Noise Measurement TrainingModule
Table of Contents • Physics of Sound (Hearing loss, terminology) • OSHA (Regulations) • Sound Level Meters • Noise Dosimetry
Noise Measurement and Dosimetry Learning Objectives • Explain sound pressure level, frequency & respective units of measure • Describe level average, time weighted average & dose • Explanation of criterion, threshold & exchange rate • Proper placement & care of microphones and instrumentation • Explain hearing conservation regulations & OSHA requirements as it pertains to noise measurement • Hands on calibration, function & measurement with the noise dosimeter, sound level meter and octave band analyzers
Why Measure Noise? I. Determine if the employee(s) are at risk for Noise Induced Hearing Loss (NIHL) & should be in a Hearing Conservation Program (HCP) • Differentiate between on-the-job & off-the-job noise exposure • Determine most effective hearing protection • Engineering Controls • Administrative Controls • Compliance with applicable Standards
Why Measure Noise? • Prolonged exposure to noise causes permanent and debilitating hearing loss • Mostly between 4 – 8 kHz • Lose ability to understand speech in many situations • Typically affects consonant reception • Individual losing hearing is often the last to know!
Why Measure Noise? • Long term health effects due to adrenaline release • More accidents in high noise environments • Measurable decrease in productivity • Measurable decrease in work accuracy • Increased fatigue
Noise Induced Hearing Loss • Intensity + Duration • Damage occurs in the cochlea • Results in loss of comprehension, not loudness • Permanent
Definition of Sound Fig. I-3 Compression & Rarefaction of air molecules
What does Sound look like? Adapted from Acoustics Animations – Dr. Dan Russell – Kettering University Applied Physics http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
Four Characteristics Of Sound • Frequency (measured in Hz) • Intensity (measured in dB) • Speed (measured in feet or meters/second) • Wavelength (measured in feet or meters)
Frequency • Measured in Hertz (Hz) • kHz for 1000xHz • Relates to the pitch of the signal • Is a measure of the cycles completed in one second
Frequency – (Pitch) Fig. I-4 1Hz and 10Hz Sine Waves
Typical Frequency Examples • Lathe 50 to 1100 Hz • Compressed air 500 to 8000 Hz • Circular saw 300 to 12000 Hz • Cars 8 to 1100 Hz • Violin 300 to 9000 Hz
Frequency Response & Weighting • The human ear is capable of responding to frequencies ranging • from 20 Hz to 20 kHz • The ear is less efficient at high and low frequencies • In the range from 500 Hz to 4 kHz a normal human ear is very sensitive
Frequency Response & Weighting Fig. I-6 “A” and “C” Weighting Curves
2 Sine Waves: Same Frequency Same Time Duration Different Intensity One is TWICE as loud as the other Sound Pressure Level Fig. I-7 Sine Waves with different sound pressures Measured in decibels (dB)
deciBels S P LdB SPL SoundPressureLevel Relates to measurements of the environment deciBels H LdB HL HearingLevel Relates to intensity of audiometric test tones UnderstandingdeciBels (dB)
100 dB 100 dB + = 103 dB Doubling Sound Source • Add 3 dB • 10 dB perceived as “twice” as loud
Addition of Decibels If the difference between two levels to be added is: - 0 to 1 dB, then add 3 dB to the higher number - 2 to 3 dB, then add 2 dB to the higher number - 4 to 7 dB, then add 1 dB to the higher number - 8 dB or more, then add 0 dB to the higher number • Decibels are logrithmic values • They can not be directly added or subtracted
Changing Distance From Source • Outside • Doubling distance • Decrease SPL 6 dB • Room • Very Near Source • Twice the distance decreases SPL by 6dB • Far from source • No change with change in distance NOTE: These are general guidelines. Each case may vary.
Octaves Fig. I-11 Octave Bands • Focus on the frequency content of the overall noise signal • Important for noise control efforts
Occupational Noise Characteristics Fig. I-12 Mixture of Source Signals • In the real world of occupational noise the overall sound is a mixture of many simultaneous sources with a variety of frequencies and intensities.
