Glossary

Terms used in this book.

A reference list of the technical vocabulary used in What is hearing?. Inline occurrences of these terms in the lessons are auto-tooltipped (dotted underline) so you can hover for a quick definition; for a fuller treatment with context, return here.

190 terms from this book.

A

A1: Primary auditory cortex, on Heschl's gyrus. The cortical entry point of the auditory pathway.
acoustic impedance: The ratio of acoustic pressure to particle velocity in a propagating wave (Z = p/v). For a plane wave in a medium of density ρ and wave speed c, Z = ρc.
acoustic reflex: Contraction of the stapedius muscle in response to loud sound (≥70–100 dB HL). Measured clinically by detecting a change in middle-ear admittance.
action potential: A brief (~1 ms) all-or-nothing voltage spike propagating along a neuron's axon. The fundamental unit of neural communication.
afferent: Neural fibres carrying signals from the periphery toward the brain. Type I afferents (95%) contact inner hair cells; Type II contact outer hair cells.
amplitude: The magnitude of a wave's departure from equilibrium. For sound, the size of the pressure fluctuation.
angular frequency: Rate of phase advance in radians per second: ω = 2πf.
antinode: A point in a standing wave where the amplitude is maximum. Located midway between nodes; the sites of maximum displacement or pressure variation.
audiogram: A graph of hearing thresholds (dB HL) vs. frequency (250–8000 Hz) for each ear. The fundamental clinical record of hearing sensitivity.
auditory nerve: The ~30,000-fibre bundle carrying spike-train information from the cochlea to the cochlear nucleus in the brainstem.
auditory streaming: The brain's perceptual organisation of acoustic input into one or more sources (streams). Depends on frequency separation, timing, and context.
auricula: Anatomical name for the pinna.
azimuth: The horizontal angle of a sound source relative to straight ahead, measured in degrees. 0° = front, 90° = right ear, 180° = behind.

B

basilar membrane: The membrane separating scala media from scala tympani. Its position-dependent stiffness gives different places different natural frequencies.
Bayes: Bayes' theorem: P(M|S) = P(S|M)·P(M)/P(S). The posterior over hypotheses given data combines likelihood with prior.
Bayes' theorem: P(M|S) = P(S|M)·P(M)/P(S). The brain's posterior over hypotheses given sensory data combines the likelihood with the prior.
Bayesian updating: Applying Bayes' rule sequentially: each posterior becomes the next prior. The foundation of online estimation and Kalman filtering.
binaural: Involving or comparing information from both ears; basis of spatial hearing in the brainstem.
bushy cell: A neuron in the anteroventral cochlear nucleus that preserves precise timing from the auditory nerve. Projects to the MSO and LSO for binaural processing.

C

capacitance: The ability to store charge per unit voltage: C = Q/V. For a parallel plate: C = εA/d. Energy stored = ½CV².
characteristic frequency: The frequency at which a given place on the basilar membrane (or auditory-nerve fibre) responds most strongly.
cochlea: The spiral, fluid-filled organ of the inner ear that performs frequency analysis on incoming sound and transduces it into neural signals.
cochlear amplifier: The active feedback process in the cochlea, driven by outer-hair-cell electromotility, that sharpens basilar-membrane tuning beyond passive mechanics.
cochlear implant: A surgically implanted device that bypasses damaged hair cells and directly stimulates auditory-nerve fibres with electrical pulses from an intracochlear electrode array.
cochlear nucleus: The first central auditory structure in the brainstem; receives all auditory-nerve fibres from the ipsilateral cochlea.
cochlear synaptopathy: Damage to inner-hair-cell synapses, especially low-spontaneous-rate fibres. Invisible on standard audiograms but causes difficulty hearing in noise ("hidden hearing loss").
cocktail-party effect: Ability to follow one voice among many competing talkers; driven by selective attention and streaming cues.
coincidence detection: A neural computation in which a cell fires maximally when inputs from both ears arrive simultaneously, indicating a specific ITD.
complex plane: The plane with real part horizontal and imaginary part vertical. Phasors, eigenvalues, and impedances all live here.
conductive hearing loss: Hearing loss caused by impaired sound transmission through the outer or middle ear (wax, fluid, ossicular fixation). Treatable medically or surgically.
contralateral: On the opposite side of the body relative to a reference structure or stimulus.
critical point: Thermodynamic state (T_c, p_c) above which liquid and vapor are indistinguishable; surface tension vanishes.
CROS: Contralateral Routing of Signal. A hearing-aid configuration for unilateral deafness: a microphone on the dead ear transmits wirelessly to a receiver on the better ear.

