The two factors that determine how hazardous noise is are:
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 relative hazard of the noise.
Loudness is measured using a Logarithmic Scale.
This means that a 10 decibel increase does not simply add 10 to the previous level. It multiplies the previous level by 10.
115 dBA has 10 million times more sound energy than 45 dBA and is capable of causing 10 million times 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 intensity and time of exposure to noise.
Different combinations of Loudness and Time can produce the same Noise Dose. The following combinations can all produce about 75% Noise Dose.
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.
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.
Exposure times (in Hours) are listed across the top of the table. (Shown here in red.)
Noise Levels (in dBA) are listed down the left side of the table. (Shown here in green.)
Estimating a Noise Dose Using the Table
Using Noise Dose Table if Noise Levels vary.
Loudness is measured in dBA
Noise Dose = Loudness + Exposure Time
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:
Time Weighted Average (TWA8)
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 the the "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 work shift, then the 90dBA PEL has not been exceeded. Trying to think of this in terms of noise dose percentages is awkward.
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