Senior Science
8.4 Humans at Work
Section 4
Ear
Protection
P\SSC\HSC\PRELIM\WORK\H Notes 04.doc
8.4 Ear Protection
8.4.1
Earmuffs and ear plugs can reduce damage by sounds in the environment
8.4.4.a
Describe structures of the ear and identify potential causes of hearing
impairment
8.4.4.b
Outline the cause of ‘industrial deafness’ and relate this to the structure and
function of the ear
8.4.4.c
Discuss sensory fatigue and the associated problems of hearing in noisy
environments
8.4.4.d
Explain the structure of earmuffs and ear plugs to the ways in which these
devices reduce sound energy reaching the auditory canal
8.4.4.i
Identify data sources and choose equipment or resources to investigate the
problems associated with hearing and learning in a noisy environment
© P Wilkinson 2002-04
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What is sound?
Sound is produced when objects vibrate. The vibrations of the object cause the surrounding
air to vibrate. The vibrations (or back-and-forth motion) of the air particles causes other air
particles to vibrate. These vibrations continue to travel through the air or other materials
(solids, liquids and gases) as long as the vibration is transferred to other particles [ie. continue
to cause the next material to vibrate]. The travelling vibrations, called sound waves, consist
of a series of compressions and rarefractions. Eventually these vibrations may reach the ear.
The ear absorbs the sound energy and the brain can then interpret the sounds.
Whenever a tuning fork is struck, it vibrates. This
produces alternate compressions and rarefractions. These
make a series of disturbances called a sound wave.
Sound is measured using a unit called the
decibel.
It is a measure of the energy
transmitted by the vibration. Zero decibels
marks the threshold of sound – the weakest
sound that a normal human ear can hear. A
sound of 140 decibels is the threshold of pain.
A whisper is about 20 Decibels.
Normal
conversation is around 65 Decibels. Since dB
are on a log scale 88dB is twice as loud as 85
dB. The risk of hearing damage is related to
‘daily personal noise exposure’. It is a measure
of the average noise energy a person is
exposed to during the working day.
130
110
90
70
50
30
10
Notes Questions
1. What causes sound?
2. Describe how sound travels from one material to another.
3. What unit is used to measure sound level?
4. How ‘loud’ is a whisper?
5. How ‘loud’ is a normal lawnmower?
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Jet engine
Circular saw
Lawn mower, moderate traffic
Loud ringing telephone
Noisy class of students
Quite conversation
Leaves rustling
8.4.1.a
Describe structures of the ear and identify potential causes of hearing
impairment
Structures in the ear
The ear is divided into three parts – the outer ear, the middle ear and the inner ear.
The outer ear consists of two parts. The structure that can be seen on the side of the head, is
called the pinna. It consists of cartilage and skin. Some people can move their outer ear.
The other part is the auditory canal. At the end of the auditory canal is the eardrum. The
function of the auditory canal is to channel sound waves to the eardrum, which can vibrate.
The middle ear is an air filled chamber inside the skull. In this chamber, there are three small
bones called the ossicles. The bones are the malleus (hammer), the incus (anvil) and the
stapes (stirrup). The function of these bones is to transmit and amplify the vibrations from the
outer ear (eardrum) to the inner ear. The Eustachian tube connects the middle ear to the
back of the nose. Its purpose is to maintain equal air pressure on either side of the eardrum.
When a person swallows this tube opens and allows air into the middle ear chamber. If the
Eustachian tube is blocked (due to a cold, or other infection) the eardrum and ossicles cannot
vibrate freely and hearing is impaired.
The organ of hearing is found in the inner ear. One of the two main parts of the inner ear is
the “cochlea”. This is a spiral shaped structure filled with fluid. Inside the cochlea is the
“organ of Corti”, the organ of hearing. This organ has sensitive cells called “hair cells”. The
function of these hair cells is to detect the vibrations in the fluid of the cochlea, and produce
electrical signals. These electrical impulses are transmitted by the optic nerve to the brain
where they are interpreted as sound. If certain parts of the ear cannot vibrate properly then
hearing is affected.
