1. HEARING Sound How the Ears Work How the Cochlea Works Auditory Pathway
Hearing Loss

2. Sound pressure variations (usually in air)
intensity: amplitude of sound waves

3. Sound Intensity
measured in deciBels (dB), sound pressure level compared to threshold
with every 6 dB increase, SPL doubles
affects perceived loudness

4. Sound Frequency Number of vibrations per second Measured in Hertz (Hz)
Affects perceived pitch

5. Sound Complexity Affects timbre (perceived sound quality)
Number of harmonics
multiples of fundamental frequency
Attack
Decay

6. The Missing Fundamental
If you play the harmonics of a fundamental without the fundamental itself, people will perceive the missing fundamental
Example: play 400, 600, 800 Hz tones; a fundamental of 200 Hz will also be heard

7. How the Ear Works
Outer Ear
Middle Ear
Inner Ear

8. Outer Ear
Pinna - outer flap; helps slightly in amplifying and locating sounds
External auditory canal - tube leading from pinna to eardrum; protects eardrum, resonates to frequencies around 3000 Hz

9. Outer Ear Tympanic membrane (eardrum) vibrates with sound waves
Transmits sound energy to middle ear

10. Middle Ear Tympanic membrane moves ossicles malleus (hammer)
incus (anvil)
stapes (stirrup)

11. Middle Ear Stapes pushes on oval window
Eustachian tube equalizes air pressure

12. Impedance Mismatch
Sound intensity decreases when the sound travels from one medium to another (e.g., air to water)
The cochlea is filled with fluid, so the sound intensity is decreased
The middle ear compensates for this by amplifying the sound

13. Middle Ear Amplification
Large surface area (tympanic membrane) to small surface area (oval window)
Ossicles work as a lever system

14. Cochlea
Part of inner ear, which also includes semicircular canals for vestibular sense
Transduces sound energy to neural responses

15. Parts of the Cochlea
Organ of Corti sits on basilar membrane, contains hair cells which are stimulated by sound
Tectorial membrane is attached diagonally over Organ of Corti

16. The Traveling Wave
The basilar membrane is narrow and stiff at the middle ear end, but wide and loose at the far end
This causes it to vibrate most at one location, depending on frequency
Higher frequencies cause vibration on middle ear end; lower frequencies cause vibration on far end

17. Hair Cells
The traveling wave creates a shearing action of the tectorial membrane over the basilar membrane
Stereocilia on top of hair cells are bent
Hair cells depolarize when stereocilia are bent toward outer part of cochlea; hyperpolarize when bent inward

18. Inner Hair Cells about 3,500 not attached to tectorial membrane
afferent fibers

19. Outer Hair Cells about 12,000 attached to tectorial membrane
afferent and efferent fibers

20. Efferent Fibers
These fibers carry messages from the brain which modulate activity in the cochlea
This may help code frequency more precisely
The efferent fibers explain cochlear microphonics and otoacoustic emissions

21. Auditory Pathway
Graded potentials in hair cells cause action potentials in bipolar cells
Axons of bipolar cells (auditory nerve) carry information into the brain

22. Auditory Pathway
Auditory nerve connects with cells in the cochlear nucleus in the hindbrain
lateral inhibition
frequency coding
Superior olivary nucleus (brain stem)
some fibers cross; binaural information
sound localization

23. Auditory Pathway Inferior colliculus (brain stem)
tonotopic organization
regulate auditory reflexes (eye blink, middle ear muscle tightening)
Some fibers connect to superior colliculus

24. Auditory Pathway Medial geniculate nucleus of thalamus
complex feature analysis
Primary Auditory Receiving Area
temporal lobe

25. Auditory Cortex Neurons
simple tone cells
complex tone cells (clicks, species-specific calls)
cells that respond to changing frequencies
cells that respond to binaural differences

26. Hearing Loss Hearing ability can be measured with audiometry
measure sensitivity in dB
normal threshold is 0
Audibility curve shows threshold for different frequencies

27. Hearing Loss presbycusis - loss of hearing with aging
high frequencies affected most
long term exposure to noise

28. Hearing Loss
conduction deafness - problem with conduting sound to the cochlea
nerve deafness - damage to cochlea or auditory nerve

29. Hearing Loss tinnitus - ringing in ears auditory nerve damage
aspirin overdose
masking may help