Noise Control
A practical approach to controlling noise in the workplace
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What’s in this booklet?
This booklet has been produced to help New Zealand businesses
manage noise in their workplace rather than only relying on
personal protective equipment as a first option. It provides an
overview of the range of approaches you can take to controlling
noise in your workplace, from simple and inexpensive in very many
cases, to more complex engineering options.
It explains:
• why excessive noise is a hazard in the workplace, and how it can
affect your employees
• the benefits to your business and your staff of controlling noise
in your workplace
• the basics of sound – some fundamental concepts about what
sound is, how it’s measured and how it behaves, designed to
help you understand some of the key approaches to controlling
noise
• an overview of how to analyse noise problems in your workplace,
based on the ‘Eliminate – Isolate – Minimise’ methodology
• noise control techniques – an overview of the main options for
managing noise (with examples to illustrate).
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Contents
1. Noise in the workplace – introduction
3
The problem
How loud is too loud?
Why earmuffs and earplugs are usually not the first response
The benefits of controlling noise in the workplace
2. What is sound and how is it measured?
6
What is sound? – sound waves
Wavelength and frequency
Amplitude (loudness)
Measuring sound
3. How sound behaves
10
Sound transmission
Damping and absorption
How sound travels
4. Noise control – strategic approach
12
Eliminate – Isolate – Minimise
Analysing the problem - observation
Analysing the problem – monitoring
Noise control techniques
5. Maintenance and repair
14
6. Absorption
15
Absorption example 1 – direct and reflected factory noise
Absorption example 2 – portable equipment noise
7. Damping
19
Damping example 1 – metal component conveyor system
Damping example 2 - piping
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8. Enclosure
22
Enclosure example 1 – ultrasonic welding machine
Enclosure example 2 – pump motor
Design principles for enclosures
9. Isolation
25
Isolation example 1 – process pump
Isolation example 2 – noise refuge
10.Design and equipment
28
11. Other sources of information
29
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1. Noise in the workplace – introduction
The problem
Noise-induced hearing loss (NIHL) is one of the most common injuries arising from work
– and the problem is rapidly increasing. The effects of excessive exposure to noise are gradual. They may not be noticed for
many years, until the damage has been done and the symptoms become a problem for
the sufferer – and those around them. That’s why it is important to proactively reduce
excessive exposure to noise to prevent the onset of NIHL.
How loud is too loud?
The 1995 Health and Safety in Employment Regulations in New Zealand state that
employers should take all reasonable steps to ensure no employee is constantly
exposed to noise above 85 decibels during an 8 hour working day, whether or not the
employee is wearing a hearing protection device (e.g. earmuffs or earplugs). 85 decibels
is roughly equivalent to road traffic noise. It’s generally agreed that prolonged exposure
above that level can result in NIHL. To give you a comparison, the level of noise in the
average shearing shed is around 96 decibels, a busy street may be 80 decibels (see diagram below).
Figure 1: Noise theometer
Jackhammer
109dB*
115dB
112dB
109dB
Hammer on Nail
104dB*
106dB
103dB
100dB
Bulldozer
99dB*
97dB
94dB
91dB
88dB
Table Saw
93dB*
85dB
Hand Saw
85dB*
0dB
Find out more at www.acc.co.nz/nihl
Normal
Conversation
60dB*
* Based on 8 hour exposures
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The Health & Safety Regulations also set out a maximum peak noise level of 140
decibels (roughly, the sound of a jet airplane taking off).
There are some simple rules of thumb you can use as indicators of noise levels in your
workplace that are likely to cause hearing loss. Common indicators of excessive noise
levels include:
•
needing to shout to be heard at arm’s length
•
background noise making it hard to communicate
•
frequent requests to repeat what has just been said
•
the need to strain to understand conversation
•
ringing in your ears after you finish work.
You can also measure sound levels more precisely with sound level meters and
dosimeters, if necessary. See section 4 for more information on analysing noise in your
workplace.
Why earmuffs and earplugs are not the first response
Most workplaces where noise is an issue rely on hearing protection devices (HPDs) such
as earplugs and earmuffs as a first option. However, they should usually be considered
as the last option, or at best as one part of an overall approach to noise control.
