Definition · Plain-language
Sound waves
Sound waves are vibrations that travel through a material medium as a pattern of compressions and rarefactions.
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Vibrations travelling through matter
A sound wave begins when something vibrates — a guitar string, a loudspeaker, a vocal cord — and pushes on the particles of the surrounding medium. Those particles bunch together (a compression) and then spread apart (a rarefaction), passing the disturbance on to their neighbours, so the wave travels outward while the particles only jiggle back and forth in place. Because the particle motion is along the same line as the wave travels, sound is a longitudinal wave, unlike the up-and-down ripples of a water surface.
Why sound needs a medium
Sound cannot travel through a vacuum because there are no particles to carry the vibration. This is why space is silent and why a ringing bell inside a jar falls quiet as the air is pumped out. Sound travels at different speeds in different media: roughly 343 metres per second in air at room temperature, faster in water, and faster still in solids such as steel, because the particles are more closely packed and pass on the vibration more quickly. Temperature and the material both affect the speed.
Pitch, loudness and frequency
Two properties of a sound wave shape what we hear. Its frequency — how many vibrations pass each second, measured in hertz — determines the pitch: high frequencies sound high, low frequencies sound low. Human hearing typically spans about 20 to 20,000 hertz; sound above this range is ultrasound and below it is infrasound. The wave’s amplitude, the size of the vibration, determines loudness: a bigger amplitude means a louder sound. Loudness is often measured on the decibel scale.
Key facts
At a glance
- Definition: vibrations travelling through a medium as compressions and rarefactions
- Wave type: longitudinal — particles vibrate along the direction of travel
- Needs a medium: cannot travel through a vacuum
- Speed in air: about 343 m/s at room temperature
- Frequency: sets pitch; human hearing is roughly 20 Hz to 20,000 Hz
- Amplitude: sets loudness, often measured in decibels
Common misconceptions
What people often get wrong
Often heard: Sound can travel through the vacuum of space.
Actually: Sound needs particles of a medium to carry its vibration, so it cannot travel through a vacuum. Space is silent; only waves that need no medium, such as light, cross it.
Often heard: Sound travels fastest through air.
Actually: Sound actually travels faster in liquids and faster still in most solids than in air, because tightly packed particles pass the vibration along more quickly.
Often heard: Louder sounds always have a higher pitch.
Actually: Loudness and pitch are independent. Loudness depends on amplitude (how big the vibration is), while pitch depends on frequency (how fast it vibrates); a sound can be loud and low, or quiet and high.
Going deeper
