• communication is essential for a rich and healthy life
  • deafness impacts social communication more than blindness does
    • deafness is far more common than blindness

hearing range

  • frequency: 20 Hz - 20 kHz
  • amplitude range: 0 dB - 180 dB

Ear Anatomy

external ear

  • this section of the ear is a long tube like structure
    • amplifies the sound, mostly speech frequencies
      • constructive interference in human speech frequencies
      • destructive interference in human speech frequencies
    • to carry it to the middle ear, where the intensity of the entire spectrum inevitably falls

rinne test

  • air borne sounds are louder than bone deducted sounds
    • due to the amplification in the external ear canal

  • air conduction is louder in the human ear

  • if the hearing through bone is louder than in the ear,
    • the hearing in that ear is most likely impaired

middle ear

  • most intensity is lost here

stapes

  • little bones
  • so light that they are moved by the tympanic membrane
  • they are light to minimize sound loss
  • the membrane also collects sound from a large area
    • to collect energy from a large area
    • and focusses it onto a oval window which is about ~15 times smaller
  • the bones are connected to two ear muscles
    • both are voluntary type muscles
    • but are controlled involuntarily by the the vibration of the stapes
  • the two muscles are
    • tensor tympani
      • pulls on the tympanic membrane
      • increases frequency of the membrane
      • automatically tightens when chewing
      • so incoming sounds will be at a higher frequency in the middle ear than in the external ear
    • stapedius
      • pulls back the stapes after a first loud sound
      • reduces the intensity of the incoming sound
      • if this reflex fails, everything sounds too loud
        • condition is called hyperacusis

inner ear

Inner Ear

  • stapes beats on an oval window
  • the oval window presses on a fluid filled structure
    • called cochlea
  • the cochlea is a spiral
    • complex 3D geometry
    • when un-rolled, a cochlear duct becomes apparent

cochlear duct

  • sound comes in through the oval and goes out of round
    • through the cochlear duct
  • the cochlear duct is an auditory sound frequency prism
    • this is true even for a dead cochlea, i.e. a cochlear extracted from the cadaver
  • responsible for auditory transduction

Cochlear Sound Prism

  • there are three layers of cilia in the cochlear duct
    • these are all suspended in the cochlear fluid
    • each hair cell is linked to the next hair cell
      • one tip to the tip of the other
  • when the hair bundles move because of the sound
    • the tip links pull open an ion channel
    • they are physically linked to the ion channels
    • this is how fluid waves get converted into electrical signals
    • this is a ‘fast’ biological transduction mechanism
  • incoming frequencies are responded to by exciting the hair cells

putting it together

  1. outer ear:
    • amplifies speech frequencies in air, cutting down other frequencies
  2. middle ear:
    • translates air waves to fluid waves through a membrane and small bone
  3. inner ear:
    • converts fluid waves to physical tension which results in ion channel opening
  • so sound waves in air are converted to electrical signals in the brain

supplementary