Sound is described as a longitudinal wave. Explain what this means and describe how sound travels through air, referring to compressions and rarefactions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a longitudinal wave the particle oscillations are parallel to the direction of wave travel. Sound waves are longitudinal: the air particles vibrate back and forth along the line in which the sound propagates.
When a sound source vibrates, it pushes neighbouring air particles together, creating a region of higher pressure called a compression. The particles then move apart, creating a region of lower pressure called a rarefaction. These compressions and rarefactions travel through the air, carrying the energy of the sound.
The air particles themselves do not move with the wave; they simply oscillate about their equilibrium positions as the compression and rarefaction pass by.
When a sound source vibrates, it pushes neighbouring air particles together, creating a region of higher pressure called a compression. The particles then move apart, creating a region of lower pressure called a rarefaction. These compressions and rarefactions travel through the air, carrying the energy of the sound.
The air particles themselves do not move with the wave; they simply oscillate about their equilibrium positions as the compression and rarefaction pass by.
Examiner tips
- Use the term "longitudinal" and state that particle motion is parallel to wave direction.
- Mention compressions and rarefactions and their pressure/density changes.
- Explain that energy is transferred but particles do not travel with the wave.
- Keep the answer concise and use correct terminology.
Common mistakes
- Confusing longitudinal with transverse waves.
- Saying particles move with the wave instead of oscillating.
- Omitting the role of compressions and rarefactions in energy transfer.
Mark scheme (4 marks)
- In a longitudinal wave, the oscillations/vibrations are parallel to (in the same direction as) the direction of travel/energy transfer.
- Sound causes air particles to be pushed together, forming compressions (regions of higher pressure/density).
- Between compressions, particles are spread further apart, forming rarefactions (regions of lower pressure/density).
- Energy is transferred through the air as these compressions and rarefactions pass through the medium, but the air particles themselves are not carried along/do not travel with the wave.
Key terms in this question
longitudinal wave · compression · rarefaction
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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