Glossary

Terms used in this book.

A reference list of the physical vocabulary used in Physics Foundations. Inline occurrences in the chapters are auto-tooltipped; this page lists every term in the book’s domain alphabetically.

185 terms from this book.

A

absorption coefficient: The fraction of incident sound energy absorbed by a surface (α = 1 − |R|²). Ranges from 0 (perfect reflection) to 1 (perfect absorption).
acoustic energy density: Energy per unit volume in a sound field: e = p²/(2ρc²) + ½ρv². Sum of potential (pressure) and kinetic (velocity) energy densities.
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.
action potential: A brief (~1 ms) all-or-nothing voltage spike propagating along a neuron's axon. The fundamental unit of neural communication.
adiabatic: A process in which no heat is exchanged with the surroundings. Sound propagation in air is adiabatic because compressions/rarefactions happen too fast for heat conduction.
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.
auditory nerve: The ~30,000-fibre bundle carrying spike-train information from the cochlea to the cochlear nucleus in the brainstem.

B

basilar membrane: The membrane separating scala media from scala tympani. Its position-dependent stiffness gives different places different natural frequencies.
Bernoulli equation: For steady, inviscid, incompressible flow along a streamline: P + ½ρv² + ρgz = const. Energy conservation per unit volume.
Boltzmann constant: k_B = 1.381×10⁻²³ J/K. Relates temperature to energy at the molecular scale: average kinetic energy per degree of freedom = ½k_BT.
Bond number: Bo = ΔρgL²/γ. Ratio of gravitational to surface-tension forces. Bo ≪ 1: surface tension dominates (small drops, capillary phenomena).
boundary layer: The thin region near a solid surface where viscous effects are significant and velocity transitions from zero (no-slip) to the free-stream value.
Brownian motion: The random motion of a particle suspended in a fluid, driven by molecular collisions. Mathematically: a continuous-time stochastic process with Gaussian independent increments.
Buckingham π theorem: If a physical law involves n variables with k independent dimensions, it can be rewritten in terms of n−k dimensionless groups (π groups).
bulk modulus: The resistance of a material to uniform compression: K = −V(dp/dV). Higher K means stiffer material and faster sound propagation.

C

capacitance: The ability to store charge per unit voltage: C = Q/V. For a parallel plate: C = εA/d. Energy stored = ½CV².
capillary length: Length scale l_c = √(σ/ρg) below which surface tension dominates gravity; ~2.7 mm for water.
capillary number: Ca = μv/γ. Ratio of viscous to surface-tension forces. Governs the shape of menisci and the dynamics of wetting and dewetting.
capillary rise: Height h = 2σ cos θ/(ρgr) a liquid climbs in a narrow tube due to surface tension (Jurin's law).
Carnot: The maximum-efficiency heat engine operating between two reservoirs: η = 1 − T_cold/T_hot. No real engine can exceed Carnot efficiency.
Cauchy stress tensor: Symmetric rank-2 tensor σᵢⱼ giving force per unit area on any internal plane in a deformed continuum.
chemical potential: μ = (∂G/∂N)_{T,P}. The free-energy cost of adding one particle to the system. Drives diffusion, phase transitions, and chemical equilibria (μ equalises at equilibrium).
Clausius-Clapeyron: dP/dT = L/(TΔv). Relates the slope of a phase boundary to the latent heat and volume change. Predicts how boiling point shifts with pressure.
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.
compressibility: The fractional volume change per unit applied pressure: κ = −(1/V)(dV/dp). The reciprocal of the bulk modulus. Air's high compressibility is why sound exists in it.
contact angle: The angle θ at which a liquid-gas interface meets a solid surface. θ < 90° = wetting (hydrophilic); θ > 90° = non-wetting (hydrophobic). Set by Young's equation.
continuity equation: The local conservation law: ∂ρ/∂t + ∇·(ρv) = 0. Any density change equals the negative divergence of the flux.
control volume: A fixed region in space through which fluid flows; used to derive conservation laws in integral form.
convective derivative: D/Dt = ∂/∂t + U·∇; rate of change following a fluid element moving at velocity U.
Coulomb's law: Force between point charges: F = kq₁q₂/r². The electric field of a point charge falls as 1/r².
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

