Hearing Function

1
Hearing Function

• Frequency range of hearing varies among
  individuals.
• Audible range 20 - 20000 Hz
• Insensitive to sounds of low frequency
• Sensitive to sounds of high frequency
• Presbycusis deterioration of perception of high
  frequencies beyond 1000 or 1200 Hz

2
Structures of the Ear

• Outer ear
• pinna
• auditory canal
• eardrum
• Middle ear
• Ossicles hammer, anvil, stirrup
• Eustachian tube
• Inner ear
• semicircular canals
• cochlea
• basilar membrane

3
Outer ear

• Pinna
• to collect sounds waves into the opening of the
  auditory canal
• acting as a funnel amplifying the sound and
  directing it to the ear canal
• to contribute to the capability to determine the
  direction of origin of sounds
• Auditory canal
• to provide a passageway for sound traveling from
  the pinna to the eardrum
• to protect the ear from infection
• 3.5 cm long and 0.7 cm wide
• Eardrum
• very important in hearing
• 1cm in diameter
• Three layers of different materials skin,
  fibrous and elastic material, and the mucus
• vibrates when impacted by sound waves and
  transfers these vibrations to the middle ear

4
The Middle Ear

• Ossicles
• Three tiny bones Hammer, Anvil, and Stirrup
• Together they act as a lever by changing the very
  small pressure on the eardrum into a much greater
  pressure on the inner ear
• To project the inner ear from very loud noises
  and sudden pressure changes
• Acoustic reflex the response of the ear to loud
  sounds
• Eustachian tube
• to let fluid drain down the throat
• to equalize pressure on either side of the ear
  drum
• the tube opens to allow air to flow in or out of
  the tube
• 3.6 cm long
• It connects the middle ear to the oral cavity

5
Inner Ear

• Oval window
• A membrane in the wall of the cochlea
• receives vibrations from the middle ear and
  transfers them to cochlea
• Semicircular canals
• no function in auditory perception
• to help maintain balance
• Cochlea

6
Cochlea

• Contains all the mechanism for transforming
  pressure variations into neural impulses
• Filled with liquid and surrounded by rigid bony
  walls
• Basilar membrane
• Along the basilar membrane, standing waves form
  caused by the vibrations of the fluid inside
  cochlea
• Depending on the frequencies of the vibrations,
  the peak of standing waves will occur at
  different points along the membrane
• Lower frequency far from the oval window
• Higher frequency close to the oval window
• Tiny hairs (cilia) pick up the movement of the
  basilar membrane and cochlear fluid and then
  convert the singals into electrical impulses.

7

• Bone conduction
• sound herd through vibrations of the skull that
  reach the inner ear
• humming or clicking ones teeth
• Why do you sound strange when you hear yourself
  from the tape?

We hear our voice by both air conduction and bone
conduction.
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Sound waves propagate through the auditory canal
Excites the ear drum and cause the mechanical
vibrations in the middle ear
The vibration of the stirrup
Pressure variations in the cochlea
Mechanical vibrations in the basilar membrane
Tiny hairs transmit electrical impulses to the
brain via the auditory nerve