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NOISE
2
What is Sound?
• Hertz (Hz)– Frequency a high or
low pitch
• Decibels (dB)– The loudness of the
sound
NOISEDefinitions• 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
الصناعية االنشطة نتيجة الصناعية الضوضاء االنشطة نتيجة الضوضاء
معينة نقطة فى الزمن و الضغط بين العالقةالصوت مستوى من
الصوتية الموجات من مختلفة اشكال
صوت مستوىمتوسط
صوت مستوىمتوسط
صوت مستوىعالى
عالى صوت مستوى
لفترة صوتصغيرة
لفترة صوتكبيرة
6
Frequency
• Humans can typically hear between 20 - 20,000 Hz
• You can hear different frequencies better than others
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Decibels
• The quietest sound most humans can detect is 0 dB
• Some humans can even hear sounds as quiet as -5 dB
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, dBA
EXAMPLE
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
11
The Loudness of Common Sounds0 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
Sound level, dBA Permissible time, T(hours) 80 32
85 16 90 8 95 4 100 2 105 1 110 0.5 115 0.25 120 0.125 125 0.063 130 0.031
البشرية لالذان مخطط
Anatomy of the Ear
Cochlea
Ear Drum
Ear Bones
Semi-Circular Canals
16
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
17
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!
18
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
Hearing LossOverview
• 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- منها الشفاء يمكن ما .-ونادرا
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
مستوياتضغط عند لالنسان السمع مدى
المختلفة الصوت
25
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.
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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 87 dB?
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• Neither, because it is a log scale• We use the following chart
• 82 dB + 83 dB = 86 dB• 87 dB + 89 dB = 91 dB
Difference in dB values
Add to Higher Value
0 or 1 dB 2 or 3 dB 4 or 10 dB 10 or more dB
3 dB 2 dB 1 dB 0 dB
دراسة حالة
Machine C82db
Difference between 2 decibel level to be added (db)
Amount to be added to larger level to obtain decibel sum (db)
0 3
1 2.6
2 2.1
3 1.8
4 1.4
5 1.2
6 1.0
7 0.8
8 0.6
9 0.5
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)
10 0.4
11 0.3
12 0.2
Scale for combined decibel
( الضوضاء ( مستوى الموجة سعة حساب اسلوب
المعدة بين الفرق صفر =(A , B ) يحسبيضاف الجدول ديسبل 3ومن
للمعدتين = الموجة سعة ديسبل 89=3+86مجموع
المعدتين بين الفرق المعدة (A,B)يحسب ديسبل C = 89-82=7 و
بضاف الجدول من ديسبل0.8و
للمعدات الموجة سعة 89+ 8 =(A,B,C) مجموع ديسبل 89.8و=
المعدات بين المعدة (A,B,C)الفرق 89.8 -12= 12= ( D) وديسبل
يضاف الجدول ديسبل 0.2من
للمعدات الضوضاء سعة =89.8+0.2=(A,B,C,D) مجموعديسبل90
العامل فى تؤثر التى الضوضاء مستوى محصلةيكون المعدات هذه به محاط مكان فى يعمل الذى
ديسبل90 .
حالة حالة دراسة دراسة
36
How does the Safety Person determine noise levels
• Sound level meter– Determine the
loudness (dB) of noise at any given moment
• Personal Dosimeters– Worn by employees– Measures the average
loudness in an 8 hour work shift “8hr. TWA” (Time Weighted Average)
– Can also measure noise dose
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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
• 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
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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.
39
Hearing Loss
• Impact - One loud bang• Cumulative - Years of a noisy environment• Tinnitus - Ringing in the ears• Presbycusis - Hearing loss due to aging
Hearing
• Hearing loss increases when exposed to over 67 dBA.
Hearing
– US occupational standard is 90 dB for 8 hours of exposure per day.
42
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
43
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
سدادات اإلذن وقاءاإلذن
Frequency Foam plug Fiberglass down High performanc muff Popular muff125 31 10 14 10250 34 12 23 15500 39 17 32 23
1000 38 19 37 312000 35 26 35 353150 42 33 38 424000 44 34 39 396300 46 37 39 378000 47 35 38 35
EARPLUG EARMUFF
من اإلذن واقيات بين المقارنةلإلذن الحقيقي الوهن حيث
المختلفة الترددات عند
46
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?
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• (NRR 25 - 7) = 18
• 18 / 2 = 9
• - 9 =
• YOU ARE ABOVE OSHA LIMITS OF 90 dB
101110 dB
48
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
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.
PerformanceSpeech 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 C) Modify sound wave
B) Modify existing D) Use personal protection
noise source
A. Plan Ahead
I. Substitute Less Noisy Processesi. Reduce the use of impact toolsii. Replace internal combustion with electric enginesiii. Replace gear transmission with belt transmission
II. Purchase Less Noisy EquipmentIII. Use Quieter Materials and Construction
i. Reduce Impactii. Reduce Vibrationiii. Reduce Turbulence iv. Reduce Transmission
A. Plan Ahead
Good: Bevel gears Poor: Spur 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
Poor Better
2. Avoid abrupt directional changes
1. Increase distance for turbulence to die
3. Avoid abrupt volume changes
A. Plan Ahead
• A. Plan AheadIV. Separate People and Noisy Equipment
i. Locate noisy equipment in an isolated room.ii. 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. Reduce the Driving Force
II. Change the direction of noise
III. 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 WaveI. 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 WaveDouble-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. Time – In the U.S. there is a 5 dBA
trade off for each doubling of time (3 dBA in Europe).
– Reduce exposure by making noise intermittent
– Consider job rotation
dBAdBA Time Time (hrs.)(hrs.)
9090 88
9595 44
100100 22
105105 11
110110 ½½
>115>115 Not Not PermittedPermitted
D. Use Personal Protection
II. Equipmenti. 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 ProtectionIV. 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! ”