The rising incidence of NIHL indicates that current approaches to noise control, which
centre mainly on the use of HPDs, may not working and this reflects international
opinion. New Zealand workers continue to suffer hearing loss as a result of their work, in
spite of wearing HPDs. There may be numerous reasons for this including the incorrect
wearing of HPDs. In addition, the use of HPDs in the workplace can be costly. As well as the cost of
purchasing and regularly replacing earmuffs and earplugs, once they are introduced to
the workplace employers are required under current regulations to monitor sound levels
and conduct regular audiometry (hearing) tests for employees. External consultants
such as occupational hygienists and audiometrists are usually required to make these
tests.
It is usually more effective - and cheaper in the longer term - to reduce the level of noise
at the source. This booklet outlines approaches for achieving that goal.
The benefits of controlling noise in the workplace
Effectively controlling noise in the workplace has obvious benefits for employees, in
terms of better health and better quality of life presently and in the future. However,
there are also significant business benefits for employers. A healthier workplace is usually a more productive one. A proactive approach to
creating a better working environment can also have a positive effect on employee
morale, again leading to increased productivity.
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An effective noise management strategy can also save money. As we have seen,
introducing hearing protection devices can have significant costs, in terms of the cost of
purchasing and replacing equipment and the regular noise monitoring and hearing tests
required. Alternative approaches that reduce the noise at source are often much less
expensive and more effective, particularly in the longer term. From a wider perspective, the cost of ACC claims resulting from NIHL is increasing
rapidly. Controlling noise in New Zealand workplaces, therefore, may also help to
reduce employer ACC levies over the longer term.
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2. What is sound and how is it measured?
Controlling noise in your workplace is easier if you understand some of the basic
concepts
aboutbooklet
sound, in particular:
ACC
Noise control
page 7 of 29
• what produces sound
2. What
is sound
soundtravels,
and and
how is it measured?
• how
•
how sound can be absorbed.
Controlling noise in your workplace is easier if you understand some of the basic concepts about
sound, in particular:
• what produces sound
What
is sound?
sound waves
• how sound
travels, –
and
• how sound can be absorbed
Sound is basically a vibration that travels in waves. A good illustration of how sound is
created
is the loudspeaker
in your home stereo. The speaker has a diaphragm or cone
What
is sound?
– Sound waves
which moves in and out in response to electrical signals. This movement creates the
Sound
is basically
vibration the
that sound. The
travels in waves.
A good
illustration
of how sound is created is
vibrations
thata produce
process
is shown
below:
the loudspeaker in your home stereo. The speaker has a diaphragm or cone which moves in and
outFigure
in response
to electrical
signals. This movement creates the vibrations that produce the
2: How
sound travels
sound. The process is shown below:
Step 1: There is no sound going through the speaker
so the diaphragm (green) is ‘at rest’.
Step 2: As the first sound goes through the speaker
the diaphragm moves forward. This vibration
compresses the air molecules in front of it. This
starts to create the first sound wave.
Step 3: As the vibration continues, the diaphragm
reverses direction through the ‘resting position’ until it
reaches the maximum point of decompression.
Step 4: When the diaphragm returns to its resting
position, one cycle of the vibration is completed,
producing one sound wave.
Step 5: In the production of sound, this cycle of
compression and decompression of air to create
sound waves is repeated many times over.
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There are three key characteristics of sound waves that it’s important to understand:
•
wavelength
•
frequency (or pitch), and
•
amplitude (or loudness).
Wavelength and frequency
We’ve seen that sound is produced by vibrations which create pressure waves
(compressions and decompressions of air), known as sound waves. The distance
between the waves is known as the wavelength.
Figure 3: A sound wave
Compression
Wavelength
+ pressure
– pressure
Decompression
The wavelength determines the frequency of the sound (also known as pitch). The
frequency of a sound is the number of cycles of compression and decompression in one
second and is measured in hertz (Hz). For example, if there were 50 cycles in one second
the frequency of the sound would be 50 Hz.
The shorter the wavelength, therefore, the higher the frequency or pitch of the sound.
