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NOISE
Sound and Noise
Sound is what we hear.
Noise is unwanted sound .
The difference between sound and
noise depends upon the listener and
the circumstances.
Ex. . Rock music can be pleasurable
sound to one person and an annoying
noise to another
In either case, it can be hazardous
to a person's hearing if the sound
is loud and if he or she is exposed
long and often enough.
• Noise does not necessarily have
any
particular
physical
characteristics that distinguish it
from
wanted
sound.
No
instrument
can
distinguish
between a sound and a noise –
only human reaction can.
PRODUCE: Sound is produced by
vibrating objects and reaches
listener's ears as waves in
air,water or other media.
the
the
HOW
•When an object vibrates
•It causes slight changes in air pressure
• These air pressure changes travel as
waves through the air and produce
sound
Ex.
Imagine striking a drum surface with a
stick
•
•The drum surface vibrates back and forth
•As it moves forward, it pushes the air in contact
with the surface
•This creates a positive (higher) pressure by
compressing the air
•When the surface moves in the opposite
direction, it creates a negative (lower) pressure
by decompressing the air
•Thus, as the drum surface vibrates, it creates
alternating regions of higher and lower air
pressure
•These pressure variations travel through the air
as sound waves
Other Examples such as Consider plate
suspended in midair.When struck, the
vibrates rapidly back and forth just like
drum .
CHARACTERISTICS OF SOUND WAVES:
As a sound energy is transmitted through a
medium in waves, it exhibits certain
properties.
1) Firstly, the longitudinal waves travel at a
velocity or speed. The speed of sound differs
depending upon medium temperature and
pressure in which it is traveling. At 00 C and
one atmosphere of air pressure, the speed
of sound is accepted as 331.3 m.s-1 As the
temperature (T) increases, the speed of
sound also increases at approximately 0.60
m.s-1 for each 10 C.
Example1). What would be the
speed of sound(c) at 200 C and 1
atmosphere?
C = (331 + 0.6T) m.s-1
= (331 + 0.6(20) )
= (331 + 12)
= 343m.s-1
2) What would be the speed of sound
© at 480 C and 1 atmosphere in
paint industry at Ankleshwar?
Pressure: Small fluctuations in air
pressure that are caused through the
vibration of air particles are known as
acoustic pressure.
• For simple sounds, the acoustic pressure
is described as a sinusoidal (sine) curve
that considers:
1. The angular nature of the wave
2. The period or time duration of the
wave(t)
3. Amplitude or maximum value of the
curve(A).
Figure of Pressure verses Time (sec)
Equation : p = A.sin 2πft
• Since the pressure of the wave changes
direction (compression and rarefaction), the
maximum amplitude (A) of the curve is both
positive and negative, when compared
against its overall amplitude would be zero
since the positive and negative amplitudes
of the wave would cancel each other out.
•Therefore, to determine the equivalent
amplitude of the pressure wave(or the
effective pressure {Peff }),the root mean
This is expressed in following equation:
Peff= Prms ~ 0.707Pamplitude
The term rms is determined by squaring
the mean values of points along the
pressure wave, and then taking the square
root of this value.
• The time taken to complete the cycle is
known as the period(t) and is measured in
seconds.
Period
and
frequency
are
inversely related shown in following
equation. t= 1/f Where: t is period (sec),f
is frequency (Hz)
• The other characteristic of a sound wave
is its wavelength(λ ), measured in meters
(m). A wavelength is the distance between
two successive crests, or two identical
points on the wave.
SOUND WAVES:Sound waves are a
particular form of a general class of waves
known as elastic waves.
• Sound waves can only occur in media
that have the properties of mass (inertia)
and elasticity.
• Because air possesses both inertia and
elasticity,a sound wave can be propagated
(spread) in air.
• One sound wave may have three times
the frequency and one third the amplitude
of another sound wave.
Show the curves of frequency
verses intensity
• Curve of normal conversation
• Curve of higher pitch than a sound
• Curve of louder (high intensity) than
the normal intensity .
Hearing
mechanism
:The
hearing
mechanism of the ear senses the sound
waves and converts them into information
which it relays to the brain. The brain
interprets the information as sound. Even
very loud sounds produce pressure
fluctuations which are extremely small (1
in 10,000) compared to ambient air
pressure (i.e., atmospheric pressure). The
hearing mechanism in the ear is sensitive
enough to detect even small pressure
waves. It is also very delicate: this is why
loud sound may damage hearing.
SOME PROPERTIES OF NOISE
THAT CAN BE MEASURED :
The properties of noise which are
important in the workplace are:
•frequency
•sound pressure
•sound power
•time distribution
FREQUENCY:
•The number of pressure variations per
second is called the frequency of the
sound,which was formerly measured in
cycles per second but is expressed in
hertz(Hz),which
represents
cycle
per
second.
