1. Hearing Anatomy

2. General
Distinguish all sounds of speech as well as 1/2 million other sounds
Locate direction in which sounds come from
We can detect a sound so weak that it moves the ear drum only 1/10 the diameter of a molecule

3. Outer Ear: Acoustic System
Two Parts:
Auricle or Pinna- Visible, sound gathering part
Ear Canal or External Auditory Canal-
Tube conveying sound waves to the ear drum;
Secrete cerumen (prevents ear canal from drying out);
Lined with cilia (small hairs) that filter dust
Tube is about an inch long
Tympanic membrane stretched across one end (separated external canal from middle ear)
Air-filled (functions as a closed tube resonator)

4. Ear Inner Ear Ear Canal Middle Ear Outer Ear Eustachian Tube Pinna
Auditory
Nerve
Middle Ear
Outer Ear
Eustachian Tube

5. Middle Ear: Mechanical System
Air-filled cavity within the temporal bone (hardest bone in the body)
Where acoustic energy of air pressure waves are converted to mechanical vibrations
Conversion occurs at the tympanic membrane which vibrates in response to the changes in air pressure that travel down the ear canal.
For the TM to vibrate properly:
Air pressure inside the middle ear must be the same as the Patm outside

6. Middle Ear: Mechanical System
But…If the acoustic pressure wave that travels down the ear canal also traveled into middle ear, the 2 waves would cancel out and the TM would not vibrate.
A mechanism that closes the middle ear cavity to vibratory pressure changes, but permits it to open and adjust to changes in Patm.

7. Eustachian Tube Middle ear pressure equalizing- Plane; “Pop”
Runs from middle ear to the nasopharynx
Normally closed at nasal entrance
Closure prevents direct transmission of breathing and speech sounds to the middle ear
Opened for pressure adjustments by velopharyngeal muscle

8. Eustachian Tube: Infections
Children- Repeated middle ear infections
Frequent & severe- Damage the auditory apparatus and interferes with the child’s language development
Reason: ET short and horizontal compared to adult
Orientation makes it easy for infections to spread from sinuses, upper resp tract or throat by way of ET
Spread assisted by enlarged adenoids & lymphoid tissue (blocks opening of ET)

9. Eustachian Tube in Infants & Adults

10. Tympanic Membrane
Ear Drum- marks boundary between outer and middle ears
Extremely thin, three layered sheet of tissue
Epithelial lining of the external canal continues as external layer of TM; inner membrane provided by middle ear
Middle layer-fibrous tissue providing the structure
TM is 55 mm2
Umbo- distant point of attachment of the inner ear TM to one of the bones of the middle ear

11. Tympanic Membrane
Umbo
“As viewed from the
EAM”
Cone of
Light

12. Auditory Ossicles Bridges gap across the middle ear
Connecting the outer ear to the inner ear by 3 tiny bones also called ossicular chain
Malleus: Largest; Provide point of attachment with TM; shaped like a primitive club or mallet; length only 9 mm
Incus: Middle bone; resembles an anvil; Intermediate communicating link; 7 mm
Stapes: Resembles a stirrup; 3rd bone of ossicular chain; smallest ossicle

13. Tympanic Membrane & Auditory Ossicles
Malleus
Incus
Stapes
Ear Canal or
External Auditory
Meatus
Middle Ear
Tympanic
Membrane

14. Ear Inner Ear Ear Canal Middle Ear Outer Ear Eustachian Tube Pinna
Auditory
Nerve
Middle Ear
Outer Ear
Eustachian Tube

15. Middle Ear Muscles Inner Ear Malleus Middle Ear Stapes Vibration
Incus
Malleus
Stapes
Vibration
Annular
Ligament
Tensor
Tympani
Tendon
Tympanic
Vibration
Stapedius
Tendon
Middle Ear
Tensor
Tympani
Muscle

16. Amplifying Sound Biggest amplification source of acoustic pressure
Difference in size of ear drum and stapes
Footplate (bottom part of stirrup) of stapes fits into oval window (Hole in the bone of inner ear)
Attached by annular ligament
Separates inner ear (fluid) from middle ear (air)
Converts mechanical energy into hydraulic waves

17. Amplifying Sound
Footplate is 14x smaller than the area of the ear drum
All air pressure waves against the large TM is concentrated on small footplate
Impedance mismatch occurs:
Must be overcome to transfer acoustic vibrations into hydraulic waves. HOW?
1) Amplification of speech frequencies (2,500-4,000) in ear canal
2) Leverage of ossicular chain
3) Mechanical force by stirrup is 14 x that of TM

18. Amplification of Sound
Ossicles
Inner Ear
Leverage Power of Ossicles

19. Amplifying Sound
So…
1) Pressure at footplate of stapes is 14 x greater than TM
2) Resonation of speech frequencies in ear canal multiplies pressure at entrance of ear canal 2-4x before it reaches ear drum
3) Therefore, Pressure multiplied 28-56x before reaching stapes
4) Result= Amplification of sound

20. Reading/Assignments
Seikel: Pgs
Maue-Dickson: Pgs