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
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
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
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!