For example, the high-pitched sound of a dog whistle might have a frequency of around
20,000 Hz (the highest frequency a normal human ear can detect). Correspondingly,
the longer the wavelength the lower the frequency or pitch. For example, the rumble of
thunder might have a frequency of around 20Hz (the lowest frequency sound a normal
human ear can detect). Frequency is a very important concept in terms of noise control. Low frequency sounds
have long wavelengths and travel great distances – for example, the doof – doof - doof
of the bass from the music at a neighbour’s party. In contrast, high frequency sounds
have short wavelengths and are less well heard over long distances. 7
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The diagram below summarises the relationship between wavelength and frequency:
Figure 4: Wavelength and frequency
Long Wavelength (Meters) 20
10
10
20
5
50
2
100
Low pitch 200
1
0.5
500
Short
0.2
1,000 2,000
0.1
5,000 10,000 20,000
Frequency (Hz) Infrasound 0.05
Range of human hearing (healthy young adult) High pitch
Ultrasound
Different approaches may be required to control noise at different frequencies. For
example, some sound-absorbing materials work better at different frequencies (for
more on this see section 6 – Absorption).
Amplitude (loudness)
While the length of the sound waves determines the frequency or pitch of the sound,
the height of the waves (known as the amplitude) determines the ‘loudness’ of the
sound. Increasing the ‘power’ of the vibrations (e.g. turning up the volume on your home
stereo) leads to an increase in the height of the sound waves, as shown by the grey
shaded areas below. The human ear perceives this increase in amplitude as an increase
in sound pressure level, which makes it sound louder.
Figure 5: How amplitude relates to volume
Wavelength
Amplitude
High pressure
Low pressure
Measuring sound
Sound is measured in decibels (dB), using sound level meters. These meters measure
the pressure of the sound waves.
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The higher the decibels, the louder the sound. However, it’s very important to
understand that decibels are measured on a logarithmic scale, not a linear scale. This means that every increase of 10dB equates to a ten-fold increase in sound intensity
(roughly equivalent to loudness) – so a 20dB sound is not twice as loud as a 10dB sound,
but ten times as loud. Similarly, a 30dB sound is 100 times louder and a 40dB sound is
1,000 times louder than a 10dB sound. That’s why an increase of a single decibel can
make a significant difference, particularly at higher decibel levels. Each extra decibel
means more energy, which can cause damage to hearing. For the same reason, when you have several sources of sound, measuring the total
sound level is not just a case of adding decibels together. For example, if you have two
machines each generating 80 decibels of sound, the total sound level is not 160 decibels
but 83 decibels. This is because decibels need to be first converted to their sound
pressures, then added and converted back to decibels. In this example, the effect of the
logarithms in the calculation means that doubling the amount of decibels only results in
a 3dB increase in the total sound level.
That’s why precise measurement is important. Cheaper sound level meters can have
a margin of error of up to 3 decibels, which can make a significant difference. To obtain
precise measurements you will usually need to engage a noise control specialist who
has specialised equipment. The apparently small difference that cheaper instruments
may not be able to measure will have a large impact.
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3. How sound behaves
Understanding how sound behaves provides useful insights into the main techniques
for controlling noise. One way of illustrating how sound behaves is to look at how sound
is produced with an acoustic guitar.
Sound transmission
As shown below, when a guitar string is plucked it vibrates, creating a sound wave.
Figure 6: Creating a sound wave
On its own, however, a guitar string does not make much sound. It is the interaction
of the string with the body of the guitar that amplifies the sound and creates the guitar
sound we recognise.
As the diagram below shows, some of the vibrations from the string are also
transmitted through the guitar’s bridge plate into the wood of the sound box, which
also vibrates. These vibrations then create additional sound waves which are reflected
within the sound box, amplifying the sound. Some of the sound emerges from the
sound hole. The vibrating surface of the sound box also creates further sound waves
(though to a lesser extent).
Figure 7: How vibrations travel
Bridge plate
Sound hole
Sound box
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Damping and absorption
We can also use the example of the guitar to illustrate some of the fundamentals of
noise control.
If we fill the sound box of the guitar with sand as shown below, we can reduce the sound
by damping and absorbing the vibrations that are created.
