibration of the skull directly moves the cochlear fluids and sensory structures, bypassing much of the outer and middle ear.
Both routes ultimately stimulate the cochlea. Comparing air- and bone-conduction thresholds during audiometry helps identify whether a hearing deficit is conductive, sensorineural or mixed.
Middle-ear muscle reflex
Loud sound can activate a reflex contraction of the stapedius, and to a lesser extent the tensor tympani. This temporarily changes ossicular movement and reduces transmission of some sustained sounds. The reflex has a delay and is not fast or strong enough to reliably protect the inner ear from sudden impulses, blasts or dangerously loud noise.
3. Cochlear mechanics
The cochlea is a fluid-filled spiral. The scala vestibuli and scala tympani contain perilymph, while the scala media (cochlear duct) contains endolymph. Movement of the stapes at the oval window sets up a pressure difference between these spaces and produces a travelling wave along the cochlear partition.
The basilar membrane is narrow and stiff near the cochlear base and wider and more flexible toward the apex. As a result, high-frequency sounds produce maximal displacement near the base, while low-frequency sounds travel farther toward the apex before producing maximal displacement. This place-based arrangement is called tonotopy.
The basal cochlea responds best to high frequencies. Its basilar membrane is relatively narrow and stiff, so the travelling wave reaches its peak closer to the oval window.
The apical cochlea responds best to low frequencies. Its basilar membrane is broader and more compliant, allowing the wave to peak farther along the cochlear spiral.
Neighbouring sound frequencies stimulate neighbouring cochlear regions. A similar ordered frequency map is maintained at several levels of the auditory pathway.
4. Hair-cell transduction in the organ of Corti
The organ of Corti rests on the basilar membrane within the scala media. Sound-related movement creates shearing between the basilar and tectorial membranes. This bends hair bundles on sensory hair cells; their stereocilia are arranged in graded rows and connected by fine tip links.
- Mechanical deflection: movement of the cochlear partition bends the stereocilia toward or away from the tallest row.
- Ion-channel gating: tension on tip links opens or closes mechanically gated channels at stereocilia tips.
- Receptor potential: the ionic environment of endolymph drives depolarising current into the hair cell when channels open.
- Neurotransmitter release: depolarisation opens voltage-gated calcium channels at the base of the inner hair cell, prompting glutamate release onto auditory-nerve endings.
- Neural firing: spiral ganglion neurons change their firing pattern, sending information through the cochlear division of cranial nerve VIII.
Hair cells do not fire action potentials in the same way as auditory-nerve fibres. Instead, they produce graded receptor potentials that regulate neurotransmitter release. The auditory nerve converts this input into patterns of neural firing that the central nervous system can analyse.
Inner hair cells and outer hair cells
They provide the main sensory output to the auditory nerve. Their neurotransmitter release communicates details of sound frequency and intensity to spiral ganglion neurons.
They change length in response to electrical signals through the motor protein prestin. This electromotility feeds mechanical energy back into the cochlear partition, amplifying faint sounds and sharpening frequency selectivity.
Outer hair cells are vulnerable to ageing, ototoxic injury and excessive noise. Their dysfunction reduces cochlear sensitivity and frequency discrimination; inner hair-cell or nerve damage can further limit sound encoding.
How the auditory system codes sound
The cochlea represents frequency mainly by the place of peak basilar-membrane movement. For lower frequencies, the timing of auditory-nerve firing can also follow individual sound cycles over a limited range.
Increasing sound intensity produces larger cochlear motion and stronger receptor responses. The brain estimates loudness from firing rate, the number of recruited auditory-nerve fibres and activity across frequency channels.
Complex tones activate several frequency regions at once. Their pattern and timing allow the auditory system to distinguish vowels, consonants, musical notes and different sound sources.
5. Central auditory pathway
Auditory-nerve fibres have their cell bodies in the spiral ganglion. Their central processes enter the brainstem and synapse in the cochlear nuclei. From there, information travels through a network with projections to both sides of the brain.
