Hearing - Wikiparency

# Hearing **Hearing**, or audition, is the sense by which an organism detects [[Sound_pressure|pressure]] fluctuations in a medium — almost always air, sometimes water or the ground — and its nervous system converts them into a perception of sound. In humans and other vertebrates a chain of mechanical and neural stages carries the fluctuation from the outer ear to the auditory cortex: the outer ear collects and focuses it, the middle ear matches its low impedance in air to the much higher impedance of the inner ear's fluid, the inner ear's cochlea sorts it by frequency along a physical map, and the auditory nerve encodes that map as a pattern of impulses. The healthy human ear is extraordinarily sensitive: the quietest audible sound carries an [[Sound_intensity|intensity]] of about 10⁻¹² W/m² at 1,000 [[Frequency|Hz]], close to the physical noise floor of air molecules jostling against the eardrum, while a sound near 1 W/m² is already painful.[^upv1-hearing] The microsim built for this article, *Hearing: the cochlea's frequency map and equal loudness*, unrolls the cochlea's basilar membrane and shows a travelling wave launched by a pure tone growing, peaking at one place along the membrane, and dying away — high tones peaking near the cochlea's stiff base, low tones near its floppy apex. Alongside it sits the family of [[Equal-loudness_contour|equal-loudness contours]], curves that trace how loud a tone at a given frequency must be to sound as loud as a reference tone at 1,000 Hz; because the ear is markedly less sensitive to bass than to the frequencies where the ear canal resonates, the two panels of the sim tell one story from two directions. [[Hearing_loss|Hearing loss]], whether from noise, aging or disease, is common enough and consequential enough that its prevention and management are treated in their own right below, alongside what the mechanism of hearing looks like away from air — underwater — and in animals other than humans. ## Hearing mechanism Hearing begins as a purely mechanical problem: air carries sound as a small, fast pressure ripple, and turning that ripple into a nerve signal takes several stages of transduction, each solving a different physical mismatch along the way. ### Outer ear The visible pinna and the ear canal it opens into gather sound and funnel it to the eardrum at the canal's inner end. The canal, roughly 2.5 cm long and closed at one end, behaves acoustically like a short tube [[Acoustic_resonance|resonator]] and is largely responsible for the ear's particular sensitivity in the 2,000–5,000 Hz range, where speech consonants carry much of their information.[^upv1-hearing] ### Middle ear Sound in air is a low-pressure, low-impedance disturbance, but the cochlea beyond the middle ear is filled with fluid, whose acoustic impedance is far higher; without help, most of the sound energy reaching the boundary between the two, an [[Acoustic_impedance|impedance]] mismatch, would simply reflect away. The middle ear solves this impedance-matching problem mechanically: the eardrum's much larger area, concentrated onto the much smaller oval window by the lever action of three tiny bones — the malleus, incus and stapes — raises the pressure delivered to the fluid by roughly an order of magnitude, the same trick a hydraulic press or a transformer uses in other domains. *Citation needed: the widely quoted area and lever ratios for the human ossicular chain, and the resulting pressure-gain figure, are standard in auditory physiology but a primary source was not directly consulted for this article.* ### Inner ear Inside the [[Cochlea|cochlea]], a coiled fluid-filled tube about 35 mm long, the stapes' motion launches a traveling [[Wave|wave]] along the basilar membrane. Because the membrane's stiffness falls steadily from the stiff base, near the oval window, to the floppy apex at the cochlea's inner end, a wave of a given frequency grows as it travels, reaches a maximum at one particular place, and then dies away rapidly past that point — high frequencies peak near the base, low frequencies near the apex. Donald Greenwood's 1990 place-frequency map fits this relationship as f = 165.4·(10^(2.1x) − 0.88), where f is frequency in hertz and x is the fractional distance from the apex; a 125 Hz tone, for instance, peaks only about 3.6 mm from the apical end.[^greenwood1990] [[Hair_cell|Hair cells]] sitting along the membrane at that peak are bent by the local motion and open ion channels that begin the electrical signal carried onward by the [[Auditory_system|auditory nerve]] — a single physical structure doing the job a spectrum analyzer does electronically. How loud a tone sounds, though, is not simply a matter of physical intensity: it depends on frequency as well, because the outer, middle and inner ear are not equally sensitive at every frequency. Equal-loudness contours, standardized as ISO 226:2003 from listening-test data, join the sound levels at each frequency that listeners judge equally loud; a 1,000 Hz tone defines its own contour's "phon" number by convention, so a 60 dB, 1,000 Hz tone is by definition 60 phons, while a 125 Hz tone must reach a considerably higher decibel level to sound just as loud.[^iso226] Because the perceived loudness in "[[Sone|sones]]" derived from the [[Phon|phon]] scale roughly doubles for every added 10 phons, small differences in sound level near a listener's threshold matter far more to how loud something feels than the same decibel difference does at a high level.