D

d-prime: d′ = (μ_signal − μ_noise)/σ. A bias-free measure of detection sensitivity in signal detection theory. Higher d′ = easier to tell signal from noise.
dB SPL: Decibels Sound Pressure Level. Referenced to 20 μPa (the threshold of hearing at 1 kHz). An absolute physical measure, unlike dB HL.
DCN: Dorsal Cochlear Nucleus. Integrates auditory-nerve input with somatosensory signals; involved in spectral-shape processing and sound localisation in elevation.
decibel: A logarithmic unit of ratio: 20·log10(amplitude ratio) or 10·log10(power ratio). Used for sound pressure level (SPL) and hearing level (HL).
delay line: A neural pathway that introduces a fixed time delay. In the Jeffress model, graded delay lines compensate for interaural time differences.
depolarization: A shift of membrane potential toward less negative values (e.g. −70 → −40 mV), making the cell more likely to fire. In hair cells, caused by K⁺ influx through MET channels.
diffraction: The bending of waves around obstacles or through apertures. Significant when the obstacle size is comparable to or smaller than the wavelength.
dispersion: The dependence of wave speed on frequency. In a dispersive medium, different frequency components travel at different speeds, distorting the waveform.
dispersion relation: The relation between frequency ω and wavenumber k for a wave. Non-dispersive: ω = ck. Dispersive: ω(k) is nonlinear.
divergence: A scalar measuring how much a vector field spreads from a point: ∇·v = ∂vₓ/∂x + ∂vᵧ/∂y + ∂v_z/∂z. Positive = source; negative = sink.
duplex theory: Rayleigh's theory (1907) that ITDs dominate low-frequency localisation and ILDs dominate high-frequency localisation.
dynamic range: The span between the softest and loudest signals a system can handle. The auditory system covers ~120 dB from threshold to pain.

E

ear canal: The tube about 25 mm long running from the pinna to the eardrum. Its closed-tube resonance amplifies frequencies near 3 kHz.
eardrum: The tympanic membrane: a thin sheet at the inner end of the ear canal that vibrates in response to pressure waves and drives the ossicular chain.
efferent: Neural fibres carrying signals from the brain toward the periphery. The medial olivocochlear efferents modulate outer-hair-cell gain.
electromotility: The voltage-driven length change of outer hair cells, mediated by prestin. Generates the active gain of the cochlear amplifier.
elevation: The vertical angle of a sound source above or below the horizontal plane. Encoded primarily by pinna spectral cues.
endbulb of Held: Giant synapse from an auditory-nerve fiber onto a bushy cell in AVCN; ensures sub-ms spike transmission fidelity.
endocochlear potential: The +80 mV potential of the endolymph in scala media relative to perilymph. Powers hair-cell transduction by driving K⁺ through open MET channels.
endolymph: The fluid in scala media; high in K⁺, low in Na⁺. Held at +80 mV (the endocochlear potential) relative to perilymph.
ERB: Equivalent Rectangular Bandwidth. The bandwidth of an idealised rectangular filter that passes the same power as an auditory filter at a given centre frequency.
Euler equation: Newton's second law for an inviscid fluid: ρ Dv/Dt = −∇p. The momentum equation of ideal fluid mechanics.
Eustachian tube: The tube connecting the middle-ear space to the nasopharynx. Opens during swallowing to equalise middle-ear pressure. Dysfunction causes negative pressure and fluid accumulation.
excitation pattern: The pattern of basilar-membrane vibration amplitude as a function of place for a given stimulus. Determines which auditory-nerve fibres are activated.
expectation: The probability-weighted average of a random variable: E[X] = ∫x·f(x)dx (continuous) or Σx·P(x) (discrete). Also called the mean or first moment.

