Sound Waves & Hearing (OCR Gateway GCSE Physics)

• Sound waves are vibrations of air molecules
• 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 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

• This is the process which converts wave disturbances between sound waves and vibrations in solids (or liquids)
• 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

• Sound waves can be heard by human beings because sound waves are transferred from the air to the solid components of the ear
• 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 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 then transfers the vibrations to the inner ear
  • In the inner ear, nerve cells detect the sound and send a message to the brain giving the sensation of sound
  • This is primarily the cochlea in the inner ear which contains nerve endings
  • The nerves produces electrical signals that pass through the auditory nerve in the brain which is then interpreted as sound

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

• Since the transmission of the vibrations is dependent on the small bones (primarily the cochlea), 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 human hearing

• The range of frequencies a human can hear is 20 Hz to 20 000 Hz
• Human hearing of high frequencies becomes worse with age
  • This is due to changes of the structure in the inner area and auditory cancel nerves
• The cochlea has small hairs, which have different lengths and vibrate at different frequencies of sound
  • The range of frequencies a human can hear depends on the range of lengths of these hairs
  • As a person ages, the shorter hairs that respond to higher frequencies stop working
  • Therefore, ageing reduces the ability to hear high frequencies