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NOISE
What is Sound?
• Hertz (Hz)
• Decibels (dB)
– Frequency a high or
low pitch
– The loudness of the
sound
2
NOISE
Definitions
• Low Frequency Sounds
– Easily travel around corners and through openings
• High Frequency Sounds
– Does not turn corners well, and it can be reflected
– Are more attenuated by distance traveled in air than low frequencies
– Are more annoying
• Pure Tone, a one-frequency sound
• Broadband noise, most industrial noise is a mixture of frequencies
known as broadband noise
• White Noise, when frequencies are equally distributed throughout the
audible range, sounds like rain
• Impulse Sound, when duration of sound is < 1 s
‫الضوضاء نتيجة االنشطة الصناعية‬
‫العالقة بين الضغط و الزمن فى نقطة معينة من مستوى الصوت‬
‫مستوى صوت متوسط‬
‫مستوى صوت متوسط‬
‫مستوى صوت عالى‬
‫صوت لفترة كبيرة‬
‫صوت لفترة صغيرة‬
‫مستوى صوت عالى‬
‫اشكال مختلفة من الموجات الصوتية‬
Frequency
• Humans can typically hear
between 20 - 20,000 Hz
• You can hear different frequencies
better than others
6
Decibels
• The quietest sound most
humans can detect is 0 dB
• Some humans can even hear
sounds as quiet as -5 dB
7
Decibels
• Decibels measure level of sound pressure.
• Sound pressure and sound power are
analogous to temperature and heat.
• Mean minimum level of hearing for the
unimpaired-hearing population is 4 dB.
Examples of Noise Levels
(Table 23.1)
DECIBEL LEVEL,
EXAMPLE
dBA
30
Quiet library, soft whisper
40
Living room, refrigerator, bedroom away from traffic
50
Light traffic, normal conversation, quiet office
60
Air conditioner at 20 ft, sewing machine
70
Vacuum cleaner, hair dryer, noisy restaurant
80
Average city traffic, garbage disposal, alarm clock at 2 ft
90
Subway, motorcycle, truck traffic, lawn mower
100
Garbage truck, chain saw, pneumatic drill
120
Rock Concert in front of speakers, thunderclap
140
Gunshot blast, jet plane
180
Rocket launching pad
Noise of leisure activities
(Table 23.2)
MEAN, dBA
ACTIVITY
90
Woodcutting, rough terrain driving
92
Motorcycling
94
Farming
95
Powerboating
96
Powered lawn equipment
98
Woodworking
99
Discotheques
101
Stock car races
110
Concerts (rock), hunting/target shooting
121
Drag races
The Loudness of Common Sounds
0 dB
Threshold of Hearing
30 dB
Soft Whisper
40 dB
Quiet Office
60 dB
Conversational Speech
80 dB
Very noisy restaurant
90 dB
Subway
110 dB
Woodworking
120 dB
Hydraulic press
140 dB
Threshold of Pain – Jet plane
180 dB
Rocket
11
Sound level, dBA
80
85
90
95
100
105
110
115
120
125
130
Permissible time, T(hours)
32
16
8
4
2
1
0.5
0.25
0.125
0.063
0.031
‫مخطط لالذان البشرية‬
Anatomy of the Ear
Ear Drum
Semi-Circular Canals
Cochlea
Ear Bones
How do we Hear?
• The outer ear collects the sound
waves
• The waves hit the eardrum, and
cause it to vibrate
• The vibrations are sent through
the ear bones to the cochlea
16
Inside the Cochlea (snail shell)
• Delicate hair cells vibrate to different frequencies
• Hair cells detect the vibration, and send a signal
to the brain
• Loud sounds destroy the hair cells, and they stop
functioning FOREVER!
17
The Ear does something else too!
• The Semi-circular canals
– Three tubes laying perpendicular to one
another
– Filled with fluid and tiny hair cells
– When your head is tilted, the fluid moves the
hair cells, and they send a signal to your brain
• Responsible for balance
18
Hearing Loss
Overview
• Conductive Hearing Loss
– Occurs in the outer and middle ear
– From wax, punctured eardrum, corrosion of the bones,
etc.
– It can be often cured with medical or surgical treatment.
• Nerve Loss
– Occurs in the inner ear
– Caused by age, viruses, drugs, and noise.
– Rarely curable- ‫ونادرا ما يمكن الشفاء منها‬-.
Hearing Measurement
• Audiograms
–
–
–
–
Most common
Tests the hearing threshold for different frequencies
Gives the total loss in the outer, middle, and inner ear
Refined tests can give the specific loss of each part of the
ear
• Audiograms should be performed annually.
Hearing Loss
• TTS – Temporary Threshold Shift
– Measured 2 minutes after exposure
– Repeated exposure can produced PTS or NIPTS
• PTS – Permanent Threshold Shift
• NIPTS – Noise-Induced PTS
Occupational Hearing Loss
• Not all hearing loss is occupational
– A series of audiograms over the years will help establish
the causes.