Notes Questions
6. Name the three parts of the ear
7. Name the two parts of the inner ear.
8. Describe the auditory canal.
9. Research Draw a diagram of the middle ear and label the three ossicles
10. Describe the Eustachian tube.
11. Describe the cochlea.
12. Copy and complete the table below naming structures in the ear and identifying the
functions of some of these structures. (One example has been completed)
Part of Ear
 Name of structure
Outer Ear

Pinna

Auditory Canal
© P Wilkinson 2002-04

Function
.
XXXXXXXXX
Channels sound towards the eardrum
4
The Ear and Hearing Impairment
How does sound reach the inner ear?
To answer this question:
 Use the diagram above to help you put the sentences in the correct order.
 The first and last sentences are already in the correct position.
a. Sound waves travel down the external auditory canal.
b. The anvil (incus) then begins vibrating
c. The eardrum begins to vibrate.
d. This in turn causes the hairs of the inner ear to bend.
e. Sound waves can also be conducted into the inner ear through the bones of the skull.
f. Finally the tiny Stirrup (stapes) begins vibrating
g. These waves hit the eardrum.
h. This creates nerve impulses to be sent along the auditory nerve.
i. The movement of the stirrup causes the fluid in the cochlea to move.
j. The vibrating eardrum causes the hammer (malleus) to vibrate
k. This process is called bone conduction.
l. [These are part of the hair cells of the organ of Corti].
m. The auditory ossicles amplify the sound transmitted to the inner ear.
n. The brain interprets the impulses as sound.
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Hearing impairment
1) Conduction hearing loss
Conduction hearing loss is concerned with the conduction
of sound from the outer ear and through the middle ear. The transmission of sound
involves transmission of vibrations from one structure to another. In conduction-hearing
loss these vibrations are not conducted completely.
Most conductive hearing loss occurs when the ossicles do not function properly.
Generally, people with conduction hearing loss have a reduction in the loudness of
sounds, no matter what the frequency. Although many sounds may become too quiet to
be heard, those that can be heard are clear and undistorted. For this reason many people
with conduction hearing loss can be helped with a hearing aid. Also, doctors can restore
many problems with conduction hearing loss. For example a ruptured eardrum, caused by
exposure to an excessively loud noise, can be repaired surgically.
Conduction hearing loss can be by such things as:
 Faulty development of the outer and middle ear.
 Blockage of the ear canal (eg by ear wax). – sound cannot reach the eardrum.
 Damage to the eardrum – the eardrum cannot vibrate.
 Damage to the ossicles (small bones of the middle ear) – the ossicles cannot vibrate.
 Failure of the Eustachian tube to let air into the middle ear cavity – reduces the ability
of the eardrum to vibrate properly.
 Collection of fluids in the middle ear cavity – reduces the ability of the ossicles to
vibrate.
 Infection of the middle ear cavity.
 Growth of extra bone around the ossicles.
2) Sensory-neural hearing loss
Sensory-neural hearing loss occurs when
 sound waves are not sensed by the inner ear (specifically by hair cells in cochlea) or
 the electrical signals are not transmitted by the auditory nerve to the brain or
 the hearing area of the brain does not interpret the signal from the auditory nerve..
It is related to the sensing of sound by the nervous system.
Most types of sensory-neural deafness are caused by damage to the hair cells in the
cochlea. These cells cannot be repaired or replaced. The hair cells detect the vibrations
and convert them into electrical impulses. Sometimes damage occurs to the auditory
nerve or the hearing area of the brain. Sensory-neural losses cause a loss of loudness
and a loss of clarity in sounds. Most problems with sensory-neural hearing loss cannot be
treated medically, but most can be helped by hearing aids. Unfortunately, the hearing aids
only restore missing loudness. They cannot restore missing clarity.
Sensory-neural hearing loss can be caused by such things as:
 Faulty development of the inner ear.
 Inherited damage to the inner ear and/or the hearing nerve from illness, drugs, or
oxygen deprivation.
 Damage to the ear from loud noises.
© P Wilkinson 2002-04
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Notes Questions
13. What is conduction-hearing loss?
14. How can people with permanent conduction hearing loss be helped?
15. Name three structures in the ear associated with conduction-hearing loss.
16. How does damage to the eardrum cause conduction hearing loss?
17. How does the collection of fluids in the middle ear cavity cause hearing loss?
18. How does the growth of extra bone around the ossicles cause hearing loss?
19. What is sensory-neural hearing loss?
20. What structure of the inner ear senses the sound vibrations?
21. What structure in the inner ear transmits signals to the brain?
22. What structure interprets the sound?
23. Research / homework
What is meant by clarity of sounds?
