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.
37 terms from this book.
A
- 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.
- amplitude
- The magnitude of a wave’s departure from equilibrium. For sound, the size of the pressure fluctuation.
- 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.
C
- 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 what passive mechanics gives.
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).
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.
- endolymph
- The fluid in scala media; high in K⁺, low in Na⁺. Held at +80 mV (the endocochlear potential) relative to perilymph.
F
- 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.
H
- HRTF
- Head-Related Transfer Function. The full frequency-dependent filter the body applies between a sound source in space and the listener’s eardrum.
I
- impedance
- In acoustics, the ratio of pressure to particle velocity. A measure of how strongly a medium resists being moved by a wave.
- incus
- The second middle-ear ossicle (the “anvil”); pivots between malleus and stapes.
M
- malleus
- The first middle-ear ossicle (the “hammer”); attached to the eardrum on one side and the incus on the other.
- MET channel
- Mechanoelectrical Transduction channel. A mechanically-gated ion channel on stereocilia that opens when the bundle is deflected, depolarising the hair cell.
O
- ossicles
- The three smallest bones in the body — malleus, incus, stapes — that transmit motion from the eardrum to the oval window of the cochlea.
- oval window
- A membrane-covered opening at the base of scala vestibuli, where the stapes footplate drives the cochlear fluid.
P
- perilymph
- The fluid in scala vestibuli and scala tympani; ionic composition resembles extracellular fluid (high Na⁺, low K⁺).
- plane wave
- A wave whose phase fronts are infinite parallel planes; idealisation of a wave from a distant source, valid locally near the listener.
- prestin
- The membrane protein in outer hair cells that changes conformation with voltage, producing the cell’s electromotility.
R
- reflection
- When a wave hits a boundary between two media, part of its energy turns back into the first medium. The reflection coefficient R = (Z2 − Z1)/(Z1 + Z2).
- resonance
- The condition where a driving frequency matches a system’s natural frequency, producing maximum response amplitude.
- ribbon synapse
- A specialised presynaptic structure in inner hair cells (and a few other sensory cells) that releases vesicles continuously at high rates with sub-millisecond precision.
S
- scala media
- The middle cochlear chamber, filled with endolymph; contains the organ of Corti.
- speed of sound
- The propagation speed of small-amplitude pressure disturbances. ≈343 m/s in air at room temperature, ≈1480 m/s in water.
- stapes
- The third middle-ear ossicle (the “stirrup”); its footplate seats in the oval window and drives the cochlear fluid.
- stereocilia
- Actin-filled rod-like protrusions on the apical surface of hair cells, arranged in a graded array with tip links connecting neighbours.
T
- 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.
- 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.
- traveling wave
- A wave that propagates while maintaining its shape (in a uniform medium) or that varies its shape as it moves (in a non-uniform one like the cochlea).
W
- wave equation
- A second-order partial differential equation describing how a disturbance propagates. For pressure in air: ∂²p/∂t² = c²∇²p.
- WKB approximation
- A method for solving wave equations with slowly-varying coefficients. Gives a solution of the form A(x)·exp(i·∫κ(x)dx) for a position-dependent wavenumber.