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).
differential equation: An equation relating an unknown function to its derivatives. ODEs involve one variable; PDEs involve several.
diffusion coefficient: D, in Fick's law J = −D∇c. Measures how fast a species spreads. For a sphere in fluid: D = k_BT/(6πμa) (Stokes-Einstein).
diffusion equation: The parabolic PDE ∂u/∂t = D∇²u describing how a concentration or temperature field spreads out over time. Solutions are Gaussian spreading profiles.
diffusivity: The constant D in the diffusion equation u_t = D∇²u; sets how quickly spatial gradients are smoothed.
dimensionless number: A ratio of physical quantities that has no units. Characterises the relative importance of different physical effects (e.g. Re, Ma, Q).
directivity: Angular dependence of a source's radiated field; D(θ) = 1 on-axis, with nulls and sidelobes off-axis.
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.
divergence theorem: Relates a volume integral of a divergence to a surface integral: ∫∫∫(∇·F)dV = ∮(F·n̂)dA. Converts between flux through a surface and source strength inside.
dynamic viscosity: Coefficient μ relating shear stress to velocity gradient in a Newtonian fluid: τ = μ(du/dy).

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.
electromotility: The voltage-driven length change of outer hair cells, mediated by prestin. Generates the active gain of the cochlear amplifier.
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.
enthalpy: H = U + PV. The heat content at constant pressure: ΔH = Q_p for isobaric processes. The natural potential for constant-pressure chemistry and engineering.
entropy: A measure of the number of microstates consistent with a macrostate: S = k_B ln Ω. Thermodynamically: dS = δQ_rev/T. Always increases in isolated systems.
equation of state: A relation among state variables (P, V, T, n) that closes the thermodynamic description. Ideal gas: PV = nRT. Real gases: van der Waals, Redlich-Kwong, etc.
equipartition: Each quadratic degree of freedom in thermal equilibrium carries average energy ½k_BT. Gives heat capacities for ideal gases and harmonic solids.
Euler equation: Newton's second law for an inviscid fluid: ρ Dv/Dt = −∇p. The momentum equation of ideal fluid mechanics.

F

far field: The region far enough from a source (r ≫ λ and r ≫ source size) that the wavefronts are effectively planar and pressure falls as 1/r.
Faraday's law: A changing magnetic flux through a loop induces an EMF: ε = −dΦ_B/dt. Foundation of generators, transformers, and electromagnetic induction.
Fick's law: The flux of a diffusing species is proportional to its concentration gradient: J = −D∇c. The diffusion analogue of Fourier's law for heat.
Fourier transform: A mathematical operation that decomposes a signal into its sinusoidal components. Time-domain ↔ frequency-domain pair.
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

Gauss's law: Maxwell equation ∇·E = ρ/ε₀; electric flux through any closed surface equals enclosed charge / ε₀.
Gibbs free energy: G = H − TS. The thermodynamic potential minimised at constant T and P. ΔG < 0 for a spontaneous process; ΔG = 0 at equilibrium.
Green's function: The response of a linear system to a unit impulse. Fully characterises propagation from source to receiver in any linear medium.
group velocity: The speed at which a wave packet's envelope (and its energy) travels: v_g = dω/dk. The physically meaningful propagation speed for a signal.

H

half-life: The time for a decaying quantity to reach half its initial value; related to time constant by T½ = τ ln 2.
Hamiltonian: The operator Ĥ = −(ℏ²/2m)∇² + V representing total energy; its eigenvalues are the allowed energies.
harmonic: An integer multiple of the fundamental frequency. The nth harmonic has frequency nf₁. Harmonics are the building blocks of periodic signals.
heat capacity: The energy required to raise a system's temperature by 1 K. C_V (constant volume) and C_P (constant pressure) differ by nR for an ideal gas.
heat equation: The parabolic PDE ∂u/∂t = α∇²u describing thermal conduction (or any diffusive process). Solutions smooth out discontinuities instantly.
Helmholtz free energy: F = U − TS. The thermodynamic potential minimised at constant T and V. Work obtainable equals −ΔF.
heterogeneous nucleation: Bubble formation at pre-existing weak points: surface crevices, impurities, dissolved gas pockets. The dominant mechanism in all practical cavitation scenarios.
homogeneous nucleation: Spontaneous formation of a vapour cavity in a pure liquid with no pre-existing defects. Requires enormous negative pressures (rarely achieved in practice).
Hooke's law: The linear relation between stress and strain for small deformations: σ = Eε (1-D) or σ_ij = C_ijkl ε_kl (general). Valid below the elastic limit.
HRTF: Head-Related Transfer Function. The frequency-dependent filter the head, pinnae, and torso apply between a sound source in space and the eardrum.
hydrogen bond: A strong (~20 kJ/mol) directional interaction between a hydrogen bonded to an electronegative atom (O, N, F) and a nearby lone pair. Dominates water's anomalous properties.
hydrophilic: Tending to attract water; contact angle < 90°. Hydrophilic surfaces wet easily.
hydrophobic: Tending to repel water; contact angle > 90°. Hydrophobic crevices trap gas pockets that serve as nuclei.

