Sound is the everyday longitudinal wave. A vibrating source pushes and pulls on the air, sending out a train of squashed and stretched regions that your ear reads as noise — and its frequency and amplitude become the pitch and volume you hear.
📘 What you need to know
Sound is a longitudinal wave, so it needs a medium and can’t travel through a vacuum
A vibrating source changes the density of the medium, sending out compressions and rarefactions
Humans hear the audible range, 20 Hz to 20 kHz; below is infrasound, above is ultrasound
Pitch depends on frequency — higher frequency, higher pitch
Volume depends on amplitude — bigger amplitude, louder sound
Sound travels at about 340 m s⁻¹ in air, fastest in solids and slowest in gases
How Sound Travels
When a loudspeaker cone (or a voice, or a guitar) vibrates, it repeatedly pushes the nearby air together and then lets it spring back apart. That creates a moving pattern of compressions (air squeezed to high density and pressure) and rarefactions (air stretched to low density and pressure). The pattern races outward to your ear, even though each air particle only jiggles back and forth on the spot.
The speaker sends compressions (dots bunched, high pressure) and rarefactions (dots spread, low pressure) through the air to the ear; the wavelength runs from one compression to the next.
The Spectrum of Sound
Sound covers a continuous range of frequencies, but human ears only pick up a slice of it. That slice — the audible range — runs from about 20 Hz to 20 kHz. Below it lies infrasound (used by elephants and whales); above it lies ultrasound (used by bats and in medical scanning).
Frequency rises left to right: infrasound (below 20 Hz), the human audible range (20 Hz–20 kHz), then ultrasound (above 20 kHz).
Pitch and Volume
Two features of the wave map onto two things you notice about a sound. The frequency sets the pitch: a high frequency (short wavelength) sounds high, a low frequency sounds low. The amplitude sets the volume: a big amplitude sounds loud, a small amplitude sounds quiet.
Top: same pitch, different volume (amplitude). Bottom: same volume, different pitch (frequency). Amplitude → loudness, frequency → pitch.
The Speed of Sound
In air at room temperature, sound travels at roughly 340 m s⁻¹. Two things change that. First, temperature: warmer air means faster-moving particles that pass the vibration on more quickly, so sound speeds up. Second, the state of the medium: sound is fastest in solids, where tightly packed particles hand the oscillation straight to their neighbours, and slowest in gases, where the spread-out particles are far less efficient.
Wave equation (for sound too)v = fλ
Quick recap: sound is longitudinal (needs a medium), heard from 20 Hz to 20 kHz; frequency sets pitch, amplitude sets volume, and it moves ~340 m s⁻¹ in air — fastest in solids.
WE 1
Middle C has a frequency of 256 Hz. Sound travels at 340 m s⁻¹ in air.
Calculate the wavelength of this note in air.
Use the wave equation
Rearrange v = fλ to λ = v/f
λ = 340 ÷ 256λ ≈ 1.3 mA low note like this has a wavelength longer than you are tall.
WE 2
A bat emits a sound of wavelength 3.4 mm in air, where the speed of sound is 340 m s⁻¹.
(a) Calculate the frequency. (b) State whether a human could hear it.
Part (a)
λ = 3.4 mm = 3.4 × 10⁻³ m, and f = v/λ
f = 340 ÷ (3.4 × 10⁻³)f = 100 000 Hz = 100 kHzPart (b)
100 kHz is well above 20 kHz, so it’s ultrasound
No — a human cannot hear it
💡 Top tips
Frequency → pitch, amplitude → volume: keep the two pairings separate
The audible range is 20 Hz to 20 kHz — memorise both ends
Sound needs a medium, so “no sound in a vacuum” is a guaranteed mark
Solid → liquid → gas is fastest → slowest for the speed of sound
The wave equation v = fλ works for sound just like any other wave
⚠ Common mistakes
Swapping the pairings — pitch is frequency, volume is amplitude, not the other way round
Saying sound can travel through space; it can’t, there’s no medium
Thinking sound is fastest in gases — it’s actually slowest there and fastest in solids
Forgetting to convert mm or kHz to base units before using v = fλ
Mixing up compression (high pressure/density) with rarefaction (low pressure/density)
Up next: the other great family of waves — Electromagnetic Waves — which are transverse, need no medium at all, and all race along at the speed of light.
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