Figure 8: Dampening a sound wave
Sand
Bridge plate
Sound box
In this example, when the string is plucked it will still vibrate and create sound waves in
the air. However, the sand reduces the vibrations that are transmitted to the sound box
and the reflections within the box, as well as absorbing the vibrations that do emerge. It also reduces the vibrations of the surface of the sound box. How sound travels
Sound decreases the further away you are from it. In a ‘free field’ (i.e. where there are
no obstructions or reflecting surfaces, for example outdoors) sound drops by 6dB for
each doubling of the distance from the source of the sound. For example, if the sound
level 10 metres away from the source is 50dB, it will reduce to 44dB at 20 metres from
the source (this is known as the inverse square law).
However, in the real world, including workplace environments, free fields hardly ever
exist. Instead, there are floors, walls, roofs, partitions, pillars and machinery. These
structures can absorb sound, but they can also reflect it, making it louder. They can also
vibrate themselves, and become an additional source of sound.
The noise levels that employees are subjected to are a combination of:
•
direct field noise i.e. the noise directly generated by machinery and other
sources nearby, and
•
reverberant field noise (reflected noise).
Noise control techniques will usually need to address both of these issues. 11
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4. Noise control – strategic approach
Eliminate – Isolate – Minimise
An effective approach to noise control approach will usually consist of the following
three steps (in order):
•
Eliminate – is the noise necessary in the first place? Is there an alternative way
to achieve the same outcome? For example, through different processes, using
different machinery or adapting existing machinery (including using different
parts)
•
Isolate – if the noise cannot be eliminated, can the machinery be enclosed to
reduce the noise? Can it be placed in another area away from people?
•
Minimise – if it is not possible to eliminate or isolate the noise, how can it be
minimised? For example, can people be moved around the plant to reduce their
exposure? Are the correct classes of hearing protection devices (HPDs) being
worn?
To help you identify the best solution for your workplace, you should first undertake
some analysis of the problem.
Analysing the problem - observation
Some simple ‘walkaround’ observation of your workplace will give you a lot of useful
information. Questions you should ask include:
•
Where is the noise coming from? What particular equipment or machines are
creating it?
•
What are the characteristics of the noise? Is it continuous or intermittent? Is it a
specific frequency or pitch? •
Is the noise direct or reflected, or both?
•
How loud is it? Are there common indicators of high noise levels? (e.g. needing
to shout to be heard at arms length, frequent requests to repeat what has just
been said, ringing in ears after work)
•
Are there specific times of the day or tasks that contribute most to the
exposure? If so, could these tasks be modified or could workers be rotated at key
times?
Analysing the problem – monitoring
If necessary, more detailed monitoring can be undertaken with specialised equipment
to provide more detail. These include:
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•
sound level meters – to measure the noise level and show how much reduction
in noise is required
•
noise dosimeters – these measure noise levels over time, which can be used
to identify specific periods of the day or tasks that contribute the most to the
exposure
•
octave band frequency analysers – these help to identify which particular
frequencies to target for noise control.
This kind of monitoring is typically undertaken by noise control specialists. Once you have gathered and reviewed this information you can identify the most
effective approach and the most appropriate techniques for your workplace.
Noise control techniques
There are a range of techniques that can be used to eliminate, isolate or minimise noise
in the workplace. Many are simple and inexpensive. For example, regular maintenance
of machinery can go a long way to reducing noise at source (as well as extending the
efficiency and working life of the machinery). Simple changes to tasks or processes, for
example rotating staff around noisy equipment, can also be effective.
Where engineering options are required, these can also be simple and inexpensive. For
example, fitting simple rubber sleeves, where possible, can significantly reduce the
noise from striking parts. Other examples include enclosing milking pumps in dairy
sheds with a simple box to reduce pump noise or adding a rubber pad to the head of a
hammer to dampen the striking of the hammer against metal.
The main noise control techniques include:
•
Maintenance and repair (see page 14)
•
Absorption (see page 15)
•
Damping (see page 19)
•
Enclosure (see page 22)
•
Isolation (see page 25)
•
Design techniques (see page 28)
These techniques are discussed in more detail in the following sections.