I

ideal gas: A model gas of non-interacting point particles obeying PV = nRT. Valid at low density and high temperature where intermolecular forces are negligible.
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.
isothermal: Process at constant temperature; Newton's (incorrect) assumption for sound gave c ≈ 280 m/s in air.

K

kinematic viscosity: Ratio ν = μ/ρ with units m²/s; a momentum diffusivity setting the Reynolds number Re = UL/ν.
Knudsen number: Kn = λ/L. Ratio of mean free path to characteristic length. Kn ≪ 1: continuum (Navier-Stokes valid). Kn ≫ 1: free-molecular regime.

L

laminar: Smooth, layered flow in which fluid parcels follow parallel streamlines without mixing. Occurs at low Reynolds number.
Laplace pressure: The pressure difference across a curved interface: ΔP = γ(1/R₁ + 1/R₂). For a sphere of radius R: ΔP = 2γ/R. Drives capillary rise and bubble stability.
latent heat: The energy absorbed or released during a phase transition at constant temperature. Latent heat of vaporisation for water: 2260 kJ/kg.
Lennard-Jones: A model pair potential V(r) = 4ε[(σ/r)¹² − (σ/r)⁶] combining short-range Pauli repulsion (r⁻¹²) with long-range van der Waals attraction (r⁻⁶).
linear mass density: Mass per unit length μ of a string; enters the 1-D wave speed as c = √(T/μ).
linearisation: Replacing a function by its first-order Taylor approximation near a point: f(x) ≈ f(x₀) + f′(x₀)(x−x₀). Valid when |x−x₀| is small.
London dispersion: The universal van der Waals attraction between any two atoms/molecules, arising from correlated fluctuations of their electron clouds. Falls as r⁻⁶.
lubrication theory: Thin-film limit of Navier-Stokes for flow between closely spaced surfaces; gives the cochlear long-wave equation.

M

Mach number: The ratio of flow speed to the local speed of sound: M = v/c. M < 1 is subsonic; M > 1 is supersonic.
malleus: The first middle-ear ossicle (the "hammer"); attached to the eardrum on one side and the incus on the other.
material derivative: The time derivative following a fluid element: D/Dt = ∂/∂t + v·∇. Accounts for both local change and advection by the flow.
Maxwell-Boltzmann: The probability distribution of molecular speeds in a gas at thermal equilibrium: f(v) ∝ v²exp(−mv²/2k_BT). Peak speed = √(2k_BT/m).
mean free path: The average distance a molecule travels between collisions: λ = 1/(√2·n·σ). In air at STP: λ ≈ 68 nm.
MEMS: Microelectromechanical Systems. Micron-scale devices integrating mechanical and electrical components on a chip. Used in accelerometers, microphones, pressure sensors.
meniscus: Curved liquid surface at a container wall or inside a capillary, shaped by adhesion and surface tension.
MET channel: Mechanoelectrical Transduction channel. A mechanically-gated ion channel on stereocilia that opens when the bundle is deflected toward the tall edge.
metastable: A thermodynamic state that is locally stable but not the global equilibrium. A superheated liquid or tensile liquid is metastable with respect to the vapour phase.
microbubble: Free gas bubble small enough to remain suspended (< 100 μm); a common cavitation nucleus.
modulus: The magnitude of a complex number: |z| = √(a² + b²) for z = a + bi. Represents the amplitude when z is a phasor.
monopole: A point source that radiates sound equally in all directions (omnidirectional). The simplest acoustic source; its field falls as 1/r.

