Download Chapter 11

Chapter 11
Function of the Ear
Perry C. Hanavan, Au.D.
Hearing and hearing tests
• Hearing range between 20 –
20,000 Hz
• Hearing tested between 250 –
8000 Hz
– Audiogram
• The ear is not equally sensitive
to all frequencies.
• Different for sound field and
ear phones…
Outer Ear
• The Mirror
• The Tube
Outer Ear
The Mirror:
• The pinna is a very interesting part of the
body!
• No one knows the function of each of the
nooks and crannies of the pinna …. such
as localization, speech perception, etc.
• Pinna serve as an “acoustic mirror”
Outer Ear
The Mirror:
• Higher frequencies (shorter sound
wavelengths) are reflected back to the ear
canal by the pinna
• Sounds above 1500 Hz are enhanced by
the “pinna effect”
Outer Ear
The Mirror:
• The pinna effect results in a natural 5-8 dB
high-frequency amplification
Outer Ear
The tube:
• The external ear canal is 25-30
mm (1 inch) long, closed at the
tympanic membrane and “open”
at the entrance
• Marshall, et al refer to this tube as
“quarter wavelength resonator”
– meaning that it has a 17 dB
resonance (gain) near 2700 Hz in
adults
Question
The child’s ear canal is adult-like by which
age?
A. 1
B. 2
C. 3
D. 4
Senteo Question
E. 5
To set the properties right click and select
Senteo Question Object->Properties...
Senteo Quiz Results (Correct answer:C)
A
8%
E
25%
C
34%
D
33%
Outer Ear
• The tube:
– People naturally hear
through this resonance.
– Infants have shorter ear
canals and their resonance
can be up to 7000 Hz.
– Children’s ears are adult-like
by age 3.
Outer Ear
• The tube:
– With mastoid cavities and large
perforations, the resonance
drops to about 1500-2000 Hz.
– Small perforations and tubes,
have no effect.
Outer Ear
Outer Ear
• The tube:
– Ear canal finishes growing by age 9, and
entire conchea by age 12.
Middle Ear
• Ear “Drum”:
– Actually refers to the tympanic membrane
AND all of the other structures down wind…
• Ossicles, muscles, tendons,…
Middle Ear
• Tympanic Membrane:
– Three layers (pars flaccida has two layers)
– Vibration pattern is rather complex
• Top portion (pars flaccida) does not transduce
sounds very effectively.
Middle Ear
• “Matching transformer”:
– The only reason we have a middle
ear is to be able to convert sound
in AIR to sound in WATER.
– Fish do not need (or have) middle
ears since they live in water
Middle Ear
• Matching
Transformer:
– Allows us to hear 3040 dB better than if
no middle ear
Middle Ear
• Matching Transformer:
– Loss of this natural transformer means that we
would have a 30-40 dB loss.
• Serous otitis media can give a 30-40 dB loss.
– Our voice is attenuated by 30-40 dB when we
are under the water.
– Still not perfect! The middle ear is only about
66% efficient….
Middle Ear
• Stapedial Reflex:
– Attached to the third stirrup-like bone is the
stapedial muscle.
– Found in all mammals.
– Protection from loud sounds (eg. our voice!)
Middle Ear
• Stapedial Reflex:
– Basis behind acoustic reflex testing in
audiometry.
– Contracts upon high level inputs.
• Own voice, loud music and noise, amplified sound.
Middle Ear
• Stapedial Reflex:
– Usually contracts with inputs of 80-90 dB
(ie., above the speech range of intensities).
– Only slightly higher for those with hearing
loss.
– Our ear has a rudimentary high-level
compression system for loud sounds.
How does this change the
speech signal?
Frequency
Mouth
Inner Ear
F1 (500 Hz)
65 dB
56 dB
F2 (1500 Hz)
60 dB
57 dB
F3 (2500 Hz)
55 dB
64 dB
Some common hearing ranges
SPECIES
Low (Hz) High (Hz)
Human
20
20,000
Chimpanzee
100
20,000
Dog
50
46,000
Dolphin
1000
130,000
Bat
3000
120,000
Goldfish
100
2000
Problems of the outer and
middle ears
• Conductive hearing loss
• Maximally 60 dB loss
• Medically and/or surgically treatable
Examples of conductive losses
• Wax (cerumen) occlusion
• Eardrum perforation
• Serous otitis media (ear infection)
• Otosclerosis (stiffening of the ossicles)
Inner Ear (Cochlea)
• Physiology:
– Over 100,000 moving parts stuffed into a
volume smaller than the tip of your baby
finger.
– Organ of Corti
– Of the 15,000 nerve endings:
• ¼ afferent (to the brain) from inner hair cells
• ¾ efferent (from the brain) to the outer hair cells
Inner Ear (Cochlea)
• Frequencies are arranged in the cochlea
similar to a piano keyboard…
– High frequencies are at the “basal” end near
the stapes footplate
– Low frequencies are at the “apical” end and
are well protected by the 2 ½ turns of the
cochlea
…. Low frequencies have a longer longevity
than higher frequencies (more peripheral).
Inner Ear
• Outer Hair Cells:
– Receives “outgoing” stimuli and this functions
to amplify the soft sounds (“motor” units).
– Usually first to die off (up to 60 dB HL, all of
hair cell damage is outer hair cell…)
– Source of oto-acoustic emissions. (OAE)
Inner Ear
• Outer Hair Cell Damage:
– Modern hearing aids helps the “motor
function”
– BUT… don’t really retune the hair cells.
– Will still need something else to help with
noise reduction (eg. directional microphones).
Inner Ear
• Outer Hair Cell Damage:
– Loss of “motor function” for amplifying the
quiet sounds.
– Loss of some “tuning” function.
– At more intense levels, “no real hearing loss”
• I can hear OK, if you just speak up!
Inner Ear
• Outer Hair Cells:
– Receives “outgoing” stimuli and this functions
to amplify the soft sounds (“motor” units).
– Usually first to die off (up to 60 dB HL, all of
hair cell damage is outer hair cell…)
– Source of oto-acoustic emissions. (OAE)
Inner Ear
• Inner Hair Cell Damage:
– Typical damage (in addition to outer hair cell
damage) for losses above 60 dB HL.
Problems of the inner ear
• Sensorineural hearing loss
• Permanent
• No maximum hearing loss
Examples of sensori-neural
hearing loss
• Presbycusis (age related loss)
• Noise/music exposure
• Certain drugs
• Hereditary conditions
Central Processing
• VIII cranial nerve takes sound up to the
auditory cortex (transverse temporal
gyrus)
• Several synapses or “weigh stations”
along the way.
Central Processing
• Poorer representation of sound in brain
– Loss of tonotopic representation?
• Loss of one-to-one correspondence
• Slower neural conduction rate
– Possible mild demylenization?
• Loss of insulation of nerves
Natural Gain Characteristics of
the Ear
•
•
•
•
•
(Pinna effect) high frequency amplification
Ear canal (high frequency amplification)
Stapedial reflex (high level reduction)
Outer hair cells (low level amplification)
Central- binaural summation (3-6 amp.)
Ear Canal Resonance
Localization
Basilar Membrane-Organ of Corti
Tonotopic
Tonotopic
Auditory Nerve
• Tuning curve for
single VIII cranial
nerve fiber
• The sharper the
tuning curve the
greater the
frequency
specificity
Central Auditory Mechanism