• The time required for each cycle is known
as the period of the wave and is simply the
reciprocal of the frequency.
•Frequency is perceived as pitch.
•Most everyday sounds contain a mixture
of frequencies generated by a variety of
sources.A sound’s frequency composition is
referred to as its spectrum.The frequency
spectrum can be a determining factor in the
level of annoyance caused by noise;high
frequency noise generally is more annoying
than low frequency noise.
• FrequenLow pitched or bass sounds have
low frequencies. High-pitched or treble
sounds have high frequencies. A healthy,
young person can hear sounds with
frequencies from roughly 20 to 20,000 Hz.
The sound of human speech is mainly in the
range 300 to 3,000 Hz.
• Also,narrow
frequency bands or pure
tones( single frequencies) can be
somewhat more harmful to hearing
than broadband noise.
WAVELENGTH:Wavelength
is
the
distance measured between two analogous
points on two successive parts of a wave.
In other words, wavelength is the distance
that a sound wave travels in one cycle.
• The Greek letter lambda( ) is used to
express wavelength,and it is measured in
feet or meters.
Wavelength is an important property of
sound. For example, sound waves that have
a wavelength that is much larger than
the size of an obstacle are little affected
by the presence of that obstacle; the sound
waves will bend around it. This bending of
the sound around obstacles is called
diffraction.
• If the wavelength of the sound is small in
comparison with the size of the obstacle
(such a wavelength is generated by high
frequency sounds), the sound will be
reflected or scattered in many directions,
and the obstacle will cast a shadow.
Actually, some sound is diffracted into the
shadow, and there is significant reflection
of the sound. As a consequence of
diffraction, a wall is of little use as a shield
against low-frequency sound ( long
wavelength), but it can be an effective
barrier against high frequency sound (
short wavelength) .
Velocity: The velocity with which the
analogous pressure points on successive
parts of the wave pass a given point is
called the speed of sound. The speed of
sound is always equal to the product of the
wavelength and the frequency
C=fƛ
In the formula,c= speed of sound(feet or
meters per second);
F = Frequency of sound
ƛ = wavelength (feet or meters)
Sound pressure :Sound pressure is
the
amount
of
air
pressure
fluctuation(pressure changes above and
below atmospheric pressure) a noise
source creates.
• Most common sounds consist of a
rapid,irregular series of positive pressure
disturbances(compression) and negative
pressure
disturbances
(rarefactions)
measured against the equilibrium pressure
value. If we were to measure the mean
value of a sound pressure disturbance, we
would find it would be zero,because there
are as many positive compressions as
negative rarefactions. Thus, the mean value
of sound
pressure is not a useful
measurement.
We
must
look
for
a
measurement that permits the effects of
rarefactions to be added to (rather than
subtracted from) the effects of compressions.
•If the drum in above example is hit very
lightly, the surface moves only a very short
distance
and
produces weak pressure
fluctuations and a faint sound. If the drum is
hit harder, its surface moves farther from its
rest position. As a result, the pressure increase
is greater. To the listener, the sound is louder.
•Sound pressure also depends on the
environment in which the source is
located and the listener's distance from
the source.
•The sound produced by the drum is louder
two meters from the drum if it is in a small
bathroom, than if it is struck in the middle
of a football field.
•Sound pressure is usually expressed in
units called pascals (Pa). A healthy,
young person can hear sound pressures as
low as 0.00002 Pa. A normal conversation
produces a sound pressure of 0.02 Pa.
•A gasoline-powered lawn mower produces
about 1 Pa. The sound is painfully loud at
levels around 20 Pa. Thus the common
sounds we hear have sound pressure over a
wide range (0.00002 Pa - 20 Pa).
•It is difficult to work with such a broad range
of sound pressures. To overcome this difficulty
we use decibel (dB, or tenth (deci) of a Bel)).
The decibel or dB scale is more convenient
because it compresses the scale of numbers
into a manageable range. The decibel is named
after Alexander Graham Bell, the Canadian
pioneer of the telephone who took great
personal interest in the problems of deaf
Sound pressure level :
Sound pressure converted to the decibel scale
is called sound pressure level (Lp). Following
figure compares sound pressures in pascals
and sound pressure levels in decibels (dB). .
Figure compares sound pressures in pascals
and sound pressure levels in decibels (dB).
The zero of the decibel scale (0 dB) is the
sound pressure of 0.00002 Pa. This means
that 0.00002 Pa is the reference sound
pressure to which all other sound pressures
are compared on the dB scale. This is the
reason the decibels of sound are often
indicated as dB re 0.00002 Pa.
