NOISE 2 What is Sound? Hertz (Hz) – Frequency a high or low pitch Decibels (dB) – The loudness of the sound.

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

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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

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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

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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

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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!

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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

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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

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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.

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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.

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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

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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

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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

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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! ”