Week 8 Noise Evaluation - WHAT’S THAT NOISE Lecture...

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Unformatted text preview: WHAT’S THAT NOISE? Lecture Reference: OSHA Technical Manual Introduction There have always been unanswerable questions in life: Why did the chicken cross the road? Which came first, the chicken or the egg? If a tree falls in the woods and nothing is around to hear it did it make a noise? What the heck is that on Donald Trump’s head?? We may never know the answer to most of them BUT after this lecture you’ll know the answer to the third one! Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations What is sound? A type of energy made by vibrations Vibration creates movement in air particles Particles bump into each other They cascade and create sound waves What is Sound? Picture a stone thrown into a still body of water. The rings of waves expand indefinitely. The same is true with sound. Irregular repeating sound waves create noise, while regular repeating waves produce musical notes. When the vibrations are fast, you hear a high note. When the vibrations are slow, it creates a low note. Terminology Frequency Wavelength Velocity Loudness Sound power level Sound pressure level Decibels Filtering Weighting Free field versus reverberant field Terminology Frequency (f) ( ) The number of complete cycles per second Measured in units of hertz (Hz). One Hz = One cycle per second Frequencies around 2,000 Hz are the most important for understanding speech Between 3,000 Hz and 4,000 Hz are the earliest to be affected by noise. Terminology Wavelength The distance traveled by a sound wave during one sound pressure cycle is called the wavelength (l). A wavelength is usually measured in meters or feet. Terminology Velocity or Speed Speed (c) of sound is determined by the density and the compressibility of the medium through which it is traveling. Speed increases with density of the medium (and compressibility decreases) Typically measured in meters or feet per second. Speed in meters per second in: Air: 344 Water: 1500 Steel: 5000 Terminology The frequency, wavelength, and speed of a sound wave are related by the following equation: c = fλ c = speed of sound in meters or feet per second f = frequency in Hertz λ = wavelength in meters or feet Terminology Sound Power Level (W) The amount of energy per unit time that radiates from a source in the form of an acoustic wave. Impossible to measure directly Sound Intensity Possible to measure but difficult and expensive Typically proportional to the sound pressure, which can be measured easily Terminology Sound pressure level Measured directly with IH instruments Measured in decibels (dB) Terminology Difference between sound pressure and sound power Sound power is: Analogous to the power rating of a light bulb. A "weak" sound source will produce low sound levels, whereas a "stronger" sound source will produce higher sound levels. Independent of the environmental surroundings. Sound pressure is: Dependent on distance from the source and environmental surroundings. Terminology Decibel (dB) A dimensionless unit Logarithmic scale (what does this mean?) Expresses sound pressure level accounting for sound power L = 10 log10 (W1/W2) L = the level in dB 10 = a multiplier W1 and W2 = quantities proportional to sound power Example Decibel Levels Filtering We have already discussed frequency Why is it important to measure frequency? High frequency noise is more disturbing and more capable of producing hearing loss than low frequency noise. Important to consider for engineering controls Lower frequency harder to control Terminology Weighting Since frequency is important, we measure noise on a “weighted” scale to account for what is perceived by the human ear A frequency scale is series of correction factors that are applied to sound pressure levels as a function of frequency. The most common weighting scales are designated A, B, and C. Terminology Weighting A-weighting is the best estimation of human hearing C-weighting measures lower frequencies, is nearly a “flat” rating Sometimes used as a tool for specialized surveys Sometimes used by OSHA to evaluate hearing protection A-rating is required by OSHA The Three Scales Terminology Free field versus reverberant field Free Field Has no reflections Sound radiates into space from a source uniformly in all directions. SPL same in every direction Inverse Square Law Rule of thumb estimate, often doesn’t work out the real world The sound pressure level decreases 6 dB for each time the distance from the point source is doubled. Example: Inverse Square Law If a point source in a free field produces a sound pressure level of 90 dB at a distance of 1 meter, the sound pressure level at 2 meters is 84 