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5. Maintenance and repair
Regular maintenance and repair of machinery, especially those elements likely to suffer
wear, can significantly reduce noise at source. Some examples of maintenance related
noise sources are:
•
slipping drive belts
•
worn drive belts
•
worn gears
•
worn or dry bearings
•
worn cutting blades e.g. saws
•
loose covers on vibrating machinery
•
damaged acoustic covers
•
worn anti-vibration mountings
•
unbalanced fly wheels, rotating shafts and fans.
The above are only some of the more common examples of elements of machinery that,
over time, will become worn and can produce increases in noise levels.
A programme of regular inspection and maintenance can do much to prevent increases
in noise. Regular inspection and maintenance can also:
•
maintain the efficiency of the machinery
•
prevent or reduce the frequency of breakdowns and loss of production
•
extend the working life of the machine.
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6. Absorption
Absorbing sound with absorbent materials is a common noise control technique
(though it is often combined with other methods).
Some materials are much better at absorbing sound than others. As the graph below
shows, mineral wool (e.g. glass fibre) has very good sound absorption qualities,
particularly at higher frequencies. In contrast, concrete and glass are poor absorbers
of sound. That’s why for example swimming pools are usually very noisy, as there are
high levels of reflected sound. In contrast, fibreboard and upholstered seats have better
absorption qualities, which is why sound levels in a cinema (before the movie starts) are
usually very quiet.
The graph also shows that different elements can change the absorption qualities
of some materials. For example with fibreboard, painting the surface of the board
decreases its absorption potential, especially at frequencies above 500Hz. However,
fixing the fibreboard to a structure which creates an air gap behind it can improve its
absorption characteristics.
Figure 9: Absorption characteristics of some materials
0.8
0.7
0.6
Absorption
0.5
0.4
0.3
0.2
0.1
0
125Hz
Concrete
Upholstered seat
Fibreboard 15mm on air gap
500Hz
Frequency
Cork
Fibreboard 15mm on solid
Fibreboard 15mm (painted) on air gap
2,000Hz
Glass 6mm
Fibreboard 15mm (painted on solid
Mineral wool (eg glass fibre
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Absorbent materials can be used to reduce both the direct sound and reflected sound in
a workplace environment.
Absorption example 1 – direct and reflected factory noise
The problem
A machine in a factory area produces high noise levels, especially in frequencies above
1000Hz. The building has concrete walls and floors and large areas of glass in the roof,
so there are high levels of reflected sound. Operators working near the machine are
exposed to high noise levels even though they do not work directly on it. There are two
significant components to the noise as shown below:
•
direct sound component (red arrows)
•
reflected sound component (blue arrows).
Figure 10: Direct and reflected sound in a building
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Key issues
The direct sound component is largely dominant over the reflected sound component,
so the direct sound must be addressed as the first priority.
Figure 11: Absorption of direct sound from the source
Acoustic barrier
The solution
Step 1 is to design an acoustic barrier of appropriate height and materials, to interrupt
and absorb as much of the direct sound component noise as possible.
Step 2 is to address the Reflected Sound component, by installing sound absorbent
panels in the roof space.
Absorbent treatments could also be applied to walls to reduce reflected sound further
Figure 12: Sound emitted from typical mobile source
Acoustic barrier
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Absorption example 2 – portable equipment noise
The problem
Many sites use portable equipment, such as pumps, that can produce high noise levels. The key issue is how to reduce the direct sound produced by the machinery itself (there
is little reflected sound unless the equipment is used in an indoor environment).
Figure 13: An example of direct sound absorption from portable equipment
The solution
The solution is to use screens to absorb direct sound from the equipment. As shown
below, flexible screening material can be fixed to movable framing and used to screen
the direct noise from operators working around the equipment.
Screens should be made from a composite to give both durability and good sound
absorption characteristics across a number of frequencies. In this case, a core of lead
sheeting, with a fiberglass quilt and an outer cover of heavy duty fabric are used.
Screens also need to be high enough to block the direct sound component.