N

Navier-Stokes: The fundamental equations of viscous fluid motion: ρ(Dv/Dt) = −∇P + μ∇²v + f. Newton's second law for a continuum with viscosity μ.
negative pressure: A liquid under tension (P < 0): molecules are pulled apart but remain cohesive. The metastable state that precedes cavitation inception.
Nernst equation: Equilibrium potential for an ion across a membrane: E = (RT/zF) ln(c_out/c_in). Sets cochlear potentials.
no-slip condition: Boundary condition requiring fluid velocity to equal zero at a solid wall; creates the boundary layer.
nucleation: The formation of a new phase (vapour bubble, crystal) from a metastable parent phase. Requires overcoming a free-energy barrier set by surface tension.
nucleation site: Preexisting weakness (crevice, free gas, impurity) in a liquid that initiates bubble formation at practical tensions.

O

Ohnesorge number: Oh = μ/√(ρσL). Ratio of viscous to inertio-capillary forces. Classifies droplet behaviour: low Oh = oscillatory breakup; high Oh = viscous breakup.
ossicles: The three smallest bones in the body — malleus, incus, stapes — transmitting motion from the eardrum to the oval window of the cochlea.
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.

P

P-wave: Primary (longitudinal) elastic wave; particles oscillate along propagation direction. Speed c_P = √((K+4G/3)/ρ).
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.
Peclet number: Pe = vL/D. Ratio of advective to diffusive transport. Pe ≫ 1: convection dominates. Pe ≪ 1: diffusion dominates.
perilymph: The fluid in scala vestibuli and scala tympani; ionic composition resembles extracellular fluid (high Na⁺, low K⁺).
phase transition: A transformation between distinct states of matter (solid, liquid, gas) or order parameters. First-order transitions have latent heat; second-order transitions are continuous.
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.
piezoelectric: A material that generates voltage when mechanically stressed (and deforms when voltage is applied). Basis of ultrasound transducers, quartz oscillators, MEMS sensors.
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's ratio: ν = −ε_transverse/ε_axial. The ratio of lateral contraction to axial extension under uniaxial stress. Ranges from −1 to 0.5; most materials: 0.2–0.4.
polytropic: A process in which PVⁿ = const for some exponent n. For gas in a bubble: n = 1 (isothermal) to γ (adiabatic) depending on thermal exchange rate.
polytropic exponent: Effective exponent κ (1 ≤ κ ≤ γ) for bubble gas compression; interpolates between isothermal and adiabatic.
prestin: The membrane protein in outer hair cells that changes conformation with voltage, producing the cell's electromotility.

R

radiation pressure: A steady force per unit area exerted by a sound wave on an absorbing or reflecting surface. Proportional to the time-averaged energy density.
random walk: A stochastic process consisting of successive random steps. In the simplest form: ±1 steps with equal probability. The mean displacement grows as √N.
Rankine-Hugoniot: Jump conditions for mass, momentum, and energy across a shock discontinuity in a compressible flow.
rate of change: How quickly a quantity varies with respect to another variable. The derivative gives the instantaneous rate of change.
Rayleigh-Plesset: The ODE governing spherical bubble dynamics: RR̈ + (3/2)Ṙ² = (1/ρ)[p_B − p_∞ − ...]. Foundation of all cavitation bubble models.
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).
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).
Reynolds number: Re = ρvL/μ. The ratio of inertial to viscous forces. Re ≪ 1: Stokes flow (viscosity dominates). Re ≫ 1: inertia dominates, turbulence possible.
ribbon synapse: A specialised presynaptic structure in inner hair cells that releases vesicles continuously at high rates with sub-millisecond precision.
root-mean-square: The square root of the time-averaged square of a signal: f_rms = √((1/T)∫f²dt). For a sinusoid, amplitude/√2.

