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Industrial Noise

Industrial Noise. Industrial Hygiene IENG 431 Dr. Carter J. Kerk Industrial Engineering Department SD Tech Spring 2009. Assignment. Plog, Chapter 9. Outline – Occupational Noise Exposure. Physics of sound Anatomy of the ear Evaluating hearing ability and hearing loss

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Industrial Noise

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  1. Industrial Noise Industrial Hygiene IENG 431 Dr. Carter J. Kerk Industrial Engineering Department SD Tech Spring 2009

  2. Assignment • Plog, Chapter 9

  3. Outline – Occupational Noise Exposure • Physics of sound • Anatomy of the ear • Evaluating hearing ability and hearing loss • Standards for occupational noise exposure • Measuring noise in the occupational setting • Controlling noise

  4. Introduction • High levels of noise cause hearing loss • Hearing loss is mostly irreversible and usually preventable • Noise can also • produce stress • reduce productivity • cause communication problems

  5. Physics of Sound • Noise – unwanted sound • Energy in the form of pressure waves • Waves can be described by frequency (f), speed (c), and wavelength () • c = f  • Sound moves at 344 m/sec in air, 6100 m/sec in steel • Some materials will amplify or reflect sound • Frequency (f) is related to pitch • Healthy, young person can detect 20 to 20,000 Hz (cycles/sec) • This declines with age and exposure history

  6. Sound Pressure Level • We can measure sound pressure • Force per unit area • SI units, pascal, Pa (N/m2) • All sound pressures are related to a reference sound pressure of 20 Pa (approximate lower threshold for human hearing at 1000 Hz) • Lp = 20 log10 (P / 20 Pa) • Where Lp is the sound pressure, in decibels (dB) • P is the measured sound pressure, in Pa • The decibel is a dimensionless quantity based on the logarithm of a ratio and gives a more convenient range of values than would Pa

  7. Weighting Scales: A, B, C • Each scale approximates the response of the human ear at different ranges of pressure • Because the human ear does not hear sound as if a machine. The human ear is more sensitive to higher frequencies • Derived from comparison experiments • Example: a noise of 1000 Hz frequency and an SPL of 20 dB sounds as loud as a noise of 25 dB at 500 Hz • A-Scale is most common and referenced by OSHA regs • B-Scale rarely used (medium sound pressure levels) • C-Scale common for evaluating explosions and impact noise

  8. Anatomy of the Ear • Outer ear and ear canal directs and amplifies the sound by 10-15 dB • Sound pressure waves impact on the ear drum and vibrate the three tiny bones in the middle ear • Malleus (hammer) • Incus (anvil) • Stapes (stirrup) • Which vibrates against the oval window leading to the inner ear

  9. Inner Ear • Cochlea (inner ear) • Basilar membrane (lining of the cochlea) • Supports 25,000 specialized hair cells • Which send characterizing nerve impulses to the brain • Three semicircular canals (in orthogonal planes) • Filled with fluid • Provides sense of balance and relative body position • Have you ever felt dizzy? • Eustachian tube • Connects middle ear to throat • Equalizes pressure • Have your ears ever popped?

  10. Hearing Loss • Conductive hearing loss • Interruptions along the pathway reducing hair stimulation • Excessive earwax, otitis media (fluid in middle ear), ruptured eardrum • Sensory hearing loss • Presbycusis (loss due to age) • Noise-induced hearing loss • Sociacusis (loss from everyday life) • Nosacusis (loss from disease, heredity, drugs, sudden and severe pressure changes, traumatic head injuries) • Tinnitus (follows traumatic exposure to loud noise: perceived ringing, roaring, hissing – may be permanent)

  11. Evaluating Hearing Ability and Hearing Loss • Audiograms • A hearing evaluation exam, called audiometry, produces a report called an audiogram • OSHA requires all workers exposed to an 8-hour TWA of at least 85 dBA (Action Level) receive a baseline audiogram and annual follow-up exam • Employee sits in soundproof booth with headphones and control button to produce HTL (Hearing Threshold Level) • Method of Limits at the following test frequencies: 500, 1000, 2000, 3000, 4000, 6000 Hz, the range most detectable by the human ear • Speech range: 1000 – 4000 Hz

  12. Reasons for Variation in Audiometric Testing • Ear wax buildup • Head cold, congestion • Confusion about response procedure • Incorrect placement of headphones • Hair under headphones • Audiometer malfunction

  13. OSHA Occupational Noise Exposure Standard • 29 CFR 1910.95 (Plog p. 242) • Requirements for maintaining and calibrating audiometric equipment and technician training • Table G-16A of Appendix A (Plog p. 247) • Relates A-weighted sound level to allowed duration • Table A-1 of Appendix A (Plog p. 248-9) • Converts % Noise Exposure (Dose) to “8-hour TWA”

  14. Quantifying Hearing Loss • Watch for changes in the HTL (Hearing Threshold Level) • STS (Standard Threshold Shift) – decrease of 10 db or more at 2000, 3000, or 4000 in either ear • Represents permanent hearing loss • Call for a re-test (after at least 14 hours of relative quiet) • TST (Temporary Threshold Shift) – a shift in HTL that disappears after the person has been in a quiet environment for a few hours

  15. Standard Threshold Shift (STS) • If an STS is identified • Notify the employee in writing • Provide additional training • Provide adequate hearing protection • Workers’ Compensation • Realize that WC laws for identifying and compensating STS will vary across the states

  16. 29 CFR 1910.95 (Plog p. 242) • Enacted in 1971 • Hearing Conservation Program is required whenever employee exposures exceed 85 dBA 8-hr TWA • This is half the allowable noise exposure for an 8 hour day or 50% Daily Noise Dose (DND) • Note: 90 dBA for an 8-hr TWA is 100% DND