Decibels and Sound Pressure Level:
•Even though the weakest sound pressure
perceived as sound is a small quantity,the
range of sound pressure perceived as
sound is extremely large.
•The weakest sound that can be heard by a
person with very good hearing in an
extremely quiet location is known as the
threshold of hearing.
• At a reference tone of 1000 Hz, the
threshold of hearing is equal to a sound
pressure of 20 microPa.(0.0002 microbar).
• The
threshold of pain, or the greatest
sound pressure that can be perceived
without pain, is approximately 10 million
times greater. It is therefore move
convenient to use a relative scale of sound
pressure rather than an absolute scale.
•Equation to calculate sound pressure
level
Lp = 20 log p/po Where, Lp = the sound
pressure level,p= rms sound pressure, po=
a reference sound pressure and log=
logarithms to the base 10.
•So, the sound pressure level of the
quietest noise the healthy young person
can hear is calculated in this way:
Lp = 20 log (0.00002/ 0.00002) = 20 log
(1) = 20 X 0 = 0 dB
• The
sound pressure level or Lp in a very
quiet room, where the sound pressure is
0.002 Pa, is calculated:
•Lp = 20 log (0.002/ 0.00002) = 20 log
(100) = 20 X 2 = 40 dB
•The sound pressure level of a typical
gasoline-powered lawn mower, which
has a sound pressure of 1 Pa, is
calculated
•Lp = 20 log (1/0.00002) = 20 log (50
000) = 20 X 4.7 = 94 dB
sound power:
•The sound power is the sound energy
transferred per second from the noise
source to the air.
•A noise source, such as a compressor or
drum, has a given, constant sound power
that does not change if the source is placed
in a different environment.
• Power is expressed in units called watts
(W). An average whisper generates a sound
power of 0.0000001 watts (0.1 W), a truck
horn 0.1 W, and a turbo jet engine 100,000
Like sound pressure, sound power (in W) is
usually expressed as sound power levels in
dB. Following figure provides examples of
sound power level calculations.
Following figure relates sound power in
watts to sound power level in decibels.
Note that while the sound power goes from
one trillionth of a watt to one hundred
thousand watts, the equivalent sound
power levels range from 0 to 170 dB.
The manufacturer can often provide the
sound power of equipment. A number
of international standards are available
for labeling machines and equipment
with their noise emission levels. From
the sound power of a compressor, one
can calculate the expected sound
pressure and sound pressure level at a
certain location and distance. This
information
can
be
helpful
in
determining possible noise exposures
and how they compare to the noise
Sound Power Level Calculations
Sound power levels or Lw are determined by the
following formula:
Lw = 10 log (Sound Power Level / Reference
Power Level )
The reference power is one trillionth of a watt
(0.000000000001 W). Therefore
Lw = 10 log (Sound Power Level /
0.000000000001)
Thus, the sound power level associated with an
average whisper, which has a sound power of
0.0000001 W, is calculated
KINDS OF NOISE :Noise can be
• continuous
• variable
• intermittent or impulsive
Continuous noise is noise which
remains constant and stable over a
given time period. The noise of boilers in
a power house is relatively constant and
can
therefore
be
classified
as
continuous.
• Most manufacturing noise is variable
or intermittent. Different operations or
different noise sources cause the
sound changes over time. Noise is
intermittent if there is a mix of
relatively quiet periods and noisy.
• Impulse or impact noise is a very
short burst of loud noise which
lasts for less than one second. Gun
fire or the noise produced by punch
presses are examples of such noise.
A-weighted decibels :
The sensitivity of the human ear to
sound depends on the frequency or
pitch of the sound. People hear some
frequencies better than others. If a
person hears two sounds of the same
sound
pressure
but
different
frequencies, one sound may appear
louder than the other. This occurs
because people hear high frequency
noise
much
better
than
low
frequency noise.
Noise measurement readings can be adjusted
to correspond to this peculiarity of human
hearing. An A-weighting filter which is built into
the instrument de-emphasizes low frequencies
or pitches. Decibels measured using this filter
are A-weighted and are called dB(A).
Legislation on workplace noise normally gives
exposure limits in dB(A).
A-weighting serves two important purposes:
1. gives a single number measure of noise level
by integrating sound levels at all frequencies
2. gives a scale for noise level
experienced or perceived by the human ear
as
Some of the Examples:
Typical Noise Levels
Noise Source
dB(A)
pneumatic chipper at 1 metre
115
hand-held circular saw at 1 metre
115
textile room
103
newspaper press
95
power lawn mower at 1 metre
92
Diesel truck 50 km per hour at 20 metres
85
passenger car 60 km per hour at 20 metres
65
conversation at 1 metre
55
quiet room
40
The three variables that affect the risk
of exposure to noise are:
• Frequency composition:
1. A healthy, young ear is able to detect
sound waves between 20 Hz and 20 kHz.