dB, at 4 meters is 78 dB, and so forth. Terminology Reverberant Field In reality, sound is reflected off of several surfaces Reverberant field is far from the source, where, the reflected sound dominates This is assuming that sound is not absorbed The real world is usually not completely free or reverberant field Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations Effects of Noise Three general categories: Primary Effects Noise-induced temporary threshold shift, noiseinduced permanent threshold shift, acoustic trauma, and tinnitus. Effects on Communication and Performance Isolation, annoyance, difficulty concentrating, absenteeism, and accidents. Other Effects Stress, muscle tension, ulcers, increased blood pressure, and hypertension. The Ear The ear is divided into three sections External outer ear Air-filled middle ear Fluid-filled inner ear Anatomy of the Ear Outer ear Anatomy of the Ear Middle ear Anatomy of the Ear Inner ear Hearing The ear is gathers, transmits, and perceives sounds Done in three stages Modification of the acoustic wave by the outer ear, which receives the wave and directs it to the eardrum. Conversion and amplification of the modified acoustic wave to a vibration of the eardrum (transmitted through the middle ear to the inner ear). Transformation of the mechanical movement of the wave into nerve impulses that will travel to the brain, which then perceives and interprets the impulse as sound. Hearing Loss The three main types of hearing loss are Conductive Sensorineural Combination of the two Conductive Hearing Loss Occurs in middle ear and prohibits sound from traveling to inner ear Can result from Excessive wax in the auditory canal. A ruptured or heavily-scarred eardrum. Fluid in the middle ear. Eustachian tube blockage. Not typically work-related except in an accident An eardrum rupture A rapid pressure change in a decompression chamber. Penetration of the eardrum by a sharp object or fragment. Often reversible through medical or surgical treatment. Sensorineural Hearing Loss Chronic noise-induced hearing loss (can be work-related) Permanent condition condition that cannot be treated medically Associated with inner ear Typically starts with a decline in sensitivity to high-frequency sounds, usually at frequencies above 2,000 Hertz (Hz). Physiological Changes in Sensorineural Hearing Loss Physiological Changes in Sensorineural Hearing Loss Occurs in hair cells Initial Twisting and swelling of hair cells. Disarray of the cilia on top of the hair cells. Detachment of the tectorial membrane from the cilia. Reduction of enzymes and energy sources in the cochlear fluids. Physiological Changes in Sensorineural Hearing Loss Increased severity (becomes irreversible) Hairs become fused into giant cilia or disappear Hair cells and supporting cells disintegrate (Ultimately) the nerve fibers disappear Hearing Loss Noise-induced hearing loss is one of the most common occupational illnesses Often ignored because there are no visible effects Usually develops over a long period of time and typically no pain. Hearing Loss Progressive loss of communication, socialization, and responsiveness to the environment. In its early stages (when hearing loss is above 2,000 Hertz (Hz)) it affects the ability to understand or discriminate speech. As it progresses to the lower frequencies, it begins to affect the ability to hear sounds in general. So… Given what we know… If a tree falls in the woods and nothing is around to hear it, does it make a noise? Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations Applicable Standards OSHA Noise and hearing conservation website 29 CFR 1910.95, Occupational Noise Exposure 29 CFR 1926.53, Occupational Noise Exposure (construction) 29 CFR 1926.101, Hearing Protection (construction) OSHA Technical Manual ACGIH TLVs NIOSH Regulations OSHA 8-hour TWA 90 dBA Action level of 85 dBA (requires hearing conservation program) ACGIH (non-enforceable) 8-hour TWA of 85 dBA Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations Instrumentation Three basic types of meters Sound level meter Octave band analyzer Noise dosimeter Sound Level Meter Most basic instrument for measuring noise levels Convenient to spot check noise levels Used to Determine hazardous noise areas Determine the need for PPE, or if existing PPE is adequate Aid in determining engineering controls (though capability is limited) Sound Level Meter Pros: Easy to use Fairly inexpensive On-the-spot results Cons: Limited applicability Difficult to estimate dose for entire shift Limited frequencies Sound Level Meter Remember frequencies? SLMs have differing capabilities Can measure dBA at a minimum Some also measure dBC, peak, impulse, and linear What do you really need? Sound Level Meter What