6.1.9 Sound Waves

Sound Waves (Physics only)

• Sound waves can travel through solids, causing vibrations within the material.
• The outer ear collects sound and directs it down the ear canal.
• As the sound wave travels through the ear canal, it remains a pressure wave in the air.

The Sound Waves Hit the Eardrum

• The eardrum is a tightly stretched membrane that vibrates when sound waves hit it.
  • Compression causes the eardrum to move inward.
  • Rarefaction causes the eardrum to move outward, due to changes in pressure.
• The eardrum vibrates at the same frequency as the incoming sound wave.
• The ossicles, a chain of small bones in the middle ear (including the stirrup bone), also vibrate at this frequency.

Vibrations of the Bones Transmitted to the Fluid in the Inner Ear

• The vibrations from the ossicles are transferred to the fluid inside the cochlea.
  • The ossicles amplify the sound waves received by the eardrum.
  • As the fluid in the cochlea moves due to these vibrations, tiny hair cells lining the cochlea also move.
  • Different hair cells respond to different sound frequencies; some move more for specific frequencies.
  • Each hair cell is connected to a nerve cell.

Limitations (Physics only)

• Humans can hear sounds between 20 Hz and 20 kHz.
  • Hair cells in the cochlea that respond to higher frequencies can be damaged or deteriorate over time.
  • This damage can be caused by prolonged exposure to loud noises, ageing, smoking, chemotherapy, or diabetes.
  • As a result, our ability to hear higher frequencies decreases with age.
• Our hearing range has evolved to provide the greatest survival advantage.