  17. Hearing Conservation Program Elements • Exposure monitoring • Audiometric testing • Hearing protective devices • Training program • Access to the written standard • Recordkeeping

  18. Hearing Conservation Program Appendices • Appendix A – Methods for computing employee noise exposure • Appendix B – Methods for determining attenuation by hearing protection devices • Appendix C – Performance of audiometric testing equipment • Appendix D – Maximum levels of noise allowable in testing rooms and booths • Appendix E – Calibration requirements for audiometric testing equipment • Appendix F – Age-correction of audiograms • Appendix G – Information on noise monitoring

  19. Measuring Occupational Noise • Sound Level Meters (SLM) • Used for area surveys • Settings for average, peak, impulse, ABC scales • Noise Dosimeters • Used for individual monitoring • Clip microphone near the ear • Wear all day • Calibrate before and after

  20. Adding Decibels • Often there is a need to combine two or more noise sources • Because decibels are logarithms, they cannot be added directly • 80 dB + 85 dB ≠ 165 dB • 80 dB + 85 db = 86.2 dB

  21. Example • Given three machines in a room measured at 80, 85, and 87 dB, respectively • SPLtotal = 10 log (1080/10+1085/10+1087/10) • SPLtotal = 89.6 dB

  22. Estimation Method (see p. 214) • Easier and reasonably accurate

  23. Example • Add 80, 85, and 87 dB using the estimation method • Start with 85 and 87, difference is 2 dB, so select 2 dB from the table • 87 + 2 = 89 dB • Add 89 and 80, difference is 9 dB, so select 1 dB • 89 + 1 = 90 dB (compare to 89.6 dB)

  24. Computing Daily Noise Doseand Calculating 8-hr TWAs • OSHA limits workers to 100% of the daily dose or 90 dBA for 8-hr TWA • D = 100 (C1/T1 + C2/T2 + … + Cn/Tn) • D = daily nose dose, in percent • C = total time of exposure at the measured noise level • T = reference allowed duration for that noise level from Table G-16a of Appendix A of 29 CFR 1910.95 (Plog p. 247)

  25. Example • A worker’s exposure was monitored for 2 hrs at 80 dBA, 2 hr at 85 dBA, and 4 hr at 87 dBA. What is the DND (Daily Noise Dose)? • D = 100 (2/32 + 2/16 + 4/12.1) = 51.8% • The worker received 51.8% of their DND. (This is OK.) • Given a DND = 51.8%, find the 8-hr TWA. • Go to Table A-1 (Plog p. 248-9). Round to 55% (conservative). Yields 85.7 dB TWA. The Hearing Conservation Program is required.

  26. Controlling Noise • Engineering Controls • Administrative Controls • PPE

  27. Engineering Controls • You can make a career out of “engineering controls” for controlling noise • Devices: insulative curtains; coverings for noise-reflective floors, ceilings, walls; vibration isolation devices • Remember sound is a wave and cannot turn around corners – this is the concept of “directivity” • “Reflection” – sound waves can bounce back and add sound pressure at the source • “Resonance” – a material vibrates at the same frequency as the emitted sound – use an “vibration isolator” or rubber mounting

  28. Engineering Controls • Some surfaces “absorb” the sound energy, or do not allow it to reflect effectively • Noise control curtains; fiber-filled cloth office partitions • Proper “preventative maintenance” (PM) on machines parts such as motors, bearings, drive belts, pumps, etc. • Adjustments, lubrication, replacement, vibration isolators • Think “outside the box” for new designs and work with suppliers • One of the best engineering controls is “distance” • The relationship between noise and distance follows the inverse square law • Doubling the distance reduces the noise by ¼ • Tripling the distance reduces the noise by 1/9

  29. Administrative Controls • Used when engineering controls are exhausted or infeasible • Limiting time in exposed areas; worker rotation; limiting the number of workers in exposed areas (limited access)

  30. Hearing Protective Devices (HPD) • After engineering and administrative controls are exhausted and infeasible • All workers exposed at ≥ 85 dBA for 8-hr TWA must be provided HPD at no cost • Employers must ensure workers actually wear the HPD • Employers must provide a variety of HPD

  31. HPD Continued • Employers must train workers to use HPD and how to care for them • HPD attenuation must effectively reduce noise exposure to below the OSHA action level (85 dBA) • Noise Reduction Rating (NRR) • Typically 22 – 30 dB NRR • Numerical attenuation value determined in a laboratory • When using the A-Scale, you must deduct 7 dB from the NRR • When using the C-Scale, no deduction is necessary

  32. Example • Worker is exposed to 98 dBA for 8-hr TWA. Earplugs are available with a 29 NRR and earmuffs are available with a 25 NRR. • Since A-Scale, Earplugs (NRR = 29-7 = 22) and Earmuffs (NRR = 25-7 = 18) • Earplugs: 98 – 22 = 76 dBA • Earmuffs: 98 – 18 = 80 dBA • Both are below the 85 dBA 8-hr TWA Action Limit

  33. Extreme Exposures • For extreme exposures with 8-hr TWA in excess of 100 dBA, it may be necessary to use both earplugs and earmuffs • Their NRRs are not additive • Tests show an additional 3 – 10 dB NRR is achieved with the second device

  34. Supplemental References • Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M. The Noise Manual, 5th Edition, AIHA Press, Fairfax, VA, 2000. • Nims DK, Basics of Industrial Hygiene, John Wiley & Sons, New York, NY, 1999.

  35. Assignment • Plog, Chapter 9

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