capabilities would you need to: Determine a hazardous noise area? Measure the sound level of a new piece of equipment? Determine the effectiveness of hearing protection? Sound Level Meter Measuring Impulse/Impact Sounds Peak vs impulse readings The true peak value is the maximum value of the noise waveform. The impulse measurement is an integrated measurement. Think gun shot, impact wrench The true peak reading should only be used when determining compliance with OSHA's 140 dB peak sound pressure level The user should not use "impulse" response when measuring true peak sound pressure levels. Sound Level Meter Considerations for Use Take measurements in the hearing zone of the employee Use the wind screen when measuring outdoors or in dusty areas Calibrate before every survey and check calibration afterward Sound Level Meter OSHA sound level meters are based on ANSI Standards (so yours probably should be too) ANSI Standard S1.4-1971 (R1976) or S1.4-1983, "Specifications for Sound Level Meters." Set performance and accuracy tolerances according to three levels of precision: Type 0 is used in laboratories Type 1 is used for precision measurements in the field (accuracy of ±1 dBA) Type 2 is used for general-purpose measurements (accuracy of ±2 dBA) Sound Level Meter Response switch settings Slow: most sound measurements Fast: short duration Peak: holds the highest sound recorded Impulse: impulse sound (gunfire) Not really recommended to use: use peak instead Sound Level Meter Weighting Switch A: most surveys B: not really used for occupational settings C: some usefulness for considering hearing protection/controls Linear: flat response dBA Range Consider noise levels you will be measuring Octave Band Analyzers What is it? A sound level meter capable of filtering different sound frequencies When would this be beneficial? May also be referred to as a “precision sound level meter” Also “real time analyzers”, which measure all frequencies simultaneously Octave Band Analyzers Can be used to: Determining noise controls (engineering and PPE) We will talk more about this under “Noise Controls” Divide noise into its frequency components Can better isolate the source of noise Octave Band Analyzers Most octave-band filter sets provide filters with the following frequencies: 31.5, 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000, and 16,000 Hertz (Hz) Remember from before: Frequencies around 2,000 Hz are the most important for understanding speech Between 3,000 Hz and 4,000 Hz are the earliest to be affected by noise Octave Band Analyzers Settings Switch weighting to flat Measure at individual frequencies Noise Dosimeters Is also a sound level meter, but is worn during entire work shift Dosimeters can be used to Make compliance measurements according to OSHA's noise standard Measure the employee's exposure to noise and automatically compute the necessary noise dose calculations. Noise Dosimeters Depending on the model, you may need to program it ahead of time Some do OSHA/ACGIH settings simultaneously Noise Dosimeters Settings Threshold 80 dBA will be used to measure compliance with action level 90 dBA will be used to measure compliance with PEL ACGIH uses 80 dBA All noise below this level is not weighed in average Noise Dosimeters Settings Criterion level Used to measure dose Set at 90 dBA since that is the PEL ACGIH set at 85 dBA Exchange Rate Rate at which sound energy is doubled Every time the sound energy doubles, the measured level increases by 3 or 5 dB OSHA uses 5 dB ACGIH uses 3 dB Effects dose reading Noise Dosimeters Other terms Lavg The average sound level measured over the run time Leq Like Lavg except that it is used when the exchange rate is set to 3dB and the threshold is set to none Noise Dosimeters Other Terms TWA Averages the sampled sound over an 8-hour period Less than the Lavg if the run time is less than eight hours Higher than Lavg after eight hours Number you report Dose Expressed in percentage of the exposure limit Noise Dosimeters Other terms MAX Level Highest sampled sound level during the instrument’s run time Peak Level Absolute highest pressure wave that is detected by the microphone Can be affected by bumping the microphone, etc Noise Dosimeters Depending on the model, you may need to program it ahead of time Many newer models report different settings simultaneously Example: Quest Dosimeters Dosimeter 1 Dosimeter 2 Dosimeter 3 OSHA Hearing Conservation Controls OSHA ACGIH Engineering Threshold 80dB 90dB 80dB Exchange Rate 5dB 5dB 3dB Criterion Level 90dB 90dB 85dB Freq. Weighting A A A Response Time Slow Slow Slow Considerations for Sound Level Meters Always check the batteries prior to use (ask me how I know!) Never kink, stretch, pinch, or otherwise damage the microphone