The cochlear division of cranial nerve VIII carries activity from the spiral ganglion into the brainstem.
Dorsal and ventral cochlear nuclei receive the first central synapse and begin separating features such as timing, intensity and frequency.
This is an early point where information from the two ears interacts. It contributes to locating sound using interaural timing and intensity differences.
Ascending fibres relay through the lateral lemniscus to the inferior colliculus in the midbrain, where auditory information is further integrated.
Signals pass through the medial geniculate body of the thalamus to the primary auditory cortex in the superior temporal region for conscious analysis and recognition.
Because auditory projections become bilateral early, a unilateral lesion above the cochlear nuclei rarely causes complete deafness in one ear. The pathway nevertheless has orderly frequency organisation and specialised circuits for timing, intensity, localisation and complex sound recognition.
Sound localisation
- Interaural time difference: a sound from one side reaches the nearer ear slightly earlier. The brain compares timing between ears, especially for lower-frequency sounds.
- Interaural level difference: the head reduces sound intensity at the far ear, creating a level difference that is particularly useful for higher-frequency sounds.
- Pinna cues: changes in the sound spectrum caused by the folds of the auricle help estimate elevation and front-back direction.
Clinical correlations
Occurs when sound transmission through the external or middle ear is impaired, such as with canal obstruction, tympanic membrane perforation, middle-ear fluid or ossicular fixation. Bone-conduction thresholds may be better than air-conduction thresholds.
Results from dysfunction of cochlear sensory cells, the auditory nerve or more central auditory structures. Air- and bone-conduction thresholds are both affected because the impairment is beyond the mechanical conducting apparatus.
Combines a conductive component with cochlear or neural dysfunction. Audiometry demonstrates an air-bone gap as well as elevated bone-conduction thresholds.
Excessive sound can injure cochlear hair cells and supporting structures. Outer hair-cell injury reduces sensitivity and frequency selectivity; significant hair-cell loss can produce permanent hearing impairment.
Quick summary
- The external ear collects and channels sound; the tympanic membrane converts sound pressure into vibration.
- The ossicles transmit vibration and improve pressure transfer from air to cochlear fluid.
- The stapes moves the oval window; the round window allows the cochlear fluids to be displaced.
- A travelling wave moves along the basilar membrane: high frequencies peak at the base and low frequencies peak nearer the apex.
- Hair-bundle movement opens mechanically gated channels. Inner hair cells provide most auditory-nerve output; outer hair cells amplify and sharpen cochlear responses.
- Auditory signals ascend from the cochlear nuclei through brainstem and midbrain relays to the thalamus and auditory cortex.
- External- and middle-ear problems cause conductive hearing loss; cochlear or neural problems cause sensorineural loss.
Self-test questions
- Why does the middle ear improve sound transfer from air to cochlear fluid?
- What is the difference between air conduction and bone conduction?
- Where along the basilar membrane do high- and low-frequency sounds produce their greatest displacement?
- What happens when a sound-induced deflection opens mechanically gated channels in a hair bundle?
- What are the main roles of inner and outer hair cells?
- List the principal central auditory relays from the cochlear nuclei to the auditory cortex.
- How can air- and bone-conduction thresholds help distinguish conductive from sensorineural loss?
References and further reading
- National Institute on Deafness and Other Communication Disorders (NIDCD). How Do We Hear?
- StatPearls. Physiology, Ear. NCBI Bookshelf.
- StatPearls. Neuroanatomy, Auditory Pathway. NCBI Bookshelf.
- StatPearls. Inner Ear Anatomy and Cochlear Physiology. NCBI Bookshelf.
- National Institutes of Health. Information about Hearing, Communication, and Understanding. NIH Curriculum Supplement.
These notes are for study and revision. Hearing tests and clinical findings should be interpreted by an appropriately trained health professional.
