Sound Waves (Cambridge (CIE) O Level Physics): Revision Note

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

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

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