• First occurs in the inner hair cells (3000 – 6000 kHz)
• Workers under occupational noises over 85 dB, set
their car stereos ~9 dB higher after work than
before work. (Evidence of TTS)
SLM 2260 Brüel &Kjaer instrument
‫مدى السمع لالنسان عند مستويات ضغط الصوت المختلفة‬
How to measure noise
• Decibels are measured on a logarithmic
scale
• Every time you add 5 dB, you double the
sound.
80 dB
85 dB
An addition or reduction of 5dB causes either an
addition or reduction of 100% in noise according to
OSHA.
25
Example
• In the field, we determined the loudness of
two compressors right next to each other
• How loud is this area?
– Do we add?
– Do we add and take the average?
89 dB
?
26
87 dB
• Neither, because it is a log scale
• We use the following chart
Difference in
dB values
0 or 1 dB
2 or 3 dB
4 or 10 dB
10 or more dB
Add to Higher
Value
3 dB
2 dB
1 dB
0 dB
• 82 dB + 83 dB = 86 dB
• 87 dB + 89 dB = 91 dB
27
‫دراسة حالة‬
Machine C
82db
Scale for combined decibel
Difference between 2 decibel level to be
added (db)
Amount to be added to larger level to obtain
decibel sum (db)
0
1
3
2.6
2
3
4
5
6
7
8
9
2.1
1.8
1.4
1.2
1.0
0.8
0.6
0.5
‫‪Scale for combined decibel‬‬
‫‪Amount to be added to larger level to obtain‬‬
‫)‪decibel sum (db‬‬
‫‪0.4‬‬
‫‪0.3‬‬
‫‪0.2‬‬
‫‪Difference between 2 decibel level to be‬‬
‫)‪added (db‬‬
‫‪10‬‬
‫‪11‬‬
‫‪12‬‬
‫اسلوب حساب سعة الموجة ( مستوى الضوضاء)‬
‫)= صفر‪A , B‬يحسب الفرق بين المعدة (‬
‫ومن الجدول يضاف ‪ 3‬ديسبل‬
‫مجموع سعة الموجة للمعدتين = ‪ 89=3+86‬ديسبل‬
‫= ‪ 7=82-89‬ديسبل‪ )C‬و المعدة ‪ A,B‬يحسب الفرق بين المعدتين(‬
‫ديسبل‪0.8‬و من الجدول بضاف‬
‫ديسبل‪8 +89 =)89.8‬و=‪A,B,C‬مجموع سعة الموجة للمعدات (‬
‫= ‪ 12‬ديسبل‪-12‬‬
‫‪ )D= ) 89.8‬و المعدة (‪A,B,C‬الفرق بين المعدات(‬
‫ديسبل‪0.2‬من الجدول يضاف‬
‫= ‪90‬ديسبل‪A,B,C,D=)89.8+0.2‬مجموع سعة الضوضاء للمعدات (‬
‫محصلة مستوى الضوضاء التى تؤثر فى العامل الذى يعمل فى مكان‬
‫محاط به هذه المعدات يكون ‪90‬ديسبل ‪.‬‬
‫دراسة حالة‬
How does the Safety Person
determine noise levels
• Sound level meter
• Personal Dosimeters
– Determine the
loudness (dB) of
noise at any given
moment
– Worn by employees
– Measures the average
loudness in an 8 hour
work shift
“8hr. TWA” (Time
Weighted Average)
– Can also measure noise
dose
36
What does BLS say?
• At 85 dB (8hr. TWA) (50%
Dose)
– Train employees
– Make hearing protection
available
– Sample for noise levels
– Do hearing tests
– Notify employees of
results
37
• At 90 dB or more
(100% Dose)
– We must keep levels at
or below 90 dB
– Or require hearing
protection that will
lower noise levels to to
90 dB
Hearing Tests
• Determine a baseline audiogram
• Hearing test every year to determine if you have
experienced a hearing loss (Standard Threshold
Shift)
• Standard Threshold Shift - A loss of 10 dB or
more at 2000, 3000, or 4000 Hz.
38
Hearing Loss
•
•
•
•
Impact - One loud bang
Cumulative - Years of a noisy environment
Tinnitus - Ringing in the ears
Presbycusis - Hearing loss due to aging
39
Hearing
• Hearing loss increases when exposed to over
67 dBA.
Hearing
– US occupational standard is 90 dB for 8 hours of
exposure per day.
Hearing Protection
• NRR - Noise reduction rating
– Ear Plug - 25 NRR
– Ear Plug - 29 NRR
– Ear Plug - 30 NRR
• DO NOT Subtract the NRR from the noise level
– WRONG (109 dB - 25 NRR = 84 dB)
• You must use the “Safety Factor.” explained on
separate slide
42
Safety Factor
• OSHA says the hearing protection is designed to reduce
the noise by the NRR, but that is unlikely to happen due
to :
– Leaks in the seal
– Vibration
– Improper insertion
43
‫وقاء اإلذن‬
‫سدادات اإلذن‬
‫المقارنة بين واقيات اإلذن من حيث الوهن‬
‫الحقيقي لإلذن عند الترددات المختلفة‬
Example of NRR Protection
• The noise from an average chainsaw is 110 dB
• You are wearing the foam plugs with an NRR of 25
• Do you have enough protection to place you below 90
dB level?
