noise basic
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MSHA STATE GRANT PROGRAM FOR MICHIGAN
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Noise Basics
The two factors that determine how hazardous noise is are:
Intensity (Loudness) measured in dBA
Time of Exposure measured in Hours and Minutes
The louder the noise, the more hazardous it is. Also, the longer the exposure time, the more
hazardous the noise is.
A Noise Dose combines both loudness and time and is a convenient way of describing the relativehazard of the noise.
Loudness is measured using a Logarithmic Scale.
Thismeans that a 10 decibel increase does not simply add 10to the previous level. It multiplies theprevious level by 10.
Threshold of Hearing............................... 0 dBA
Quiet Room................................................. 45 dBA
Conversation.............................................. 55 dBA = 45 dBA x 10
Car (50 mph at 50 ft).................................. 65 dBA = 45 dBA x 100
End Loader (In Good Cab)...................... 75 dBA = 45 dBA x 1,000
Haul Truck (In Good Cab)....................... 85 dBA = 45 dBA x 10,000
Crusher........................................................ 95 dBA = 45 dBA x 100,000
Old Dozer (No Cab)................................... 105 dBA = 45 dBA x 1,000,000
Air Track Drill (No Controls).................... 115 dBA = 45 dBA x 10,000,000
115 dBA has 10 million times more sound energy than 45 dBA and iscapable of causing 10 milliontimes more damage to hearing.
Allowable Noise exposure is based on Noise Dose.
Noise Dose is expressed as a percentage of the allowable limit. How high it goes depends on both
noise intensityand time of exposure to noise.
Louder (More dBA) = More Noise Dose
Longer (More Time) = More Noise Dose
Absolute Quiet = 0% Noise Dose
85 dBA for 8 Hrs = 50% Noise Dose
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90 dBA for 8 hrs = 100% Noise Dose
95 dBA for 8 Hrs = 200% Noise Dose
95 dBA for 12 Hrs = 300% Noise Dose
Different combinations of Loudness and Time can produce the same Noise Dose. The followingcombinations can all produce about 75% Noise Dose.
84 dBA for 14 Hrs = 76%
85 dBA for 12 Hrs = 75%
87 dBA for 9 Hrs = 74%
88 dBA for 8 Hrs = 76%
93 dBA for 4 Hrs = 76%
95 dBA for 3 Hrs = 75%
98 dBA for 2 Hrs = 75%
103 dBA for 1 Hr = 76%
108 dBA for 30 Min = 76%
113 dBA for 15 Min = 76%
Note: The above Noise Doses are based on using the 80 dBA (Low) threshold.
Noise Dose Table
If you have not already done so, choose one of the options below to either view the Noise Dose Table in a separate browser
window or print it out.
Noise Dose Table - Adobe PDF version Noise Dose Table - MS Word 2000 version
A Noise Dose table is used to look up the Noise Dose when 'noise-level & exposure-time' are known.
For the following several items, please refer to the Noise Dose Table in your printed materials.
(Please note that the examples shown below are not an exact duplication of the real Noise Dosetable.)
Exposure Time
Exposure times (in Hours) are listed across the top of the table. (Shown here in red.)
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Noise Levels
Noise Levels (in dBA) are listed down the left side of the table. (Shown here in green.)
Estimating a Noise Dose Using the Table
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Measure the Decibel Level. (We will discuss methods for doing this later.)1.
Measure or estimate Exposure Time.2.
The Noise Dose is found at intersection of Decibel Level and Exposure Time. (Yellow Box)3.
Using Noise Dose Table if Noise Levels vary.
If Noise Levels vary, divide shift into separate tasks.1.
Determine Dose for each task, then add the doses.2.
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Reveiw
Loudness is measured in dBA
Quiet Room = 45 dBA
Crusher = 95 dBA
Air Track Drill = 115 dBA
Noise Dose = Loudness + Exposure Time
Quiet (45dBA)for 8 Hrs = 0% Noise Dose
Crusher (95dBA) for 8 Hrs = 200% Noise Dose
Air Track Drill (115dBA) for 8 Hrs = 3,200% Noise Dose
The dBA level alone does not define noise hazard!
Use Noise Dose Table to determine the hazard.
Noise Dose defines the noise hazard.
Noise Dose Table Exercise
Use the Noise Dose Table to figure out the Noise Dose for the following conditions:
Noise exposure is 88 dBA for 10 Hr - What is Noise Dose ?1.Noise exposure is 84 dBA for 4 Hrs and 94 dBA for 6 Hrs. (Remember: Figure the two noise doses2.
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separately and then add them.) What is the total Noise Dose ?Noise exposure is 79 dBA for 2 Hrs, 92 dBA for 3 Hrs, 81 dBA for 3 Hrs, 87 dBA for 30 Min, 90
dBA for 1 Hr, and 82 dBA for 30 Min. What is Noise Dose ?
3.
Click here for Noise Table Exercise Answers
Time Weighted Average (TWA
8
)
The 'TWA8' is the Noise Level (dBA) which, if constant over 8 hours, would result in the same Noise
Dose as the one calculated using the actual exposure time.
Example: If a worker's Noise Dose from a 10 hour shift is 400%, what constant Noise Level (in dBA)over 8 hours would produce the same Dose? The answer is on the Noise Dose Table. Find "400" in thethe "8 hour" Exposure Time column. Follow the row (left) to the Noise Level, which, in this case, is
100 dBA. The TWA8
value for any Noise Dose equaling 400% is 100 dBA .
Why use TWA8? Why not just use Noise Dose?
Because the "TWA8" (which is expressed in dBA, rather than a percentage) is easier to use when
applied to noise controls.
Example: If one knows that the noise level in an area is kept below 90 dBA, for the full 8 hour workshift, then the 90dBA PEL has not been exceeded. Trying to think of this in terms of noise dosepercentages is awkward.
Action Level = 85 dBA (TWA8)
Permissible Exposure Limit = 90 dBA (TWA8)Double Hearing Protection Level = 105 dBA (TWA
8)
CONTINUE ON TO NEXT SECTION - SOUND MEASURING EQUIPMENT
For Mine Safety Training in Michigan - Contact Dave Carlson at [email protected]
to MTU-MS Web Staff This site last updated 01/31/08
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