Glossary
Terms used in this book.
A reference list of the technical vocabulary used in What is sound?. 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.
30 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
- characteristic frequency
- The frequency at which a given place on the basilar membrane (or auditory-nerve fibre, or cortical neuron) 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 what passive mechanics gives.
- convolution
- A mathematical operation that combines two signals: the output at time t is the weighted sum of one signal across all times, weighted by the other shifted to t.
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.
F
- 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.
- Fourier series
- Decomposition of a periodic signal into a sum of sinusoids at multiples of its fundamental frequency.
- 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.
- impulse response
- A system’s output when given a single, brief impulse as input. Fully characterises any linear time-invariant system.
- inverse-square law
- Radiated power from a point source falls as 1/r². A consequence of energy spreading over an expanding spherical surface.
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.
P
- 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.
- plane wave
- A wave whose phase fronts are infinite parallel planes; idealisation of a wave from a distant source, valid locally near the listener.
- precedence effect
- The perceptual rule that, given the same sound arriving from multiple directions with small delays, the listener attributes the source to the first-arriving wavefront.
Q
- quality factor
- Q. A dimensionless measure of resonance sharpness. Equal to the peak amplitude (relative to DC) of a damped driven oscillator.
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.
- reverberation
- The collection of reflections that follows a direct sound in a room, gradually decaying. Characterised by reverberation time, T60.
S
- speed of sound
- The propagation speed of small-amplitude pressure disturbances. ≈343 m/s in air at room temperature, ≈1480 m/s in water.
- spherical wave
- A wave radiating outward from a point source, with amplitude falling as 1/r.
W
- wave equation
- A second-order partial differential equation describing how a disturbance propagates. For pressure in air: ∂²p/∂t² = c²∇²p.