Sound Waves (Cambridge International IGCSE Physics)

Describing sound

Spec point — 3.4.1 · Describe the production of sound by vibrating sources Describe the longitudinal nature of sound waves Know that a medium is needed to transmit sound waves

Describing sound

• Sound waves are produced by vibrating sources

• Sound waves are longitudinal

  • So a medium is needed to transmit sound waves
  • This means if there are no particles, such as in a vacuum, then the sound can’t travel through it

• When a sound wave comes into contact with a solid, the longitudinal wave vibrations are transferred to the solid

  • For example, sound waves can cause a drinking glass to vibrate
  • If the glass vibrates too much the movement causes the glass to shatter

Representing longitudinal waves

• Longitudinal waves are usually drawn as several lines to show that the wave is moving parallel to the direction of energy transfer

  • Drawing the lines closer together represents the compressions
  • Drawing the lines further apart represents the rarefactions

Representing a longitudinal wave with compression and rarefaction

Longitudinal waves are represented as sets of lines with rarefactions and compressions

Compression & rarefaction

Spec point — 3.4.1 · Describe compression and rarefaction

Extended Tier Only

• Longitudinal waves consist of compressions and rarefactions:

  • A compression is a region of higher density i.e. a place where the particles are close together
  • A rarefaction is a region of lower density i.e. a place where the particles are spread further apart

Sound is a longitudinal wave consisting of compressions and rarefactions - these are areas where the pressure of the air in the pipe varies with the wave

• These compressions and rarefactions cause changes in pressure, which vary in time with the wave

  • Therefore, sound is a type of pressure wave

• When the waves hit a solid, the variations in pressure cause the surface of the solid to vibrate in sync with the sound wave

When sound waves hit a solid, the fluctuating pressure causes the solid to vibrate

Exam Hint When describing compressions and rarefactions, make sure to use the correct terms. You must explicitly state that it is the particles (or molecules/atoms) of the medium that are closer together or further apart, not the wave.