Figure 14: Screen to absorb direct sound
25mm fibreglass quilt
Heavy duty fabric outer
eg. canvas
0.1–1mm lead sheets
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7. Damping
As we’ve seen, sound is created by vibrations. By damping or reducing the vibrations
which create sound, we can reduce noise levels.
There are a number of ways to use damping to control noise in workplace environments.
These techniques are best illustrated by some examples.
Damping example 1 – metal component conveyor system
The problem
Metal components are delivered by conveyor to a metal storage bin. The components
are guided into the bin via a metal chute, as shown below. Noise is produced by the
metal components hitting the metal wall of the chute, as well as the metal base and
walls of the storage bin.
Key issues
•
The walls of the chute and bin vibrate when struck by the metal components. This produces an ‘amplified’ sound (similar to the effect produced by a guitar see example on page 10).
•
The distance the metal components fall also affects the amount of sound
produced. The further they fall, the more energy they develop. This energy is
released as sound when the components hit the surface of the chute and bin.
Figure 15: Metal component conveyor system
Conveyor
Metal chute
Metal bin
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The solution
Two techniques can be used here, as shown in the illustration.
First, the metal chute and bin can be lined with a resilient material to dampen the
vibrations produced when the components strike them. In this example the chute and
bin are lined with 50mm polyurethane foam and finished with an inner skin of heavy
duty rubber, PVC or nylon.
Second, by reducing the drop heights between the conveyor and chute, and between
the chute and bin, we can reduce the noise levels further.
Figure 16: Dampened metal component conveyor system
Reduced drop height
Resilient lining (eg. 50mm polyurethane foam)
Reduced drop height
Durable lining eg. rubber, PVC, nylon
Damping example 2 – piping
The problem
Pneumatic conveying systems can be very noisy. The noise is often heard as a hiss
and sometimes accompanied by a ringing sound. Pipes can also ‘breakout noise’ from
machinery attached to the end of the pipe (e.g. pumps). Air conditioning systems and
local exhaust ventilation systems can create similar noise problems. The problem is
often compounded by rigid fixing of the pipes to walls and other structures, which can
amplify the vibrations
Key issues
•
Noise is produced by the friction of material being conveyed against the metal
pipe.
•
The walls of the pipe vibrate and if pipes are of the right length between fixings a
ringing sound can be produced, (similar to the effect produced by tubular bells).
•
Vibrations can also be transmitted into structures to which the pipe hangars are
fixed.
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Figure 17: Fixed pipe
Often a rigid fixing
The solution
By using flexible and absorbent materials as shown in the illustrations, we can dampen
the vibrations from the pipe or duct and reduce noise levels.
The steps shown here need to be completed as a ‘complete package’ but not necessarily
in the order shown.
Figure 18: Options for baffling pipe vibrations
Mount brackets at irregular levels
Step B
Step A
Resilient material to reduce
transmission of vibrations
inner resilient layer eg. mineral
wool minimum 50mm thick
Outer mass skin eg. metal
Wall
Flange
Mineral wool packing
Pipe/Duct
Flexible ‘Mastic’ seal
Step C
An example of damping on pipework to
reduce vibration.
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8. Enclosure
Machinery noise can often be reduced by the use of acoustic enclosures fitted around
either all or part of a machine. Whatever method is employed the techniques are largely
the same.
The key difference between enclosures and isolation techniques (see section 9) is that
with enclosures, operators tend to be working in the same area as the noise source,
whereas with isolation techniques the operators and the machinery tend to be well
separated.
With enclosures, attention to detail in the design is essential
Enclosure example 1 – ultrasonic welding machine
Ultrasonic welding machines are in common use in many industries. While the
ultrasonic frequencies are outside the audible range for most people, machines using
ultrasonic frequencies often produce harmonics in the 10 – 20KHz range which are
perceived as piercing, sometimes painful noise. This is especially true for young adults
whose hearing is usually very good in this frequency range.
Machines can often be easily enclosed using steel or even glass casings, coupled with
acoustic absorbing ‘blankets’ to line the enclosure.
