Noise Basics
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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 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.

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 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.

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
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 following combinations 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 Dose table.) 

Exposure Time

Exposure times (in Hours) are listed across the top of the table. (Shown here in red.)

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

  1. Measure the Decibel Level. (We will discuss methods for doing this later.)

  2. Measure or estimate Exposure Time.

  3. The Noise Dose is found at intersection of Decibel Level and Exposure Time. (Yellow Box)

Using Noise Dose Table if Noise Levels vary.

  1. If Noise Levels vary, divide shift into separate tasks.

  2. Determine Dose for each task, then add the doses.


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:

  1. Noise exposure is 88 dBA for 10 Hr - What is Noise Dose ?
  2. Noise exposure is 84 dBA for 4 Hrs and 94 dBA for 6 Hrs. (Remember: Figure the two noise doses separately and then add them.) What is the total Noise Dose ?
  3. 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 ?

Click here for Noise Table Exercise Answers

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.

Action Level = 85 dBA (TWA8)
Permissible Exposure Limit = 90 dBA (TWA8)
Double Hearing Protection Level = 105 dBA (TWA8)


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