Sound Waves (Cambridge O Level Physics)

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Describing Sound

  • Sound waves are produced by vibrating sources
  • When a sound wave comes into contact with a solid, those vibrations can be 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

 

longitudinal-wave-definition, IGCSE & GCSE Physics revision notes

Sound waves are longitudinal: the molecules vibrate in the same direction as the energy transfer

  • Sound waves require a medium to travel through
    • This means that if there are no molecules, such as in a vacuum, then the sound can’t travel through it

  • The range of frequencies a human can hear is 20 Hz to 20 000 Hz

Compression & Rarefaction

  • Longitudinal waves consist of compression and rarefactions:
    • A compression is a region of higher density i.e. a place where the molecules are bunched together
    • A rarefaction is a region of lower density i.e. a place where the molecules are spread out

Compressions and Rarefactions of Air in a Column

Compressions and rarefactions, IGCSE & GCSE Physics revision notes

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

Compressions and Rarefactions of Sound Reflecting from a Solid

pressure-waves, IGCSE & GCSE Physics revision notes

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

Examiner Tip

When describing compressions and rarefactions, make sure to use the correct terms. It is best to refer to them as regions of high and low densities of particles instead of the particles are more 'bunched up' or 'far apart', as this is too vague and not very scientific!

Investigating Sound in a Vacuum

Sound Waves in a Vacuum

  • Sound waves are longitudinal waves
    • All longitudinal waves require a medium through which to travel
  • A vacuum is a region of space that does not contain air (or any other matter)
    • This means that, in a vacuum, there is no medium for sound waves
    • So sound waves cannot travel in a vacuum

Using a Bell Jar

  • This can be easily demonstrated using a piece of equipment called a bell jar
    • This is a glass container from which air can be pumped out, creating a vacuum (or nearly a vacuum)
  • A sound-emitting object is used, such as a battery-operated ringing bell or alarm
  • This is placed in a bell jar, which still contains air
    • The ringing bell can be heard despite the bell jar's glass walls
  • However, as the air begins being pumped out, the volume of the sound heard starts decreasing
  • When the air is completely removed from the bell jar, the ringing bell cannot be heard at all

Sound in a Bell Jar Demonstration

3-4-bell-jar-cie-ol-rn

In the absence of air, sound waves are unable to travel and leave the bell jar

Examiner Tip

It is very difficult to make a perfect vacuum. This means that you may well still hear a very faint ringing in this experiment, even when as much air as possible is removed from the jar. 

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Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.