Download Module 20: Hearing

HEARING
The Nature of Sound
• Sound, like light, comes in waves
• Sound is vibration
• Features of sound include:
– Pitch / Hertz
– Loudness / Decibels
Pitch
• A sound’s highness or lowness
• Dependent on the frequency of the
sound wave
• Is measured as hertz (Hz)
High pitched sounds
Low pitched sounds
Hertz (Hz)
• A measure of the number of sound
wave peaks per second; measures
“frequency”
• Determines the pitch of the sound
• Human hearing goes from 20 Hz to
20,000 Hz
Frequency of Sound Waves
• The frequency of a sound wave is
measured as the number of cycles
per second (Hertz)
– 20,000 Hz
– 4,186 Hz
– 1,000 Hz
–
100 Hz
–
27 Hz
Highest Frequency we can hear
Highest note on a piano
Highest pitch of human voice
Lowest pitch of human voice
Lowest note on a piano
Test You what frequency you can hear at this site
Decibel (dB)
• A measure of the height of the sound wave
• Determines the loudness of the sound
• Sometimes called amplitude
Loud sounds
Soft sounds
Who hits the
higher pitch?
Christina or
Mariah?
• Above are examples of Frequency & Amplitude/decibels
• Timbre – distinctive quality of a sound determined by
the complexity of the wave and its different combinations
of frequencies. (Figure C is more complex than Figures
A or B)
The Structure of the
Auditory System
Hearing: Sound Waves
• Auditory perception occurs when sound waves
interact with the structures of the ear
• Audition (sense of hearing) results in sound
waves being collected in the outer ear,
amplified in the middle ear and converted to
neural messages in the inner ear.
Major Divisions of the Ear
• Outer Ear—acts as a funnel to direct sound
waves towards inner structures
• Middle Ear—consists of three small bones
(or ossicles) that amplify the sound
• Inner Ear—contains the structures that
actually transduce sound into neural response
Divisions of the Ear
Anatomy of Ear
Purpose of the structures in the
ear:
– Measure the frequency (pitch) of
sound waves
– Measure the amplitude (loudness) of
sound waves
Parts of the Ear – Sound Waves
enter through the Pinna
Pinna
Hearing:
Sound Localization
Localization of Sound
• Locating where sound is originating
from
• Done through two cues:
– Which ear hears the sound first?
– Which ear hears the louder sound?
Localization of Sound
Parts of the Ear – Auditory Canal
Auditory Canal
• Sound Waves enter through the pinna
then travel through the auditory canal.
• The opening through which sound
waves travel as they move into the ear
for processing
• Ends at the tympanic membrane
(eardrum)
Parts of the Ear – Tympanic
Membrane (Eardrum)
Tympanic Membrane (eardrum)
• The tissue barrier that transfers sound
vibration from the air to the tiny
bones of the middle ear
• Can be damaged by objects in the ear
or exceptionally loud noises
Parts of the Ear - Ossicles
Ossicles
• Three tiny bones that transfer sound waves
from the eardrum to the cochlea
• Hammer, anvil and stirrup
• In old age they may become brittle or
damaged resulting in conduction deafness
Parts of the Ear - Cochlea
Cochlea
• A hearing organ where sound waves
are changed into neural impulses
• The major organ of hearing
• Filled with fluid; a snail shaped body
tube
Parts of the Ear – Oval Window
Oval Window
• The point on the surface of the
cochlea which receives the sound
vibration from the ossicles
• As the oval window vibrates, the fluid
in the cochlea vibrates moving hair
cells along the basilar membrane.
Hair cells along the Basilar Membrane
move as the fluid vibrates
Review how this works with this short video
Anatomy of the Cochlea
Another View
Outer ear Middle ear
Inner ear
Cochlea,
partially
uncoiled
Tectorial
membrane
Hair cells
Hammer Anvil
Basilar
membrane
Stirrup
Oval window
Sound
waves
Auditory
canal
Eardrum
Round window
A sound causes
the basilar
membrane to wave
up and down.