F

false alarm rate: The probability of reporting 'signal present' when only noise is present; one axis of the ROC curve.
formant: A resonant peak in the spectrum of a vowel. The configuration of mouth and tongue produces F1, F2, F3… which together identify the vowel.
free-energy principle: Friston's proposal that self-organising systems minimise variational free energy — a bound on surprise under their generative model.
frequency: The number of oscillation cycles per second, measured in hertz (Hz). For sound, this is what the brain perceives as pitch.
frequency selectivity: The ability to resolve individual frequency components in a complex sound. Determined by cochlear mechanics and the sharpness of basilar-membrane tuning.

G

generative model: Internal probabilistic model the brain uses to predict sensory input; perception inverts it via Bayes' theorem.
glutamate: The excitatory neurotransmitter released by inner hair cells at ribbon synapses onto auditory-nerve fibres. Drives post-synaptic spiking.
Greenwood function: The empirical fit relating cochlear place x to characteristic frequency: f(x) = A(10^{α(1−x/L)} − K). For humans: A=165.4, α=2.1, K=0.88, L=35 mm.

H

harmonic: An integer multiple of the fundamental frequency. The nth harmonic has frequency nf₁. Harmonics are the building blocks of periodic signals.
head shadow: The attenuation of sound at the far ear caused by the head acting as an acoustic obstacle. Most effective at high frequencies (short wavelengths).
hearing aid: An electronic device that amplifies and processes sound to compensate for hearing loss. Modern aids are digital, programmable, and fit behind or inside the ear.
helicotrema: The small opening at the cochlear apex where scala vestibuli and scala tympani communicate, short-circuiting DC pressure.
Helmholtz equation: The time-independent wave equation: ∇²u + k²u = 0. Arises from separating the time dependence out of the wave equation for steady-state oscillations.
Heschl's gyrus: Transverse temporal gyrus on the superior temporal plane where primary auditory cortex (A1) is located.
Hopf bifurcation: The critical point at which a damped oscillator becomes self-sustained. The mammalian cochlea operates near this bifurcation, giving it compressive nonlinearity.
HRTF: Head-Related Transfer Function. The frequency-dependent filter the head, pinnae, and torso apply between a sound source in space and the eardrum.
hyperpolarization: A shift of membrane potential toward more negative values, making the cell less likely to fire. In hair cells, occurs when stereocilia deflect away from the tall edge.

I

ILD: Interaural Level Difference. The intensity difference between the two ears, dominant at frequencies whose wavelength is smaller than the head.
impedance: The ratio of a driving quantity to a flow quantity. In acoustics: pressure/velocity. A measure of how strongly a medium resists being moved by a wave.
impedance mismatch: A difference in acoustic impedance between two media at a boundary. The greater the mismatch, the more energy is reflected and the less is transmitted.
incus: The second middle-ear ossicle (the "anvil"); pivots between malleus and stapes.
inferior colliculus: A midbrain nucleus and obligatory waystation for ascending auditory information. Where spatial cues, spectral features, and temporal patterns converge.
inner hair cell: The cochlea's actual sensory transducer. ~3,500 along the basilar membrane, each contacting 10–30 auditory-nerve fibres.
interaural level difference: The intensity difference between the two ears, dominant at frequencies whose wavelength is smaller than the head.
interaural time difference: The arrival-time difference for a sound between the two ears. Humans resolve differences as small as 10 μs.
ipsilateral: On the same side of the body as the reference structure or stimulus.
ITD: Interaural Time Difference. The arrival-time difference for a sound between the two ears. Humans resolve differences as small as 10 μs.

J

Jeffress model: Lloyd Jeffress (1948) proposed delay-line coincidence detectors as the neural mechanism for ITD measurement; canonical picture for the MSO.

L

lateral lemniscus: The white-matter tract carrying the ascending auditory pathway from cochlear nucleus and superior olive to the inferior colliculus.
lateral superior olive: LSO. Brainstem nucleus that computes interaural level differences from excitatory ipsilateral and inhibitory contralateral inputs.
law of large numbers: The sample mean converges to the true mean as sample size grows: (1/n)Σxᵢ → E[X]. The mathematical basis for empirical measurement.
likelihood: In Bayesian inference, P(S|M): how probable the observed sensory input is given that hypothesis M is true.
likelihood ratio: The ratio f₁(x)/f₀(x) of probability densities under two hypotheses. The optimal statistic for distinguishing signal from noise.
LSO: Lateral Superior Olive. Brainstem nucleus that computes interaural level differences.

