Sound Waves (Edexcel GCSE Physics)

Higher Tier Only

• Sound waves are vibrations of molecules
• When a sound wave travelling through a gas or a liquid comes into contact with a solid, some of the energy it is transferring is reflected from the surface of the solid, and some is transmitted through or absorbed by the solid 
  • A reflected sound wave is called an echo

• Sound is an example of a longitudinal wave, hence it consists of:
  • Compressions - regions of higher density
  • Rarefactions - regions of lower density

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

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

Higher Tier Only

• Different solids have a tendency to vibrate at different frequencies 
• This is called the object's natural frequency
• As a result, sound waves with a frequency that is close to a particular solid's natural frequency will cause larger vibrations than for sound waves with frequencies much larger or smaller than the solid's natural frequency
• This means some frequencies of sound are transferred much more efficiently to the solid than others

Higher Tier Only

• Sound waves can be heard by human beings because sound waves are transferred efficiently from the air to the solid components of the ear
• The transmission of sound to the human ear only works over a limited range of frequencies
  • This limits the range of sound frequencies a human can hear

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

• In the case of the human ear, the sound waves are transferred by two main solid components:
• • The eardrum which is made of tissue and skin
  • Three small bones

The human ear is made up of several components which turn sound waves into signals which the brain can interpret

• The sound wave travels down the auditory canal towards the eardrum
  • The pressure variations created by the sound wave exert a varying force on the eardrum causing it to vibrate
  • The vibration pattern of the sound waves creates the same pattern of vibration in the eardrum

• The eardrum vibration is transferred to the three small bones
• The vibration of these small bones amplifies the vibrations and then transfers the vibrations to the liquid in the cochlea located in the inner ear
  • Tiny hairs inside the cochlea detect the vibrations and create electrical impulses which travel along neurones in the auditory nerve to the brain giving the sensation of sound