AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Longitudinal waves show areas of compression and rarefaction. Sound waves…
Full specification wording
Transverse and longitudinal waves: Longitudinal waves show areas of compression and rarefaction. Sound waves travelling through air are longitudinal.
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. Sound travelling through air is longitudinal: air particles vibrate back and forth along the same line as the sound travels, creating regions where particles are crowded together (compressions) and regions where they are spread apart (rarefactions). These pressure variations pass through the air and are detected as sound. A compression is a region of higher pressure and density; a rarefaction is a region of lower pressure and density. The pattern moves forward, but the air particles themselves oscillate about fixed positions and do not travel with the sound. You can represent this with a slinky spring pushed and pulled along its length, or by a series of alternating dense and sparse dots.
What earns the marks
- State that longitudinal waves have oscillations parallel to the direction of energy transfer.
- Define compression as a region where particles are closer together, with higher pressure or density.
- Define rarefaction as a region where particles are further apart, with lower pressure or density.
- Describe sound travelling through air as a longitudinal wave in which air particles vibrate back and forth along the direction of travel.
- Explain that the compressions and rarefactions travel through the air while the air particles oscillate about fixed positions.
- Use a slinky spring or a particle diagram to model alternating compressions and rarefactions.
Common mistakes and how to put them right
Labelling compressions and rarefactions as crests and troughs.
Put it right by using compression and rarefaction for longitudinal waves.
Claiming that air particles travel from the source to the listener.
Put it right by stating that energy is transferred through particle oscillations while the particles remain near fixed positions.
Describing sound as transverse because it can be drawn as a wave.
Put it right by stating that sound in air is longitudinal, with oscillations parallel to energy transfer.
Exam tips
- Define compression and rarefaction in terms of particle spacing and pressure before applying them to sound.
- Use a slinky spring or a dot diagram to show how compressions and rarefactions move along the direction of travel.
- State clearly that sound in air is longitudinal, and avoid using crest and trough language for it.
Try the lesson
The start of the Transverse and longitudinal waves: Longitudinal waves show areas of compression and rarefaction. Sound waves travelling through air are longitudinal. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of wave shows areas of compression and rarefaction?
Listen
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. Sound travelling through air is longitudinal: air particles vibrate back and forth along the same line as the sound travels, creating regions where particles are crowded together (compressions) and regions where they are spread apart (rarefactions). These pressure variations pass through the air and are detected as sound. A compression is a region of higher pressure and density; a rarefaction is a region of lower pressure and density. The pattern moves forward, but the air particles themselves oscillate about fixed positions and do not travel with the sound. You can represent this with a slinky spring pushed and pulled along its length, or by a series of alternating dense and sparse dots. A strong answer earns marks for these points. State that longitudinal waves have oscillations parallel to the direction of energy transfer. Define compression as a region where particles are closer together, with higher pressure or density. Define rarefaction as a region where particles are further apart, with lower pressure or density. Describe sound travelling through air as a longitudinal wave in which air particles vibrate back and forth along the direction of travel. A common slip: Labelling compressions and rarefactions as crests and troughs. Instead, using compression and rarefaction for longitudinal waves. Exam tip: Define compression and rarefaction in terms of particle spacing and pressure before applying them to sound.
Key words
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Compression
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Rarefaction
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Sound as a longitudinal wave
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Particle oscillations and energy transfer
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See every definition
- Compression
- A region where particles are crowded closer together, with higher pressure and density.
- Rarefaction
- A region where particles are spread further apart, with lower pressure and density.
- Sound as a longitudinal wave
- Sound travelling through air is longitudinal, with air particles vibrating back and forth along the same line as the sound travels.
- Particle oscillations and energy transfer
- The pattern of compressions and rarefactions moves forward while air particles oscillate about fixed positions and do not travel with the sound.
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