[^iso226] *Try: sweep the tone frequency and watch where the traveling wave peaks on the unrolled membrane on the left, then raise the sound level and watch which equal-loudness contour on the right the tone's point sits closest to; switch on the age-related loss overlay to see the high-frequency contour rise.* ### Neuronal [[Hair_cell|Hair cells]] convert the basilar membrane's motion into a graded electrical signal that triggers the release of neurotransmitter onto auditory-nerve fibers, each of which is tuned by its point of contact on the membrane to a narrow range of frequencies — the tonotopic map established mechanically in the cochlea is preserved, largely unchanged, all the way to the auditory cortex. At low frequencies the nerve's firing can also lock to particular phases of the sound wave itself, a timing cue that, pooled across many fibers by the "volley principle," extends well above the rate any single neuron could fire alone and underlies fine pitch and timing discrimination at low frequencies. ## Hearing tests The standard clinical test of hearing is pure-tone audiometry: a listener reports the faintest level they can detect at each of several standard [[Frequency|frequencies]], and the results are plotted as an [[Audiogram|audiogram]] — hearing level in decibels relative to a normal-hearing reference, against frequency — that shows at a glance which part of the range, if any, has become less sensitive. ## Hearing loss ### Causes Hearing loss has two broad physical origins: conductive loss, where something in the outer or middle ear blocks sound from reaching the cochlea efficiently (wax, fluid, a damaged eardrum, fixed ossicles), and sensorineural loss, where the cochlea's hair cells or the auditory nerve itself are damaged or lost. Sensorineural loss is far more often permanent, because unlike many other tissues the mammalian [[Cochlea|cochlea]] does not regrow hair cells once they die. The two dominant causes are age (presbycusis) and cumulative noise exposure, which damages hair cells mechanically and metabolically in proportion to sound level and duration. ### Prevention Noise-induced hearing loss is preventable because it is dose-dependent: the U.S. Occupational Safety and Health Administration sets a permissible occupational noise exposure of 90 dBA averaged over an 8-hour day, with an action level of 85 dBA at which hearing-conservation measures must begin, using a 5 dB exchange rate (each 5 dB increase in level halves the allowed exposure time); the U.S. Department of Defense's hearing-conservation program instead applies an 85 dBA action level with the steeper 3 dB exchange rate the National Institute for Occupational Safety and Health recommends.[^osha-fra] Outside occupational settings, the same physics applies to recreational exposure — loud concerts, personal audio players at high volume, and firearm discharge (which can exceed 140 [[Decibel|dB]] in a single impulse) all carry the same risk per unit of energy delivered to the cochlea. ### Management Where hearing loss is conductive, surgery or a hearing aid that simply amplifies sound can often restore useful hearing. Sensorineural loss, once hair cells are gone, cannot be reversed by amplification alone; a cochlear implant instead bypasses the damaged hair cells altogether, converting sound directly into electrical pulses delivered to the [[Auditory_system|auditory nerve]] through an array of electrodes threaded into the cochlea, restoring a coarse approximation of the frequency map described above. ### Relation to health Hearing loss is not only a sensory limitation. The Lancet Commission on dementia prevention, intervention and care identified midlife hearing loss as the single largest modifiable risk factor it considered for later dementia, estimating that addressing it could in principle reduce population dementia risk by a larger share than any of the other eleven factors in its 2020 model.[^lancet2020] The mechanism is debated — a noisier signal reaching the brain, the cognitive load of constant compensation, or the social withdrawal that often follows hearing loss are all proposed contributors — but the epidemiological association between [[Hearing_loss|hearing loss]] and dementia risk is one of the better-replicated findings in the field. ## Hearing underwater Hearing works differently submerged. [[Water|Water]] is a much denser, stiffer medium than air, so sound travels through it about four times faster — roughly 1,480 m/s against 343 m/s in air at room temperature — and a human head, whose tissue has an acoustic impedance close to water's, no longer casts the same sharp interaural time and level differences it does in air; sound instead reaches both ears largely by bone conduction, degrading a swimmer's or diver's ability to localize a sound's direction even though detection itself remains possible.[^upv1-speed] ## In vertebrates ### Frequency range The audible [[Frequency|frequency]] range varies widely across vertebrates and reflects the size of the animal's head and ear structures as much as its ecology: the young healthy human range is conventionally given as about 20 Hz to 20,000 Hz, dogs and cats hear well above that into the tens of kilohertz, and echolocating bats produce and hear calls extending past 100 kHz. At the other end, elephants and some other large animals communicate using infrasound below 20 Hz, frequencies too low for a human to perceive as a tone at all rather than as a felt vibration. *Citation needed: the specific upper-frequency figures commonly quoted for dogs, cats and bats, and the lower figures for elephant infrasound communication, are widely repeated in comparative-hearing literature but a primary source was not directly consulted for this article.