24. Name two causes of sensory-neural hearing loss
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8.4.4.b
Outline the cause of ‘industrial deafness’ and relate this to the structure and
function of the ear
8.4.4.c
Discuss sensory fatigue and the associated problems of hearing in noisy
environments
Introduction
Sounds that are pleasing to the ear normally consist of regular vibrations [eg the vibrating
string of a cello]. Most noises are generally produced by objects, which send out irregular
vibrations at irregular intervals [eg a window breaking]. Of course noise to one person can be
music to another. Noise can be defined as unwanted sound. Noises are usually regarded as
unpleasant, annoying or distracting.
Most of the hearing losses, before the C20th, were caused by medical reasons. In recent
years the development of large machinery, amplifiers (loud music), gun powder, and other
mechanical disturbances have made the hearing environment a dangerous place for our ears.
People disregard loud noises because the exposure is not causing instant pain. But extended
periods of exposure to loud sounds can cause long-term damage to the inner ear. The
volume of home stereos is of particular concern. In two recent studies it was found that 1 in
10 of Germany’s 18 year olds had suffered ear damage severe enough to handicap them in
normal conversation.
For the first time in history people are starting to care and discover why our hearing
decreases with age. It is not necessarily just part of the aging process.
Noise hazards
Noise is one of the most widespread hazards facing Australian workers. People are
continually being exposed to loud noises, from building sites, aeroplanes and household
appliances. Headaches, irritability, ear damage, permanent hearing loss can all be caused by
noise. A noise of 140 dB can make the body ill. Sudden sounds can lead to an increase in
blood pressure and heart rate; saliva production is reduced; muscles contract and
perspiration increases. Therefore, in many situations it is important to control unwanted
sounds, and to reduce the amount of sound energy reaching the ear.
Noise in a work place may be a safety hazard if workers cannot hear safety instructions or
warnings. Of course it is just as important to remember that ear protection should not
eliminate all sound – this is potentially dangerous for the same reason. There is an
acceptable or useful level of noise. Researchers have shown that noise level alone does not
cause the greatest distraction. It is the difference between background noise and the problem
noises that cause the most distraction. If the background noise is too high that is a distraction
in itself. If the background noise is too low, then another small noise becomes a distraction.
In many situations background noise should be pleasant to listen to.
© P Wilkinson 2002-04
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There are many everyday examples of controlling or reducing the energy of noise hazards.
Noise hazards can be controlled in three ways:
 Quieting the source of noise (mufflers on car exhausts)
 Blocking the passage of noise (large heavy walls)
 Absorbing noise energy (carpet and heavy curtains)
8.4.1.b
Discuss sensory fatigue and the associated problems of hearing in noisy
environments
Sensory Fatigue
The effort to concentrate and listen to a sound or voices causes fatigue. The longer the time
a person is exposed to sounds of a particular frequency or loudness the greater the fatigue.
The hairs responsible for the detection of sound waves in the cochlea constantly move in
response to the sound vibrations. When they become fatigued they can no longer respond
with the same degree of movement. The louder the sound the more quickly the hair cells
become fatigued. Constant fatigue due to constant exposure to loud sounds results in
permanent damage to the hair cells. This damage is the basis of noise induced hearing loss.
Generally our bodies need rest for growth and repair. Sleep provides the body with its best
opportunity to rest. The psychological effects of being woken from sleep by noise leads to a
person who awakes unrefreshed and psychologically strained. For this reason, anti -noise
laws are in force at various times of the late afternoon through to early morning and on
weekends, to allow uninterrupted sleep.
Notes Questions
25. What is noise?
26. What was the main reason for hearing loss before the C20th?
27. Revision What is a hazard?
28. Why don’t people regard loud noises as a hazard?
29. Identify one piece of evidence to suggest the volume of home stereos is of concern.
30. Name two affects on the body caused by noise?
31. “If a worker could work in silence by wearing earmuffs that eliminated all sound, he would
be much better off”. Explain why this statement is dangerous.