M

malleus: The first middle-ear ossicle (the "hammer"); attached to the eardrum on one side and the incus on the other.
McGurk effect: A multisensory illusion: visual articulation of one syllable combined with audio of another produces a third perceived syllable.
medial geniculate body: MGB. The thalamic relay for auditory information, between the inferior colliculus and primary auditory cortex.
medial superior olive: MSO. Brainstem nucleus that computes interaural time differences using coincidence detection of binaural inputs.
membrane time constant: Product RC of membrane resistance and capacitance; sets how quickly a neuron's voltage responds to input.
MET channel: Mechanoelectrical Transduction channel. A mechanically-gated ion channel on stereocilia that opens when the bundle is deflected toward the tall edge.
mismatch negativity: ERP component (150–250 ms) elicited by a deviant stimulus in a repetitive sequence; indexes automatic change detection.
MSO: Medial Superior Olive. Brainstem nucleus that computes interaural time differences.

N

natural frequency: The frequency at which an undamped system oscillates freely: ω₀ = √(k/m) for a mass-spring. The intrinsic resonance of the system.
Nernst equation: Equilibrium potential for an ion across a membrane: E = (RT/zF) ln(c_out/c_in). Sets cochlear potentials.
newborn hearing screening: Universal testing of newborns (typically OAE or automated ABR) to detect hearing loss early enough for intervention before 6 months of age.

O

octave: Interval corresponding to a 2:1 frequency ratio; the fundamental unit of musical pitch perception.
organ of Corti: The sensory structure on the basilar membrane containing inner and outer hair cells, supporting cells, and the tectorial membrane.
ossicles: The three smallest bones in the body — malleus, incus, stapes — transmitting motion from the eardrum to the oval window of the cochlea.
otoacoustic emission: A faint sound generated by the cochlear amplifier and emitted outward through the ear canal; used in newborn hearing screening.
outer hair cell: Active mechanical element of the organ of Corti. ~12,000 of them; they shorten and lengthen with voltage, sharpening basilar-membrane tuning.
oval window: A membrane-covered opening at the base of scala vestibuli, where the stapes footplate drives the cochlear fluid.
overtone: Any mode of vibration above the fundamental; for strings and tubes the overtones are integer harmonics.

P

particle velocity: The oscillatory velocity of a fluid element as a sound wave passes through it. Related to pressure by Z = p/v in a plane wave.
perilymph: The fluid in scala vestibuli and scala tympani; ionic composition resembles extracellular fluid (high Na⁺, low K⁺).
phase locking: The tendency of an auditory-nerve fibre to fire at a preferred phase of a low-frequency stimulus cycle, up to ~5 kHz.
phase velocity: The speed at which a single-frequency wave's phase fronts travel: v_p = ω/k. May exceed c in dispersive media without violating causality.
phoneme: The smallest unit of sound that distinguishes meaning in a language (e.g. /b/ vs /p/). English has ~44 phonemes.
phonemic restoration: A perceptual effect in which a phoneme replaced by noise is heard as intact, restored by context-driven top-down priors.
piezoelectric: A material that generates voltage when mechanically stressed (and deforms when voltage is applied). Basis of ultrasound transducers, quartz oscillators, MEMS sensors.
pinna: The visible outer flap of the ear. Its folds and ridges encode elevation by introducing spectral notches that depend on the sound source's vertical angle.
place coding: Encoding of frequency by which location on the basilar membrane (or fibre in the auditory nerve) is most active.
plane wave: A wave whose phase fronts are infinite parallel planes; idealisation of a wave from a distant source, valid locally near the listener.
Poisson process: A stochastic process in which events occur independently at a constant average rate λ. The number of events in any interval is Poisson-distributed with mean λt.
posterior: In Bayesian inference, P(M|S): the probability of hypothesis M given sensory input S. What the brain perceives.
precedence effect: The perceptual rule that the listener attributes the source to the first-arriving wavefront, even when reflections arrive from other directions.
prediction error: Difference between actual sensory input and the top-down prediction; drives model updating in predictive coding.
predictive coding: A theory of cortical processing in which higher layers predict lower-layer activity, and only prediction errors propagate upward.
prestin: The membrane protein in outer hair cells that changes conformation with voltage, producing the cell's electromotility.
primary auditory cortex: A1. The first cortical processing stage for sound, on Heschl's gyrus. Tonotopically organised.
prior: In Bayesian inference, P(M): the probability of a hypothesis before any sensory input. Shaped by context, expectation, and learned experience.
pure tone: A sinusoidal sound at a single frequency. Used in audiometry to measure frequency-specific hearing thresholds.