cable. Use the microphone windscreen to protect the microphone outdoors or in dirty areas. Never use any type of covering over the microphone Considerations for Sound Level Meters Never try to clean a microphone (other than wiping it off) Remove the batteries when the dosimeter will be stored for more than 5 days. Protect dosimeters from extreme heat and humidity. No field maintenance is required other than replacement of batteries and calibration Calibrate before every survey and check calibration afterward Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations Types of Surveys Types of surveys Walkaround survey Workshift sampling Walkaround Survey Screening tool to determine if workshift sampling is necessary Use a sound level meter What is done Tour the facility and get an idea of operations A floor plan is useful Take spot readings in potential problem areas and mark on the diagram Estimate worker exposure If results indicate possible exposure > 80 dBA then workshift sampling should be accomplished Types of Surveys Workshift Sampling (noise dosimetry) Sample from representative employees from each job classification that may be overexposed Length of time should be sufficient to establish whether exposures are above the allowed limits Best to sample for entire workshift Noise Dosimetry Tell the employee being monitored: The dosimeter should not interfere with normal duties Purpose of the dosimeter, and that it is not a recording device (though this can be fun!) Not to remove the dosimeter or accidentally cover the microphone with a coat or outer garment Noise Dosimetry When setting up the dosimeter: The microphone should be located in the employee's hearing zone. Use the windscreen Excess microphone must be positioned so that it is not in the way Noise Dosimetry During Monitoring: Check the dosimeter periodically to make sure everything is ok Keep a timeline Tie readings to processes Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations OSHA Exposure Limits Action Level 85 dBA Requires hearing conservation program PEL 90 dBA Require feasible engineering or administrative controls Other If employee has had a Standard Threshold Shift ( STS) in their hearing, hearing protectors must reduce noise exposure to below 85 dBA Exposure Limits OSHA PELs for Noise Based off of 5 dB exchange rate Duration per day, hours dBA 8 90 6 92 4 95 3 97 2 100 1.5 102 1 105 Noise Evaluation Principles of noise Effects on hearing Applicable standards Instrumentation Types of surveys Limits Noise calculations Noise Calculations First of all, remember Noise is on a logarithmic scale What does this mean? So, 80 dBA is not twice as loud as 40 dBA Exchange rates Doubling rate (OSHA: 5 dB) Noise Calculations Adding identical sources SPLtotal = 10 log n + SPL1 SPLtotal = 10 log (3) + 89 SPLtotal = 5 + 89 SPLtotal = 94 89 dBA 89 dBA 89 dBA Noise Calculations Time Limits T = (480)(2[(criterion level-La)/exchange rate]) Where T is time in minutes La is A-weighted sound level Noise Calculations How long can a person work in a generator room at 92 dBA without PPE? Criterion level = 90 dBA (OSHA) Exchange Rate = 5 dB (OSHA) T = (480)(2[(criterion level-La)/exchange rate]) T = (480)(2[(90-92)/5]) T = (480)(2[(-0.4]) T = (480)(0.76) T = (480)(2[(90-92)/5]) T = 365 minutes (Does this make sense?? Think about the exchange rate) Noise Calculations Remember this? 1910.95 Table G-16 Duration per day, hours dBA 8 90 6 92 4 95 3 97 2 100 1.5 102 1 105 Noise Calculations Estimating noise exposure Noise Dose C/T Calculation C=Time exposed T=Time allowed If result is greater than 100%, do noise dosimetry C1 C2 Cn Dose 100 Tn T1 T2 Noise Calculations A carpentry worker uses a: Circular saw at 95 dBA 2 hours per day Drill at 92 dBA 1 hour per day Lathe at 97 dBA 1 hour per day Noise Calculations Duration per day, hours dBA 8 90 Circular saw: 95 dBA 6 92 4 hours (240 min) 4 95 3 97 2 100 1.5 102 1 105 Step 1: How long can he be exposed to each? Drill: 92 dBA 6 hours (360 min) Lathe: 97 dBA 3 hours (180 min) Noise Calculations Step 2: Plug into calculation Circular saw: 95 dBA Uses for 120 min Allowed 240 Drill: 92 dBA Uses for 60 min Allowed 360 min Lathe: 97 dBA Uses for 60 min Allowed 180 min Ready?? Noise Calculations Step 2:Let’s plug it into the calculation C1 C2 Cn Dose 100 Tn T1 T2 60 120 60 Dose 100 240 360 180 Dose 100 0.5 0.17 0.33 Dose 100% Noise Calculations Calculating TWA from Dose (D) D TWA 16.61Log 90 100 Noise Calculations You performed 8 hours of noise dosimetry on an antiquated dosimeter and received a dose of 220%. What is the 8-hour TWA? D TWA 16.61 log 90 100 220 TWA 16.61log 90 100 TWA 16.61log 2.2 90 TWA 16.61 0.34 90 TWA 5.6 90 TWA 95.6 Questions? ...
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