S

S-wave: Secondary (shear) elastic wave; particles oscillate perpendicular to propagation. Cannot propagate in fluids.
scala media: The middle cochlear chamber, filled with endolymph; contains the organ of Corti.
scalar field: A function assigning a single number to each point in space (temperature, pressure, potential). Visualised as contour maps.
scaling law: A power-law relationship between quantities that holds across scales. Often derivable from dimensional analysis without solving the full equations.
shear modulus: G = τ/γ. The ratio of shear stress to shear strain. Measures resistance to shape change at constant volume. Related to E by G = E/2(1+ν).
shear stress: The component of stress acting tangentially to a surface: τ = F_tangential/A. In a Newtonian fluid: τ = μ(dv/dy).
shock wave: A thin region of abrupt pressure, density and temperature change propagating supersonically. Nonlinear steepening of a large-amplitude wave into a discontinuity.
sonoluminescence: Light emission from a collapsing cavitation bubble, caused by extreme heating of the gas during the final stage of inertial collapse.
specific heat ratio: γ = cₚ/cᵥ; ratio of heat capacities. For diatomic air γ = 1.4; enters sound speed as c = √(γRT/M).
spectrum: The frequency-domain representation of a signal: the set of amplitudes and phases at each frequency component.
speed of sound: The propagation speed of small-amplitude pressure disturbances. ≈343 m/s in air at room temperature, ≈1480 m/s in water.
spinodal: The thermodynamic limit of metastability beyond which a phase becomes absolutely unstable. On the P-V diagram: where (∂P/∂V)_T = 0.
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).
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.
Stokes flow: Fluid flow at Re ≪ 1 where inertia is negligible and the Navier-Stokes equations reduce to ∇P = μ∇²v. Reversible, linear, dominated by viscosity.
Stokes-Einstein: D = k_BT/(6πμa). Relates the diffusion coefficient of a spherical particle of radius a to temperature and fluid viscosity.
strain: The fractional deformation of a material: ε = ΔL/L for linear strain. Dimensionless; linked to stress via the constitutive law (e.g. Hooke's law).
stress: Force per unit area within a material (σ = F/A). A tensor quantity: normal stresses (tension/compression) and shear stresses act on different faces of an element.
stria vascularis: Metabolically active epithelium on the lateral wall of scala media that maintains the endocochlear potential.
superposition: The principle that solutions of a linear equation can be added to give new solutions. The foundation of Fourier methods and modal analysis.
surface tension: The energy per unit area (or force per unit length) at a liquid interface, arising from the cohesive deficit of surface molecules. Units: N/m or J/m².
surfactant: A molecule with a hydrophilic head and hydrophobic tail that accumulates at interfaces, lowering surface tension. Critical in lung mechanics and microfluidics.

T

tangent line: The straight line that touches a curve at a single point and matches its slope there. Its slope equals the derivative at that point.
Taylor expansion: An expansion of a smooth function f near a base point x0 as an infinite sum f(x0) + f′(x0)(x−x0) + ½f″(x0)(x−x0)² + … . Truncating gives a polynomial approximation of any required order.
tensile strength: The maximum negative pressure a liquid can sustain before rupturing into vapour. Theoretical: ~GPa for pure water; practical: ~MPa due to pre-existing nuclei.
tensor: A multilinear map that transforms under coordinate changes according to specific rules. Scalars are rank-0, vectors rank-1, stress/strain are rank-2 tensors.
thermal diffusivity: Coefficient α = k/(ρcₚ) governing heat conduction rate; determines whether bubble oscillation is isothermal or adiabatic.
tip link: A molecular thread connecting the top of one stereocilium to the side of its taller neighbour. Stretching opens the MET channel.
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.
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.
turbulence: Chaotic, three-dimensional fluid motion characterised by eddies at many scales, enhanced mixing, and energy cascade from large to small scales.

V

van der Waals: Weak intermolecular forces (dipole-dipole, dipole-induced, London dispersion) that fall as r⁻⁶. Responsible for condensation, surface adhesion, and deviations from ideal-gas behaviour.
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.
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.
von Kármán vortex street: Alternating pattern of vortices shed behind a bluff body in a flow; source of aeolian tones.

W

wave equation: A second-order PDE describing how a disturbance propagates. For pressure in air: ∂²p/∂t² = c²∇²p.
wavenumber: The spatial frequency of a wave: k = 2π/λ. Higher k means shorter wavelength and more rapid spatial oscillation.
Weber number: We = ρv²L/γ. Ratio of inertial to surface-tension forces. Governs droplet breakup, jet instability, and splash dynamics.
wetting: The tendency of a liquid to spread on a solid surface. Determined by the balance of solid-liquid, solid-gas, and liquid-gas surface energies (Young's equation).
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.

Y

Young-Laplace: The equation relating pressure jump across a curved interface to surface tension and curvature: ΔP = γ(1/R₁ + 1/R₂). Governs bubble equilibrium size.
Young's modulus: E = σ/ε for uniaxial stress. The stiffness of a material: how much stress is needed to produce a given strain. Units: Pa.