Figure 19: Ultrasonic welding machine with enclosure
Neoprene seals between sections
Fastening latches
Removable steel cover
Inner screen descends
when unit operates
Neoprene seal
Porous sound absorbing lining
Operating buttons
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This example of an ultrasonic welder has a sliding inner screen which is electronically
linked to the machine operation. When the two operating buttons are pressed together,
the inner screen descends. Shortly after the screen has closed the ultrasonics are
applied to the work piece.
Enclosure example 2 – pump motor
In this example a pump motor emits a significant amount of noise. Enclosure is an
option but care needs to be taken to ensure sufficient ventilation to prevent the motor
overheating. Provision of ventilation slots will reduce the sound reduction achieved by
the enclosure but there are designs that can cut down the ‘leakage’ through ventilation
slots – see the following design principles.
Figure 20: Pump motor with and without enclosure
Motor noise
Potential for vibration isolation
mountings at these points
Resilient strip to dampen
casing vibration
Ventilation slot
Close fitting to frame with
flexible rubber flanges to
minimise air gaps
Outer steel
enclosure with
acoustic lining
Design principles for enclosures
Attention to detail is critical in the design of enclosures to ensure the close fitting of
each element. Even the smallest air gaps will leak sound into the surroundings. In the
case of ultrasonics (as in example 1), air gaps can almost negate any effect the rest of
the enclosure produces, because the ultrasonic wavelength is very short and penetrates
gaps easily.
When designing enclosures, try to include as many of the following design points as
possible (some of these are illustrated in the diagram below):
•
mechanical isolation between the noise source and the enclosure
•
vibration isolation mountings between the machine and the floor
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•
air gaps reduced to an absolute minimum (this includes seemingly small gaps)
•
services entering or leaving the enclosure should be mechanically isolated and
have air gaps sealed
•
enclosure walls must be fabricated from appropriate materials for the
frequencies you are trying to address
•
wherever possible controls should be relocated so they are on the outside of the
enclosure. This may mean modifying some aspects of the machine
•
access hatches must seal well when closed
•
where air vents need to be provided to prevent overheating acoustic baffles and
silencers may need to be fitted
•
where mechanical ventilation is required for cooling purposes the ventilation
system will need to be fitted with a silencer system.
The design team should include the operators who use the machines. They know what
access is required during routine operation.
Remember, even the smallest air gap reduces the effectiveness of the enclosure
considerably – just think about how much noise comes into a room through the smallest
gap created by an open window. Careful measuring up and close fitting of enclosure
structures will maximise the sound reduction achieved.
Figure 21: Cut away of pump motor with enclosure
Resilient flanges to seal
entry point of equipment
Air vent (silencer design)
Steel outer casing
Air vent for cooling air
Acoustic absorbent lining
Resilient vibration mounting
Sealed air gap between enclosure and floor
using neoprene or similar resilient material
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9. Isolation
In some processes the size of the machinery and the number of points where noise is
generated makes the use of screens, barriers or enclosures impractical.
This is common in operations such as:
•
sawmills
•
dairy factories
•
pulp and paper mills
•
power generation stations
•
engine testing facilities.
In these situations isolating the noise source from the operator, or the operator from
the source, can be a viable option. The following are two examples of the techniques.
Isolation example 1 – process pump
The problem
A process pump generating an excessively high level of noise was enclosed in an
acoustically treated plant room. This enabled operators to walk along the corridor to
other process areas without being exposed to noise from the pump.
However, several times a day the operator was required to enter the pump room to
carry out visual checks on the plant. Although the operator used ear muffs to protect
themself from the noise, even the short period of exposure during the checks (as shown
by red circle on the graph) was a significant component of the operator’s overall noise
exposure.
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Figure 22: Pump room that requires entry for montitoring
Accoustically treated enclosure
Window
z
The solution
By installing a CCTV camera focused on the area of plant requiring the visual check
the need for the operator to enter the plant room can be almost eliminated. This is a
significant reduction of risk of hearing damage and easily justifies the resources involve
in installing a CCTV system.
Figure 23: Pump room with CCTV installed
Accoustically treated enclosure
CCTV camera for
remote monitoring
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Isolation example 2 – noise refuge
This example illustrates what is commonly known as a ‘noise refuge’. It is frequently
used when operators need to have ready access to process equipment but much of the
process is fully or semi automatic.