Hair Cells
• The receptor cells for hearing in the cochlea that
change sound vibrations into neural impulses.
When they move they trigger action potential in the
base of the hair cell (transduction).
• Similar to the rods and cones within the eye except
hair cells are sensitive to vibrations rather than
light.
• If these are damaged (due to prolonged loud noises)
then you have nerve deafness (sensorineural hearing
loss) which cannot be helped by a hearing aid.
Parts of the Ear – Auditory Nerve
Auditory Nerve
• The nerve that carries sound
information from the ears to the
thalamus then to the auditory cortex
in the temporal lobes of the brain
• The auditory nerve is stimulated by
the hair cells in the basilar membrane
of the cochlea.
Anatomy of
the Ear:
A final look
Transduction of Sounds
• Sound waves are captured by the Pinna and sent down
the ear canal where they stimulate the eardrum.
• The eardrum’s vibrations are amplified by the ossicles
(hammer, anvil, stirrup).
• These vibrate the oval window on the cochlea which
in turn vibrates the fluid around the basilar membrane.
• The fluid bends the hair cells on the basilar membrane
triggering action potential in the base of the hair cells.
• This message is transmitted to the auditory nerve
which carries the info to the thalamus and then to the
auditory cortex of the temporal lobe.
• Review using this Nobel Prize site on Hearing
Review how the ear works with this short video
How Can I Remember This?
•
•
•
•
•
•
•
•
•
•
•
•
Please – Pinna
Eat – Ear Canal
Everything – Eardrum
Offered – Ossicles
On – Oval Window
Cruise– Cochlea
Buffet – Basilar Membrane
Helpful – Hair Cells
Attendants – Auditory Nerve
Take – Thalamus
Away – Auditory Cortex
Trash – Temporal Lobe
Or Maybe This Works Better…
Portenga- [Pinna]
evaluates- [ear canal]
exams- [eardrum]
on- [Ossicles]
ominous- [oval window]
charts- [cochlea]
between- [basilar membrane]
hungry- [Hair Cells]
architects- [Auditory nerve]
that- [Thalamus]
always- [Auditory cortex]
Twitter- [Temporal lobe]
Frequency Theory
• The Basilar Membrane vibrates according to the same
Frequency of the sound wave hitting the oval window.
– The higher the frequency wave the faster the firing of hair cells
– Theory used to explain how you hear low frequencies
– Volley Principle – neural cells alternate firing for higher
frequencies
Oval window
Direction of traveling wave
Proximal
end
Distal
end
Basilar
membrane
Place Theory
• Different frequencies cause larger vibrations at different
locations along the basilar membrane
• Different pitches stimulate different areas on the basilar
membrane
• The brain receives these messages and interprets them as different
pitches.
• Theory used to explain how you hear high frequencies.
Use both Frequency & Place theories when you listen to sounds with
high and low frequencies. See this website to see how it works.
Cochlear Implants to Eliminate
Nerve Deafness
Hear What it
sounds like
to hear with
one of these.
Click
HERE.
Which
theory is
being used
by this
implant?
P
L
A
C
E
Coding and Auditory Masking
• The way in which waves travel down the
Basilar Membrane causes some sounds
to interfere with (or mask) our ability to
hear other sounds
• Low frequency sounds provide better
masking than high frequency sounds
Auditory Masking
• Low frequency
sounds effectively
mask high frequency
sounds
• High frequency
sounds cannot
effectively mask low
frequency sounds
Vibration
amplitude
of basilar
membrane
Bassoon, loud
Piccolo, soft
Distance along basilar membrane
(a)
Effect of bassoon on basilar membrane
Vibration
amplitude
of basilar
membrane
Piccolo, loud
Bassoon, soft
Distance along basilar membrane
(b)
Effect of piccolo on basilar membrane