2. Exposure
at some of the higher
frequencies can produce permanent
hearing loss.
3. Occupational noise consists of many
frequencies.This is known as broadband
sound.
4)Where sound has only one frequency, it
is referred to as a pure tone. These are
found less frequently in industrial
settings.
• Amplitude:
1. The amplitude of sound can be measured
as an intensity level (IL); although
more commonly as the sound pressure
level (SPL).
2. For an ideal point source in a free field
(where the source is in open air or where
reflection is limited) the intensity of sound
I = W/4π r2
Where: ‘I’ is intensity of radiated sound
‘W’ is power (Watts)
‘r’ is the distance from the source (m)
3. Example of this type of noise may be a
small loud speaker operating at low
frequency.
• Continuousness:
The continuousness of sound relates to
whether
the
sound
is
produced
intermittently or constantly.
BASIC RULES OF WORKING WITH
DECIBEL (DB) UNITS
• The decibel [dB, and also dB(A)] is a
logarithmic scale.
• For mathematical calculations using dB
units,
we
must
use
logarithmic
mathematics . However, in our day-to-day
work we do not need such calculations.
•The use of dB unit makes it easy to deal
with the workplace noise level data
provided we use a set of simple rules as
summarized in following Table.
Table
Decibel (dB) basics
Change in dB
Change in sound energy
3 dB increase
Sound energy doubled
3 dB decrease
Sound energy halved
10 dB increase
Sound energy increased
by factor of 10
10 dB decrease
Sound energy decreased
by factor of 10
20 dB increase
20 dB decrease
Sound energy increased by
factor of 100
Sound energy decreased
by factor of 100
SOUND PRESSURE WEIGHING
• Simple to build an electronic circuit
whose sensitivity varied with frequency in
the same way as the human ear.
• Resulted in three different internationally
standardized
characteristics
termed
weighting networks “A”,”B”,”C”.
• The A –network was designed to
approximate the equal loudness curves at
low sound pressure levels.
•The “B” –network was designed
medium sound pressure levels
for
•The “C”-network was designed for high
levels.
•The
very
low
frequencies
are
discriminated(Distinguish),attenuated
or
filtered out quite severely the Anetwork,moderately filtered out by the Bnetwork and hardly attenuated at all by the
C-network.
• The
A-weighted
sound
level
measurement has become popular in the
assessment of the overall noise hazard
since this level is thought to provide a
rating industrial broadband noises that
indicates the injurious effects such noise
has on the human ear.
• As a result of its simplicity in rating the
hazard to hearing, the A-weighted sound
level
has
been
adopted
as
the
measurement of assessing noise exposure
by the ACGIH.
• The A-weighted sound levels have also
been shown to provide reasonably good
assessments of speech interference and
community disturbance conditions and
have been adopted for these purposes.
NOISE LEVEL ADDITION METHOD:
•Sound pressure levels in decibels (dB) or
A-weighted decibels [dB(A)] are based on
a logarithmic scale .
• They cannot be added or subtracted in
the usual arithmetical way. If one machine
emits a sound level of 90 dB, and a second
identical machine is placed beside the first,
the combined sound level is 93 dB, not 180
dB.
•Following table shows a simple way to add
noise levels.
Addition of Decibels
Numerical difference
Amount to be added
between
to the higher of the
two noise levels [dB(A)]
two noise levels [dB
or dB(A)]
0
3.0
0.1 - 0.9
2.5
1.0 - 2.4
2.0
2.4 - 4.0
1.5
4.1 - 6.0
1.0
6.1 – 10
0.5
10
0.0
Step 1: Determine the difference
between the two levels and find the
corresponding row in the left hand
column.
Step 2: Find the number [dB or
dB(A)]
corresponding
to
this
difference in the right hand column of
the
table.
Step 3: Add this number to the higher
of the two decibel levels.
For instance, using the example of two
machines each emitting a noise level
of 90 dB:
•Step 1: The numerical difference between
the two levels is 0 dB (90-90= 0), using
the first row.
•Step 2: The number corresponding to this
difference of 0, taken from the right hand
column, is 3.
•Step 3: Add 3 to the highest level, in this
case 90. Therefore, the resulting noise
level is 93 dB.
When the difference between two noise
levels is 10 dB(A) or more, the amount to
be added to the higher noise level is zero.
In such cases, no adjustment factor is
needed because adding in the contribution
of the lower in the total noise level makes
no perceptible difference in what people
can hear or measure. For example if your
workplace noise level is 95 dB(A) and you
add another machine that produces 80
dB(A) noise, the workplace noise level will
still be 95dB(A).