Types or Characteristics of Sound“Measure all continuous, variable and impact/impulse sound” A fan is an example of a continuous noise source A forklift driver is an example of a variable exposure An explosion or gunshot are examples of an impulsive noise A hammer striking a nail would create an impact noise signal
Response Time • Slow 1 second rise and fall • Fast 0.125 second rise and fall • Impulse 35 millisecond rise 1.5 second fall • Peak 50 microsecond rise and fall
Types of Hearing Protectors • Muffs • Plugs Foam formable Pre formed Custom • Others Plugs on a head band Noise cancellation
Hearing ProtectionNRR Noise Reduction Rating
NRR Devaluation • NRR- 7 dBAdjustedAttenuationthen divide this value by 2to determine if hearing protector’s are adequateto forego noise control • The OSHA method is described well on the OSHA Noise & Hearing Conservation e-Tool website http://www.osha.gov/dts/osta/otm/noise/hcp/attenuation_estimation.html
Hearing Protection NRR - 7 2
NIOSH subject fit Model • Muffs NRR less 25% • Formable Plugs NRR less 50% • All Other Plugs NRR less 70%
GRAPHIC ILLISTRATION • Assume you need 10 dB reduction • Assume NRR is 24 for all • De-rate muff by 25% • De-rate formable by 50% • De-rate anything else by 70%
Plugs + muffs • REMEMBER: • Earmuffs over earplugs provide a maximum of 5dB additional attenuation no matter what the muff rating.
Critical Terminology Criterion - If exposed to SPL on average for eight hours, it would result in a maximum allowable exposure. [90dB for 8 hrs] Threshold – dB level below which, all SPL’s are assigned a value of zero. [80dB] Exchange Rate - Results in a doubling or halving of the maximum allowable exposure.
Critical Terminology Average Level (LAVG and Leq) - If present continuously, would generate the same amount of energy as the varying levels that are present in the environment measured in decibels. Time Weighted Average (TWA) - A level average with an assumed fixed sample period of eight hours measured in decibels. Dose - The allowable daily exposure value. A maximum allowable exposure is equal to 100% dose. [90dB x 8hrs = 100% Dose]
A Basic Concept… • TWA = Lavg @ exactly 8 hours • DOSE = Follows the same line! Lavg TWA 8 hours
Exchange Rates • SOUND LEVEL METERS • 3dB exchange rate • When averaged, Leq (level equivalent) • NOISE DOSIMETERS • 5dB exchange rate • When averaged, Lavg (level average)
Criterion and 5 dB Exchange rate • 90dB x 8hrs = 90dB TWA = 100% Dose • 95dB x 8hrs = 95dB TWA = 200% Dose • 100dB x 8hrs = 100db TWA = 400% Dose • 100dB x 4hrs = 95dB TWA = 200% Dose • 100dB x 2hrs = 90dB TWA = 100% Dose • 100dB x 1hr = 85dB TWA = 50% Dose
Section II OSHA 21-25
The Occupational Noise Exposure Standard • Derived from the Walsh-Healey Public Contracts Act. • 1971 adopted under the Occupational Safety and Health Act. • Permissible Exposure Level (PEL) of 90 dBA. • Noises with a higher level than 90 dBA can be sustained for periods of less than 8 hours. • Sounds with average levels less than 90 dBA can persist for periods of more than 8 hours.
Permissible Noise Exposure Hours per Day Sound Level dBA slow response 8 90 6 92 4 95 2 100 1.5 102 1 105 0.5 110 0.25 or less 115
Hearing Conservation Amendment • Published in the Federal Register on March 8, 1983. • The Hearing Conservation Amendment requires the employer to perform five key tasks: • Measure Noise • Audiometric Tests • Hearing Protectors • Education & Training • Record Keeping
Table G-16a (abbreviated) A-Weighted Duration Sound Level (Hours) Measuring Threshold 80 32 H.C. Action Level (50% exp.) 85 16 8 Hour Criteria 90 8 95 4 100 2 105 1 110 0.5 115 0.25 120 0.125 125 0.063 Minimum Upper Range 130 0.031 24 Section II
Measuring Method for OSHA Surveys Original Rule H.C.A. Weighting/Response A/Slow A/Slow Exchange (Doubling) Rate 5 dB 5dB Criterion (LC=100%) 90dB 90dB Threshold (Cut Off) 90dB 80dB Limit 90dB/100% 85dB/50% (PEL) Action Level
Conclusion • The focus of the Occupational Noise Exposure Standard and the Hearing Conservation Amendment, is to set minimum requirements to protect hearing for those workers in a noisy environment. • The Keys to Success • Positive attitude on the part of the management • Clear communication of the value of hearing • Hearing protection takes care of the problem of noise for the short term, but the key to real hearing protection is education and communication.