* ### Time discrimination Directional hearing in air depends on tiny differences in arrival time and level between the two ears, and the human auditory system is remarkably precise at extracting them: listeners can detect interaural time differences on the order of tens of microseconds, far finer than the timing resolution of any single [[Auditory_system|auditory-nerve]] fiber, by pooling information across many fibers and across frequency. The physical basis of these interaural cues — the geometric path-length difference around a head of a given size — is the subject of [[Sound_localization|sound localization]]. ### In birds Bird ears lack an external pinna but otherwise share the vertebrate [[Cochlea|cochlear]] plan in a shorter, straighter tube called the basilar papilla; unlike the mammalian cochlea, the avian papilla can regenerate hair cells after damage, a capacity mammalian hearing research has long tried to understand and reproduce. Barn owls carry directional hearing to an extreme among birds: an asymmetric arrangement of the two ear openings gives each ear a different sensitivity pattern in elevation as well as azimuth, letting the owl localize a rustling target in complete darkness with an accuracy neurophysiologists have mapped down to single neurons tuned to particular combinations of interaural time and level difference.[^konishi] ## In invertebrates Many invertebrates detect sound without anything resembling a vertebrate ear. Moths and crickets carry a tympanal organ — a thin, tensioned membrane backed by an air sac, often on the thorax or a leg — that responds to the ultrasonic calls of hunting bats or to a mate's calling song, and mosquitoes and other flies instead sense the air particle velocity of nearby sound with feathery antennae driven by Johnston's organ at the antennal base, a fundamentally different physical strategy from the pressure-sensing ear of a vertebrate. ## Minnesota *This section is specific to Wikitube.* Mayo Clinic, based in Rochester, Minnesota, runs one of the country's larger clinical audiology and cochlear-implant programs, evaluating and treating both conductive and sensorineural hearing loss described above under Hearing loss.[^mayo] The clinic's presence ties the Acoustics portal's hearing-loss section to a real, sourced Minnesota institution rather than only to the underlying physics and physiology, which — the cochlear traveling wave, the equal-loudness contours, the OSHA exposure limits — are the same wherever they are applied. ## See also - [[Sound_localization|Sound localization]] - [[Bioacoustics]] - [[Equal-loudness_contour]] - [[Cochlea]] - [[Hearing_loss]] ## References [^upv1-hearing]: OpenStax, *University Physics Volume 1* (2016), §17.3 "Sound Intensity," subsection "Human Hearing and Sound Intensity Levels" (threshold and pain intensities, the ear canal's 2,000–5,000 Hz resonance range, Figure 17.14 anatomy of the ear, Table 17.2 sound intensity levels). https://openstax.org/details/books/university-physics-volume-1 [^greenwood1990]: Greenwood, Donald D. (1990). "A cochlear frequency-position function for several species — 29 years later." *Journal of the Acoustical Society of America* 87 (6): 2592–2605. https://doi.org/10.1121/1.399052 [^iso226]: International Organization for Standardization. *ISO 226:2003 — Acoustics: Normal equal-loudness-level contours.* The standard covers roughly 20–90 phon; loudness in sones follows Stevens' relation, sone = 2^((phon − 40)/10). https://www.iso.org/standard/34222.html [^osha-fra]: U.S. Occupational Safety and Health Administration, 29 CFR 1910.95, "Occupational noise exposure" (90 dBA permissible exposure limit for an 8-hour time-weighted average, 85 dBA action level, 5 dB exchange rate). https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.95 · U.S. Department of Defense, DoDI 6055.12, "Hearing Conservation Program" (85 dBA action level, 3 dB exchange rate, following NIOSH's recommended criteria). https://www.cdc.gov/niosh/docs/98-126/ [^lancet2020]: Livingston, Gill; Huntley, Jonathan; Sommerlad, Andrew; et al. (2020). "Dementia prevention, intervention, and care: 2020 report of the Lancet Commission." *The Lancet* 396 (10248): 413–446. https://doi.org/10.1016/S0140-6736(20)30367-6 [^upv1-speed]: OpenStax, *University Physics Volume 1* (2016), §17.2 "Speed of Sound," speed of sound in air (about 343 m/s at room temperature) and in water; the roughly fourfold ratio quoted in this article follows from the book's tabulated speeds. https://openstax.org/details/books/university-physics-volume-1 [^konishi]: Knudsen, Eric I.; Konishi, Masakazu (1978). "A neural map of auditory space in the owl." *Science* 200 (4343): 795–797. https://doi.org/10.1126/science.644324 [^mayo]: Mayo Clinic. "Hearing Loss" and "Cochlear Implants" patient-care pages, Rochester, Minnesota. Company/institutional release. https://www.mayoclinic.org/  **Microsim — three.js (Wikitube framework):** *Hearing* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/acoustics/Hearing.html" data-title="Hearing"></div> *Built from `MICROSIM_GUIDE/specs/acoustics/sims/Hearing.json`; part of the [[PORTAL_Acoustics|Acoustics portal]] spine (section sims and See-also variants).*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Hearing) : [Wikitube](https://en.wikitube.io/wiki/Hearing) - skeleton pinned to revision 1369615421 (2026-09-11).