Figure 24: Typical noise refuge
Acoustic treated roof
Acoustic door
Floating floor
Acoustic
treated walls
Double glazing
The main features are:
•
walls and roof that are acoustically treated
•
a ‘floating floor’ to reduce structure-borne vibrations being transmitted into the
refuge
•
a double door entry system to reduce noise infiltration when people enter and
leave the refuge. This system also provides an area where hearing protection
can be stored and put on prior to entering the noisy area
•
all doors designed with double glazing and airtight seals
•
workstations for the process controls etc
•
air conditioning would usually be provided to the refuge and this would also
have to be acoustically treated to prevent sound being carried through the
system into the refuge.
Typically refuges, if well designed and fabricated, can reduce sound levels by around
30dB.
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10. Design and equipment
Many noise control issues can be overcome with good design. While design changes
may not always be practical unless you are building new structures or purchasing or
designing new equipment, here are some things to consider in terms of design:
•
The building e.g. minimisation of reflective materials, inclusion of sound
absorbing materials, use of vibration isolation for floors and walls
•
Layout of machinery e.g. collection of noisy machinery in one area, increasing
distance between machinery, avoiding placing machinery near reflective
surfaces
•
Process flow/design e.g. locating ‘quiet’ tasks away from noise sources,
reducing the frequency with which operators have to approach noisy machinery,
gluing instead of nailing, belt conveyors rather than roller conveyors
•
Equipment e.g. ‘buy quiet’ purchasing machinery that has been validated as
producing lower noise levels, use of nylon gears as opposed to metal gears,
low pressure air nozzles, modified circular saw blades, modified cutting blades,
‘acoustic’ cabs on vehicles especially heavy plant.
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11. Other sources of information
This booklet has outlined the essential elements of noise reduction other than by the
use of hearing protection devices such as ear plugs and ear muffs.
There are other sources of information, some more detailed and technical than others.
The following covers the main sources easily accessible and downloadable via the
internet.
1. New Zealand Department of Labour – Management of Noise at Work –
Control Guide
(http://osh.dol.govt.nz/order/catalogue/738.shtml)
Step by step guidance to assist organisations to effectively manage workplace noise and
prevent noise-induced hearing loss. The chapters are conveniently listed as separate
downloadable files.
2. National Occupational Health & Safety Commission Australia – Control Guide:
Management of Noise at Work
(http://www.safeworkaustralia.gov.au/NR/rdonlyres/DFD85E29-E3A3-4E49-95C9DD3F29E2DD70/0/noise_control.pdf)
A general guide covering many of the aspects of noise control. Contains some examples
of engineering control methods.
3. Health and Safety Executive UK – Index of case studies
(http://www.hse.gov.uk/noise/casestudies/csindex.htm)
An index page listing more recent ‘real world’ noise control case studies. Each case
study is on a separate sheet. Non-technical with good illustrative examples of a variety
of techniques.
4. Health and Safety Executive UK – Sound Solutions
(http://www.hse.gov.uk/noise/casestudies/soundsolutions/index.htm)
60 noise control case studies first published in 1995 in 'Sound Solutions' HSG138. Each
case study is on a separate sheet. Non-technical but good illustrative ‘real world’
examples of a variety of techniques.
5. World Health Organisation – Occupational exposure to noise: evaluation,
prevention and control
(http://www.who.int/occupational_health/publications/occupnoise/en/index.html)
This book provides an overview of the evaluation, prevention and control of exposure
to noise at the workplace, with a view to preventing noise-induced hearing loss. The
chapters are conveniently listed as separate downloadable files. Very technical in
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places and aimed at occupational hygienists and other occupational health and safety
personnel as an introduction to the subject.
6. National Institute of Occupational Safety and Health US – Industrial Noise
Control Manual
(www.cdc.gov/niosh/79-117pd.html)
A 1978 publication containing extensive case studies. Very technical in places and aimed
at 0ccupational hygienists and other occupational health and safety personnel.
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Notes
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Notes
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www.acc.co.nz
0800 844 657
ACC5376 Printed June 2010 ISBN: 978-0-478-31491-5
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