32. What noises cause the greatest distraction?
33. Identify two methods (with examples) of controlling noise hazards
34. What is the difference between normal ‘hairs’ and those that are fatigued?
35. How can hair cells be damaged?
© P Wilkinson 2002-04
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Hearing loss caused by noise
Exposure to loud noise beyond a critical value can damage your hearing. The critical value
varies from person to person. Factors such as age and previous noise exposure affect the
critical value. Generally, the louder a sound and the longer it continues, the more likely it is to
damage your hearing.
The table below sets out the limits to maximum exposure levels.
Maximum daily exposure
Sound level (dB)
8 hours
4 hours
2 hours
1 hours
30 minutes
15 minutes
7 minutes
3 minutes
85
88
91
94
97
100
103
106
This means that workers exposed to average sound levels greater than 85db for 8 continuous
hours, must be provided with protection, such as earplugs of earmuffs. To protect hearing,
safe work practices, such as wearing ear protection when using power tools (eg a power saw)
are compulsory.
There are two main types of hearing loss due to excessive noise

Temporary threshold shift: This is usually a temporary dullness in hearing after
exposure to loud noise. This dullness occurs because the hair cells of the inner ear that
sense noise vibrations are fatigued. They cannot respond to the vibrations properly
transmitted from the middle ear. The hair cells and hearing will eventually recover
depending on the noise that caused it. Repeated exposure to hazardous noise can lead
to Permanent Threshold shift.

Permanent threshold shift: Permanent hearing loss occurs if hearing does not recover
after 48 hours. This most likely means that the hair cells of the inner ear have been
permanently damaged. There are two categories of permanent threshold shift.
 Industrial deafness
Occupational hearing loss happens when you have been
regularly exposed to hazardous levels of noise over a long period of time. You
gradually acquire a sensory-neural hearing loss that is usually most severe in the
high frequencies around 3-4 kHz. The hearing loss will be similar in each ear and
will get worse with continued exposure to noise.
 Acoustic trauma
This occurs when exposed to a very high level sound for a
short period of time, for example if you are close to an explosion, or gunfire. This
type of sound can cause a sudden hearing loss that is often more severe in the ear
closest to the sound.
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Tests of a hearing loss caused by exposure to noise show a characteristic pattern. The
typically pattern consists of a dip in your hearing in the high frequencies – at around 3-4kHz.
The ear is most sensitive to, and therefore most damaged by frequencies in the 1000 –
4000H z parts of the audio spectrum. If the noise exposure continues, this notch in your
audiogram – the chart that shows your hearing levels when you have a hearing test – will
broaden and spread to affect lower and higher frequencies too. The hearing loss will continue
to increase for up to five years, after which it gradually stops getting worse unless you are
exposed to increased noise levels. Because hearing loss typically begins at the higher
frequencies of 4000 – 6000 cycles per second, the effects of noise exposure may go
unnoticed until the hearing loss spreads to the lower frequencies of 1000 – 2000 cycles a
second. As it gets worse and spreads to affect a wider frequency range, an awarness
develops of difficulty in following conversations if there is background noise.
The graphs below show audiograms of Temporary and Permanent Threshold shift.
Temporary Threshold Shift (short exposure to 100dB)
Intensity
(decibels)
0 dB
10dB
20dB
30dB
40dB
250Hz
500Hz 1000Hz 2000Hz 4000Hz 8000Hz
Frequency (Hz)
Permanent Threshold Shift (constant exposure to 100dB)
Intensity
(decibels)
© P Wilkinson 2002-04
0 dB
10dB
20dB
30dB
40dB
250Hz
500Hz 1000Hz 2000Hz 4000Hz 8000Hz
Frequency (Hz)
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Notes Questions
36. What is the maximum exposure level to a noise level of 88dB?
37. What should a person do if they know they will be exposed to loud noises?
38. How long could a person listen to a 97dB sound without causing permanent damage to
their ears?
39. How would a person know they suffered a temporary threshold shift in hearing?
40. What structure in the ear is damaged for both temporary and permanent threshold shifts?
41. Identify the type of hearing loss (conduction or sensory-neural) associated with both
temporary and permanent threshold shifts
42. What is the difference between hearing losses due to industrial deafness and hearing
losses due to acoustic trauma?
