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Communication
Topic 12: Processing Sound
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT Point(s)
 outline the path of a sound wave through the external, middle
and inner ear and identify the energy transformations that occur
 describe the relationship between the distribution of hair cells
in the organ of Corti and the detection of sounds of different
frequencies
 outline the role of the sound shadow cast by the head in the
location of sound
Introduction
In this section we are going
to trace the path of a sound
wave through the ear to the
organ of Corti where it is
processed. I suggest you
draw a flowchart to outline
the path of sound in the
next few slides. There’s 9
steps to you may not get it
all on a single line.You
could do it vertically like
this.
www.hearingtestlabs.com
The Path of Sound
Sound is transmitted as a wave through air in the auditory canal
to the outer layer of the tympanic membrane.
auditory canal
(Eardrum) tympanic membrane
www.digitalhearing.com.au
The Path of Sound
Vibrations from the tympanic membrane are conveyed through
this air-filled chamber via the movement of the interconnecting
ear ossicles to the oval window of the inner ear.
(Ossicles) malleus
incus
stapes
www.cochlear.com
The Path of Sound
The stapes vibrates the oval window setting up a pressure wave in
the perilymph of the upper canal of the Cochlea. This causes
Reissner’s membrane to move.
oval window
cochlea
deanoyr12bio.wikispaces.com
The Path of Sound
The movement of Reissner’s membrane transfers the kinetic
energy to the endolymph of the middle ear canal. This vibrates
the basilar membrane, stimulating the hair cells of the organ of
Corti.
Hair cells (Organ of Corti)
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The Path of Sound
The hair cells send messages along
nerve fibres to the brain where they
are interpreted. The pressure waves
continue to the round window at
the end of the lower canal.
Auditory Nerves
Brain
www.cidpusa.org
Energy Transformations
Sound waves pass in air along the auditory canal. Sound energy is
converted to mechanical (kinetic) as the vibration is set up in the
tympanic membrane. The mechanical energy is transmitted
through the three ear ossicles to the oval window.
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Energy Transformations
As it passes into the perilymph, as a pressure wave, the
mechanical energy is transferred via Reissner’s membrane to the
endolymph to the organ of Corti. Mechanical energy is now
converted to electrochemical energy as information is
transmitted, as nerve impulses, from the hair cells by the
auditory nerve to the brain.
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Energy Transformations
Lets review what our flow-charts should look like shall we…
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The Organ of Corti
 The organ of Corti rests on top of the basilar membrane. It is
composed of supporting cells and about 15 500 hearing
receptor cells called cochlea hair cells. Unlike other cells of the
body, there is a finite number and they are not replaced as they
die.
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The Organ of Corti
There is one row of inner hair cells and three rows of outer hair
cells. These are sandwiched between the tectorial and basilar
membranes of the cochlea.
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The Organ of Corti
The fibres of the cochlea nerve are coiled around the bases of the
hair cells. The ‘hairs’ of the hair cells (cilia) protrude into the
potassium (K+) rich endolymph and the longest of them are
embedded in the overlying gel of the tectorial membrane.
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The Organ of Corti
 Activation of the hair cells
occurs at points of vigorous
vibration of the basilar
membrane. Hair cells nearest
the oval window (base) are
activated by the highest
pitched sounds while those
furthest away at the narrow
end of the cochlea are
stimulated by low frequency
sounds.
www.daviddarling.info
Measurement of Sound
Sound is measured by its relative intensity. The unit of
measurement is the decibel (dB). A sound, 10 time as powerful as
another is said to be 10 decibels more intense. A decibel level of
zero (0dB) represents the faintest sound audible to the average
person. The usual level for conversation is about 60dB. Sound
becomes physically painful to humans above 130dB.
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Dangers
Noise is probably the most common occupational hazard facing
people today. Even leisure activities produce harmful noise. Many
everyday sounds may not be recognised as hazardous as they do
not intrude our comfort zone.
www.takepart.com
Dangers
A bulldozer, idling at 85dB, can cause permanent damage to
hearing in one workday. Standard volume music, transmitted
directly vie earphones, reach 100dB and can cause permanent
damage after only 15 minutes per day.
www.hardwarecanucks.com
Dangers
Avoid noise which causes
discomfort. Above 120dB, noise
can be felt on the eardrum as a
tickling sensation. Beyond
130dB the sensation turns to
pain.
dB
Source
10
Normal breating
20
Whisper
50
Rainfall
60
Conversation
70
Washing machine
80
Busy city traffic
90
Hair dryer
100
Mp3 player
110
Rock concert
120
Ambulance
130
Jet plane from 30 metres away
140
Fireworks
160
Shotgun
180
Space rocket at liftoff
Sound Shadows
The phenomenon caused by the
obstruction or absorption of a
sound wave by an object is its
path is called a sonic or sound
shadow. This is perceived as a
reduction in amplitude or
volume. The effect will be
greatest when the sound source,
the absorbing object and the
person hearing the sound are all
aligned.
inthefray.org
Sound Shadows
 The sonic shadow is the
region which does not receive
the direct sound as the head is
blocking the vibration
As humans are binaural (have
two ears), the head creates a
sonic shadow for the ear further
away from the sound source.
www.ssc.education.ed.ac.uk
Sound Shadows
 Animals use the sonic shadow
to determine the direction of
the source of the sound.
The difference in loudness and
time of arrival of the sound at
each ear can be interpreted by
the brain to determine location.
www.kangaroosofthescrubbybush.com
Sound Shadows
Many humans will turn their
head when trying to determine
the source of a sound. Turning
the head increases the difference
in time of arrival at each ear and
increases the ability to
determine location.
www.cochlear.com
Sound Shadows
The effect has been shown to
be less important than the
split-second time difference in
each if the two ears receiving
the same sound. Visually
impaired people use the sonic
shadow effect, together with
echolocation and other cues for
orientation.
www.lowvision.com
Hearing Aids
Some people are born with an
inability to hear. Others may
lose their sense of hearing
suddenly or gradually. The causes
of hearing loss are generally
grouped into such categories as
heredity, diseases, accidents,
prescription drugs, aging and
acoustic trauma.
Some day your partner may tell
you that they have selective
hearing loss…. ;)
hiddenhearingblog.com
Hearing Aids
The most avoidable of these causes is acoustic trauma or the
chronic exposure to sound. Noise-induced hearing loss (NIHL)
may result from a one-time exposure to loud sound or from
repeated exposure to sounds of various amplitudes over an
extended period of time.
www.exceptionalhearing.com
Hearing Aids
The tympanic membrane may rupture and/or the ear ossicles
could fracture or be displaced by short exposure to a loud noise
such as an explosion.
drugline.org
Hearing Aids
Short term hearing loss is caused by fatigue of the hair cells of the
cochlea. ‘Ringing’ in the ears is called tinnitus. It happens when
delicate cells inside your ear that send sound messages to your
brain are injured or over-stimulated. Usually your ears ring for a
brief time after you’ve been exposed to loud noise.
www.hdiwudoaawebtest.com
Hearing Aids
Continual exposure to excessive
noise (over 85dB) causes
damage to the cilia of the hair
cells. The hair cells die and are
not replaced. If enough of these
cells are damaged, permanent
hearing loss results. The high
frequency receptors are more
often damaged by loud sound.
mashable.com
Activity/Homework
-Handout Table 6.3 Comparison of hearing aid with cochlear
implant
-Handout DOT Point 6.9 Hearing Aids and Cochlear Implants
-Students to complete DOT Point 6.9 and evaluate a hearing
aid and a cochlear implant