Sound is a longitudinal mechanical wave produced by vibrating objects and transmitted through a medium as a series of compressions and rarefactions. Unlike light, sound cannot travel through a vacuum — it needs particles to carry the energy from place to place. This is a core KS3 physics topic, typically taught in Year 8.
What is a longitudinal wave?
Waves transfer energy without transferring matter. Waves come in two main types:
- Transverse waves — the particles oscillate perpendicular to the direction the wave travels. Light and water ripples are transverse waves.
- Longitudinal waves — the particles oscillate parallel to the direction the wave travels. Sound is a longitudinal wave.
In a longitudinal sound wave, regions where particles are pushed close together are called compressions (high pressure), and regions where they spread apart are called rarefactions (low pressure). These alternating bands of pressure move outward from the source through the medium.
| Property | Transverse wave | Longitudinal wave |
|---|---|---|
| Particle motion | Perpendicular to wave direction | Parallel to wave direction |
| Example | Light, water ripples | Sound |
| Can travel in a vacuum? | Yes (light can) | No |
| Visible compressions? | No | Yes |
How does sound travel?
When a loudspeaker cone vibrates, it pushes neighbouring air molecules together (compression), and those molecules push the next layer, passing the disturbance along. The energy travels outward in all directions, but the air molecules themselves only oscillate back and forth in place — they do not travel with the wave.
Sound needs a medium. It travels through gases, liquids and solids, but not through a vacuum. This is why space is completely silent: there are no particles to compress. A famous classroom demonstration places a ringing bell inside a vacuum jar and pumps out the air — the bell becomes inaudible even though it is still vibrating.
Sound travels at different speeds depending on the medium:
| Medium | Approximate speed of sound |
|---|---|
| Air (20 °C) | ~340 m/s |
| Water | ~1,480 m/s |
| Steel | ~5,000 m/s |
Sound travels faster in solids and liquids than in gases because the particles are closer together and can pass on vibrations more quickly.
What are frequency and amplitude?
Every sound wave can be described by four key properties:
- Frequency (f) — the number of complete wave cycles per second, measured in hertz (Hz). A higher frequency means more vibrations per second. Frequency determines pitch: a high-frequency wave sounds high-pitched; a low-frequency wave sounds low-pitched.
- Amplitude (A) — the maximum displacement of a particle from its undisturbed (equilibrium) position. Amplitude determines loudness: a larger amplitude carries more energy and sounds louder.
- Wavelength (λ) — the distance between two successive compressions (or two successive rarefactions), measured in metres.
- Wave speed (v) — how fast the wave travels through the medium, measured in m/s.
These properties are connected by the wave equation:
wave speed = frequency × wavelength v = f × λ
Worked example: calculating wavelength
A sound wave travels through air at 340 m/s with a frequency of 170 Hz. What is its wavelength?
Rearranging the wave equation:
λ = v ÷ f = 340 ÷ 170 = 2 m
So the wavelength of this sound is 2 metres.
What is the human hearing range?
The human ear can detect sounds with frequencies between approximately 20 Hz and 20,000 Hz (20 kHz). Sound below 20 Hz is called infrasound (inaudible to humans but felt as vibration — elephants use it to communicate). Sound above 20,000 Hz is called ultrasound (used in medical scanning and by bats for echolocation).
Hearing range typically narrows with age: many adults cannot hear frequencies above 15–16 kHz. The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to understand the difference between the speed of sound and the speed of light, the range of human hearing, and the properties of longitudinal waves.
How does the ear detect sound?
The human ear converts sound waves (pressure vibrations) into electrical signals that the brain interprets:
- Outer ear (pinna and ear canal) — funnels sound waves towards the eardrum.
- Eardrum (tympanic membrane) — a thin membrane that vibrates when compressions hit it. Higher amplitude → larger vibration → louder perceived sound.
- Three ossicles (malleus, incus, stapes) — tiny bones in the middle ear that amplify and transmit the vibrations to the inner ear.
- Cochlea — a fluid-filled, spiral-shaped structure in the inner ear. Vibrations travel through the fluid and stimulate tiny hair cells, which generate electrical impulses.
- Auditory nerve — carries the electrical impulses to the brain, where they are perceived as sound.
BBC Bitesize KS3 Physics covers sound, hearing, and wave properties as a key section of the Year 8 curriculum, with interactive diagrams of the ear and wave animations.
What is the difference between loudness and pitch?
Loudness is determined by amplitude — the bigger the wave's amplitude, the more energy it carries, and the louder the sound. Pitch is determined by frequency — the faster the vibrations, the higher the pitch. It is possible to have a very loud, low-pitched sound (a large-amplitude, low-frequency wave) or a quiet, high-pitched sound (a small-amplitude, high-frequency wave). The two properties are completely independent.
Frequently asked questions
What is the difference between loudness and pitch?
Loudness depends on the amplitude of the sound wave: a bigger amplitude means the particles are displaced further, carrying more energy per second, and the sound seems louder. Pitch depends on the frequency: more vibrations per second produce a higher-pitched sound. Doubling the amplitude does not change the pitch, and doubling the frequency does not change the loudness. On an oscilloscope, a loud sound shows a tall wave, and a high-pitched sound shows a wave with many peaks close together.
Why can't sound travel in space?
Sound is a mechanical wave — it transfers energy by making particles vibrate and pass those vibrations to neighbouring particles. In space, the density of matter between stars is so low (near-perfect vacuum) that there are essentially no particles to vibrate. With nothing to compress and rarefy, the wave cannot propagate. This is why explosions in space films are scientifically inaccurate — a real explosion in orbit would be completely silent from a short distance away.
What is the speed of sound?
In air at around 20 °C, sound travels at approximately 340 m/s (about 1,225 km/h). This is roughly a million times slower than the speed of light (300,000,000 m/s), which is why you see lightning before you hear thunder. Sound travels faster through denser, more rigid media: about 1,480 m/s in water and around 5,000 m/s in steel. Temperature also matters — warmer air is less dense and sound travels slightly faster through it.
How does the ear detect sound?
The ear converts pressure waves in air into electrical nerve signals. Sound enters the ear canal and makes the eardrum vibrate. Three tiny bones (the ossicles) amplify and transmit these vibrations to the fluid-filled cochlea. Hair cells lining the cochlea move in response to different frequencies — cells near the base respond to high frequencies; those deeper in respond to low frequencies. The movement of the hair cells generates electrical impulses that travel along the auditory nerve to the brain, where the pattern is decoded as sound.
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