43. A Senior Science student loves loud music. He is a regular visitor to Rock Concerts and
discos. Recently he has noticed his hearing is a little dull for one or two days after the
concerts. [4 marks]
a. What scientific information would you give this person about hearing impairment?
b. What would you suggest this person do to protect his hearing?
44. Name three events that could cause an acoustic trauma?
45. What is an audiogram?
46. Describe the audiogram of a person who suffers a temporary threshold shift in their
hearing.
47. What is the difference in the audiograms of a person suffering a temporary threshold shift
and someone suffering a permanent threshold shift?
48. Describe what happens to your hearing if you are continually exposed to a hazardous
level of noise.
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8.4.4.d
Explain the structure of earmuffs and ear plugs to the ways in which these
devices reduce sound energy reaching the auditory canal
Reducing Sound Energy
Soundproofing materials.
Sound energy is absorbed when it causes a material to vibrate. Soundproofing occurs when
the vibration is stopped completely or stopped from being transmitted in a particular direction.
A sound absorbing material is a material that can absorb the back-and forth motion of
particles, without radiating a new sound wave. The more surfaces that begin to vibrate
because of the sound wave, the more energy that is absorbed. The sound energy is
converted into mechanical energy, which is finally converted into heat. Some materials
absorb sound better than others because of their structure.
Materials that absorb sound (acoustically soft materials) have:
 Many small surface pores (holes) to allow entry of sound
 Interlocking fibres that provide many separate surfaces to vibrate.
 Relatively light
Diagram 1
Thermal batts can act as sound proofing material.
Diagram 2
Street noise into a building can be
limited by installing double panes with
an air space between the glass.
Materials that reflect sound (acoustically hard materials) have:
 Few pores or openings on the surface providing a solid wall to the sound wave.
 Fibres that are closely packed which will not readily vibrate
 Heavy and hard to move.
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The part of the ear that protrudes from the side of the head, the pinna, directs sound into the
ear. The sound waves enter the auditory canal, and set the eardrum into motion. Ear plugs
work because they absorb the energy of the sound vibrations before they reach the
auditory canal. They do this because they are made of sound absorbing materials.
Therefore, by the time the sound wave (vibration) enters the auditory canal the energy level
has been greatly reduced. Ear muffs, on the other hand, are made of a smooth, hard
outer surface that reflects sound rather than transmitting it. Padding and other material
ensures the ear is totally enclosed allowing only a small amount of sound to enter.
Notes Questions
49. How do materials absorb sound energy?
50. What energy changes occur when sound energy is absorbed?
51. Name two features of materials that absorb sound.
52. Name two features of materials that reflect sound.
53. How do ear plugs work?
54. How do ear muffs work?
55. What type of materials are used to make ear plugs?
© P Wilkinson 2002-04
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56. Read the advertisement about the Ultimate 10 Dual Earmuff.
The Ultimate 10 Dual Earmuff
The Peltor patented twin cup Ultimate
10 features a dual ear cup shell that
minimizes resonance thereby achieving
maximum sound reduction.
This
design ensures sustained, uniform
sound reduction for the most noisy
situations.
Ultimate 10 features include:
 A unique twin cup design
 Foam ear cushions for all day long wearing comfort.
 Simple, independent head band height adjustment for fitting all head sizes
and shapes.
 Unequaled independent stainless steel spring wires with two low tension
points to provide consistent pressure distribution.
 Soft padded headband forms to the shape of the head.
 Anatomical design cup with smooth surface provides very easy cleaning.
 Soft and wide ear cushions offer low pressure and provide effective seal.
a. What features listed reduce the sound energy that reaches the auditory canal?
b. Several features do not effect sound but are important in the design of the earmuff.
i. Name THREE of these features.
ii. Suggestion a reason why each of these other features are important for ear
protection.
c. Name the different materials used in this product.
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57. One way of reducing the amount of sound energy reaching your ears is to use earplugs or
earmuffs. However this is not always practical. People are exposed to unwanted sounds
in many situations. These include aircraft noise and excessive noise in offices as people
move about. Many materials have been developed to reduce noise in all sorts of
situations.
