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Longitudinal waves
Beginner
Oscillation parallel to propagation, like sound.
Particles oscillate parallel to the propagation direction, creating compressions and rarefactions: sound, P-waves, a slinky push. They can't be polarised.
A loudspeaker cone pushes air back and forth.
Wavelength is the distance between successive compressions.
In more detail
In a longitudinal wave, particles oscillate parallel to the direction of travel, forming compressions and rarefactions. Sound in air is the classic example.
Real-world uses
- Sound from a loudspeaker is a longitudinal wave in the air.
- Ultrasound scans use longitudinal waves in body tissue.
- Ultrasound scanners and seismic P waves are longitudinal, with the particles moving back and forth along the direction of travel.
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Frequently asked questions
What are compressions and rarefactions?
Regions where particles are closer together and farther apart, respectively.
Can longitudinal waves be polarised?
No. Polarisation requires transverse oscillation.
How do I recognise a longitudinal wave?
Compressions and rarefactions form along the direction of travel, as in a slinky pushed and released.
Related topics in Waves & Sound
What is a wave? · Mechanical vs electromagnetic waves · Transverse waves · Wavelength, frequency, speed · Amplitude and intensity · Wave equation (travelling)