46
•
(NRR 25 - 7) = 18
• 18 / 2 = 9
•
110 dB- 9 =
101
• YOU ARE ABOVE OSHA LIMITS OF 90 dB
47
Final Thoughts
• Hearing is important
• In time, noise levels at 85 dB can
permanently damage your hearing
• Wear your hearing protection both at work
and at home
• Choose hearing protection with a proper
NRR, and wear it properly
48
Effects of Noise
(in work stations)
Comfort and Annoyance
• Noise increases fatigue.
• Noise requires more concentration.
• Annoyance has increased over the years.
• Noise reduction is required regardless of high cost or
low benefits
Comfort and Annoyance
• Community reaction to industrial noise is highly
variable.
• Variability of noise increases annoyance.
Performance
• No firm evidence of lower
productivity at high noise
level (100dBA) unless max.
mental capacity used.
• Assumption: speech
communication is not an
important part of the job.
Performance
Speech Interferance Levels (SIL)
Note: Levels above 60 dB, masking spreads over a wider range
Noise Reduction
Noise Reduction
• Reduce signal/noise ratio by increasing noise (masking
signal)
• In offices, coworkers’ conversations are the main source of noise.
• Reducing noise on hearing:
A) Plan ahead
B) Modify existing
noise source
C) Modify sound wave
D) Use personal protection
A. Plan Ahead
I.
Substitute Less Noisy Processes
i.
ii.
iii.
II.
III.
Reduce the use of impact tools
Replace internal combustion with electric engines
Replace gear transmission with belt transmission
Purchase Less Noisy Equipment
Use Quieter Materials and Construction
i.
ii.
iii.
iv.
Reduce Impact
Reduce Vibration
Reduce Turbulence
Reduce Transmission
A. Plan Ahead
Poor: Spur gears
Good: Bevel gears
Better: V-belt Transmission
Drumheads with holes:
Perforated guards have less
vibration
A. Plan Ahead
Pipe isolation reduces
noise produced by
vibration
A. Plan Ahead
1. Increase distance
for turbulence to die
2. Avoid abrupt
directional changes
3. Avoid abrupt
volume changes
Poor
Better
A. Plan Ahead
•
A. Plan Ahead
IV. Separate People and Noisy Equipment
i.
ii.
Locate noisy equipment in an isolated room.
Penetrations (ducts and pipes) should be sealed airtight.
iii.
Inverse square law: Each doubling in distance from
source reduces 6 dB in noise.
iv.
High frequency sounds have high magnification ratio (Q)
therefore avoid locations at higher Q.
–
–
–
corner Q= 8
Junction of walls Q= 4
Wall Q= 2
B. Modify the Noise Source
•
Start with the loudest noise
I.
II.
III.
Reduce the Driving Force
Change the direction
of noise
Minimize Velocity and Turbulence of Air
B. Modify the Noise Source
YES
NO
Exert force over a
longer time
Reduce sharp edges
C. Modify the Sound Wave
•
•
Confining and absorbing of sound waves
are expensive and reduce dB very little
Use if no ahead planning nor source
modification were possible.
I.
Confine
i.
Problems:
• Long wavelength
• Requires total or near to total enclosure
C. Modify the Sound Wave
I. Confine (cnt.)
ii.
Goal: Confine and absorb before transmission of sound
occurs
–
Sound absorbing materials also transmit, yielding little
attenuation
–
Hard materials transmit little but also absorb little
–
Transmission is less for over 1000 Hz
–
Absorb inside, prevent transmission on the outside
www.owenscorning.com/around/sound/whatis.asp
C. Modify the Sound Wave
Double-wall construction gives good transmission
losses (TL)
C. Modify the Sound Wave
II. Absorb
– Once the sound is out, using absorbent panels is
not cost effective
– Remember logarithmic properties
– In most rooms 5 dB is the maximum reduction
possible
D. Use Personal Protection
I.
dBA
Time
(hrs.)
90
8
95
4
Reduce exposure by making
noise intermittent
100
2
105
1
Consider job rotation
110
½
>115
Not
Permitted
Time
–
–
–
In the U.S. there is a 5 dBA
trade off for each doubling
of time (3 dBA in Europe).
D. Use Personal Protection
II. Equipment
i. Hearing Protector Devices (HPD)
• Give greater attenuation of higher frequency noise
• Noise Reduction Rating (NRR) summarizes attenuation
regardless of frequency
• Earmuffs tend to give better protection than earplugs
D. Use Personal Protection
III. Earmuffs
Advantages:
–
–
Once size fits all
Easy to supervise
Disadvantages:
–
–
–
They are hot, heavy
Glasses reduce their seal
Apply pressure to head
D. Use personal Protection
IV. Earplugs
Advantages:
- Better accepted if fitted properly
- Allows hearing better in noise
- Light, compact and do not affect appearance
Disadvantages:
- Need to be individually fitted
- Harder to supervise
- Easy to lose
D. Use Personal Protection
V. Recommendations
- Use a cord on earplugs
- Be sure to have spears
- In very high noises use earmuffs
over earplugs
“Goal is ears alive at 65! ”