Q

quality factor: Q: a dimensionless measure of resonance sharpness. Equal to center frequency divided by bandwidth. Higher Q = sharper tuning.

R

rate coding: Encoding of stimulus intensity (or frequency above the phase-locking limit) by the firing rate of a neuron.
reactance: The imaginary part X of a complex impedance Z = R + iX; stores energy without dissipating it.
receptor potential: Graded voltage change in a hair cell produced by MET-channel current; drives neurotransmitter release.
reflection: When a wave hits a boundary between two media, part of its energy turns back into the first medium. R = (Z2 − Z1)/(Z1 + Z2).
reflection coefficient: The ratio of reflected to incident wave amplitude at a boundary: R = (Z2 − Z1)/(Z1 + Z2). Ranges from −1 (soft boundary) to +1 (rigid boundary).
Reissner's membrane: Thin membrane separating scala vestibuli from scala media; mechanically near-transparent to the traveling wave.
resonance: The condition where a driving frequency matches a system's natural frequency, producing maximum response amplitude.
resting potential: The membrane voltage of a cell at rest, typically −60 to −70 mV for neurons. Set by the balance of K⁺ leak and Na⁺/K⁺-ATPase activity.
restoring force: Force directed toward equilibrium, proportional to displacement for small perturbations (F = −kx).
reverberation: The collection of reflections that follows a direct sound in a room, gradually decaying. Characterised by reverberation time T60.
ribbon synapse: A specialised presynaptic structure in inner hair cells that releases vesicles continuously at high rates with sub-millisecond precision.
round window: A flexible membrane at the base of scala tympani that bulges outward to accommodate the incompressible cochlear fluid displaced by stapes motion.

S

sample rate: Samples per second f_s when digitising a signal; must exceed 2× the maximum frequency to avoid aliasing.
scala media: The middle cochlear chamber, filled with endolymph; contains the organ of Corti.
scala tympani: The lower of the three cochlear chambers, filled with perilymph; terminates at the round window.
scala vestibuli: The upper of the three cochlear chambers, filled with perilymph; the stapes pushes into it at the oval window.
Shepard tone: An auditory illusion of continuously rising (or falling) pitch, created by stacking octaves with a fading amplitude envelope.
short-time Fourier transform: Windowed Fourier transform giving local spectral content; the basis of spectrogram computation.
signal detection theory: A framework for analysing decisions under uncertainty: the observer must decide whether a noisy observation came from a signal-present or signal-absent distribution.
sinusoid: A function of the form A sin(ωt + φ) or equivalently A cos(ωt + φ). The basic periodic waveform; all Fourier components are sinusoids.
spectral cue: Frequency-dependent modifications imposed by the pinna that encode source elevation and front/back location. Mainly notches and peaks above 4 kHz.
spectrogram: Time-frequency image formed by computing short-time Fourier transforms of successive windowed signal segments.
spectrotemporal: Relating jointly to frequency and time; describes receptive fields and stimulus representations in auditory cortex.
spectrum: The frequency-domain representation of a signal: the set of amplitudes and phases at each frequency component.
speech banana: The banana-shaped region on an audiogram (250–6000 Hz, 20–50 dB HL) where the phonemes of conversational speech typically fall.
speed of sound: The propagation speed of small-amplitude pressure disturbances. ≈343 m/s in air at room temperature, ≈1480 m/s in water.
spiral ganglion: The collection of neuronal cell bodies in the cochlea's modiolus whose axons form the auditory nerve.
spontaneous rate: The firing rate of an auditory-nerve fibre in silence. High-SR fibres (~60 sp/s) have low thresholds; low-SR fibres (~1 sp/s) have high thresholds and wider dynamic range.
standing wave: A wave pattern formed by superposition of two waves traveling in opposite directions. Characterised by fixed nodes (zero amplitude) and antinodes (maximum amplitude).
stapedius reflex: A bilateral reflex that contracts the stapedius muscle in response to loud sounds, attenuating ossicular transmission at low frequencies by 10–15 dB.
stapes: The third middle-ear ossicle (the "stirrup"); its footplate seats in the oval window and drives the cochlear fluid.
steady state: The long-time behaviour of a driven system after transients have died away. For a sinusoidally driven linear system, it oscillates at the driving frequency.
stereocilia: Actin-filled rod-like protrusions on hair cells, arranged in a graded staircase with tip links connecting neighbours.
STRF: Spectro-Temporal Receptive Field. The 2-D function (frequency × time) characterising what stimulus features drive a cortical neuron.
stria vascularis: Metabolically active epithelium on the lateral wall of scala media that maintains the endocochlear potential.
superior olivary complex: A brainstem region where information from both ears first converges. Contains the MSO (timing) and LSO (level) nuclei.
superior temporal gyrus: STG. Temporal cortex extending forward from A1; contains regions specialised for phonemic and speech processing.

