Sound waves and hearing

• Sound Waves in Solids

  • 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
• Sound Waves in the Ear

  • 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 ear:

  A) Inner

  B) Three

  C) Auditory

  D) Ear Drum

  
• 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