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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! ”