T

tectorial membrane: A gelatinous strip overhanging the organ of Corti. Couples basilar-membrane motion to the stereocilia of the outer hair cells.
thalamus: A central diencephalic structure relaying most sensory information to cortex. The medial geniculate body is its auditory subdivision.
timbre: Perceptual quality distinguishing sounds of the same pitch and loudness; determined by spectral envelope shape.
time constant: τ = 1/α: the time for an exponentially decaying quantity to fall to 1/e ≈ 37% of its initial value.
tinnitus: The perception of sound (ringing, buzzing, hissing) without an external source. Often associated with cochlear damage and central gain changes.
tip link: A molecular thread connecting the top of one stereocilium to the side of its taller neighbour. Stretching opens the MET channel.
tonotopic map: Spatial arrangement preserving the cochlea's frequency-to-place mapping through each stage of the auditory pathway.
tonotopy: Spatial organisation in which neighbouring elements respond to neighbouring frequencies. Preserved from cochlea through brainstem to cortex.
transfer function: The frequency-domain ratio of output to input phasor: H(ω) = Y(ω)/X(ω). Fully characterises a linear system's frequency response.
trapezoid body: The white-matter tract carrying axons from the ventral cochlear nucleus across the midline to the contralateral superior olivary complex.
traveling wave: A wave propagating along the basilar membrane from base to apex, growing in amplitude until reaching its characteristic-frequency place, then dying abruptly.
tuning curve: A plot of the minimum sound level needed to excite a neuron as a function of frequency. The tip indicates the characteristic frequency; the shape shows selectivity.
tympanic membrane: The eardrum: a thin sheet that vibrates in response to pressure waves and drives the ossicular chain.

V

variance: The expected squared deviation from the mean: Var(X) = E[(X−μ)²]. Measures the spread of a distribution; its square root is the standard deviation.
vector strength: A measure of phase-locking: |⟨exp(iφ)⟩| averaged over spikes. 1 = perfect synchrony; 0 = random firing phase.
vesicle: A membrane-bound sphere (~40 nm) containing neurotransmitter molecules. Fusion with the presynaptic membrane releases transmitter into the synaptic cleft.
viscoelastic: A material exhibiting both viscous (time-dependent, dissipative) and elastic (instantaneous, recoverable) response to deformation. Characterised by a complex modulus.
viscous damping: Energy loss from viscous shear at the bubble wall; term −4μṘ/R in the Rayleigh-Plesset equation.
volume velocity: The product of particle velocity and cross-sectional area (U = v·A). The acoustic analogue of electric current in lumped-element circuit models.

W

wave equation: A second-order PDE describing how a disturbance propagates. For pressure in air: ∂²p/∂t² = c²∇²p.
wavefront: A surface of constant phase in a propagating wave. Plane waves have flat wavefronts; point sources have spherical wavefronts.
wavenumber: The spatial frequency of a wave: k = 2π/λ. Higher k means shorter wavelength and more rapid spatial oscillation.
WKB approximation: A method for solving wave equations with slowly-varying coefficients. Gives A(x)·exp(i·∫κ(x)dx) for position-dependent wavenumber κ.