The graphs below show the performance of specific materials in reducing unwanted
sounds. Answer the questions using information from the graph.
Graph 1
a. Which building material has the best sound reduction for 500Hz?
b. What is the effect on the sound reduction capacity of using strip ties with cavity
brick? [Hint– compare to cavity brick without ties] Suggest a reason for this effect.
c. Which frequency is reduced the most – 500Hz or 1000Hz?
d. Describe the sound reduction capacity of Double brick – plastered both sides for
the frequency range 125Hz to 2000Hz.
e. Generally, what is the relationship between frequency and sound reduction?
Graph 2
f.
g.
h.
i.
j.
Which floor covering has the best sound absorption for 500Hz?
Of the materials named, which is the worst floor covering for sound absorption?
Do floor coverings tend to absorb sounds with a frequency of 250Hz or 1000Hz?
Describe the sound absorption capacity of Wilton carpet for the frequency range
63Hz to 4000Hz.
State the relationship between frequency and sound absorption.
© P Wilkinson 2002-04
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8.4.4.i
Identify data sources and choose equipment or resources to investigate the
problems associated with hearing and learning in a noisy environment
4.8 Learning in a noisy environment
Problem
What are some problems associated with hearing and learning in a noisy
environment? Can people learn as well in a noisy environment as they can in a
quiet environment?
Information
What to do
1. Write a heading
2. Copy the problem into your book
Method [VGMANS]
3. Discuss a possible method to perform a
a. The independent variable is the amount
first hand investigation of the problem.
The questions below will help in the
of noise.
discussion
b. The dependent variable is the amount
a. How can the noise level be varied?
of learning.
b. What noise would be used?
c. Some variables need to be controlled
c. What is going to be learnt?
so that results from different groups can
d. What variables need to be controlled?
be compared (ie results are valid).
e. Possible procedure – Game of “Recall”
using two similar lists or items; learn
d. Two groups are compared
one list with noise; learn one list
i. Learning in a noisy environment
without noise; one group of subjects.
ii. Learning in a quiet environment
f. How will data be measured and
d. The amount of learning is measured.
recorded?
e. The activity is a practical investigation
g. How can people be distracted when
f. The experiment can be repeated a
trying to learn in a quiet environment?
How do people react to distractions?
number of times for reliability. The best
h. How many subjects are necessary for
way to do this is to use a large number
reliable results?
of subjects (people).
g. Noise may be a safety issue in this first 4. Write a possible method. It needs to be
read by at least ONE other person.
hand investigation.
5. Complete the activity and collect data on:
a. how much was learnt and
b. how people react to various noises.
6. Record data
7. Write a conclusion
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Marking Criteria
Factors
Mark Allocation
H13 Present information by
 Writing a heading
3–0
/3
3-0
/3
2-0
/2
 Steps outlined in logical order
1–0
/1
 Investigation can be repeated
1–0
/1
 Clearly describe what is being measured
2–0
/2
 Demonstrate use of terms dependent & independent
2–0
/2
 Identify variables that need to be kept constant
2–0
/2
 Method contains control and experimental groups
1–0
/1
 Results are reliable (large number of trials suggested)
1–0
/1
 Complete a risk assessment
2–0
/2
 Identify safe working practices
2-0
/2
a. Using the report scaffold
e. Correctly draw and labelled diagrams to present information
H11.3 choose equipment
a Identify equipment needed
H11.2 plan first –hand investigations
12.1
Perform first-hand investigation
Discussion questions
/20
Total
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/40
Discussion Questions
1. Name the independent variable.
[1 mark]
2. Name the dependent variable.
[1 mark]
3. In order to make valid comparisons certain variables must be controlled.
a. Identify the controlled variables in this investigation. [2 marks]
b. Identify any variables that are not controlled in this investigation.
[2 marks]
4. Two groups are being compared in this experimental investigation.
a. Name the two groups being compared.
[1 mark]
b. Identify the Control Group.
[1 mark]
5. Outline how the dependent variable is measured in this investigation.
6. a. What type of activity is this investigation?
[1 mark]
b What other types of scientific investigations are there?
[2 marks]
[2 marks]
7. What features of the method makes this investigation reliable? [2 marks]
8. Discuss if a valid conclusion can be drawn?
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[5 marks]