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Appendix III:A-5. Dosimeter Settings
Settings

According to OSHA's noise standard 29 CFR 1910.95, the noise dosimeter is the primary instrument for making compliance measurements. The following dosimeter settings must be utilized:
  • Exchange rate: 5 decibels (dB)
  • Frequency weighting: A
  • Response: slow
  • Criterion level: 90 dBA
  • Threshold: 80 dBA or 90 dBA
A dosimeter with a threshold of 80 dBA as well as one with a threshold of 90 dBA should be used to measure noise exposures (most modern dosimeters utilize simultaneous 80 and 90 dBA threshold settings), as follows:
  • The 80 dBA threshold dosimeter is used to measure the noise dose of those employees identified during the walkaround whose exposure may exceed the 85 dBA time-weighted average (TWA) limit.
  • The 90 dBA threshold dosimeter is used to measure the noise dose of those employees identified during the walkaround whose exposure may exceed the 90 dBA permissible exposure level (PEL).
  • Note: Paragraphs 29 CFR 1910.95(a) and 29 CFR 1910.95(b) of OSHA's noise standard date back to the 1969 Walsh-Healey Act. This early standard predated noise dosimetry and OSHA had no instructions for taking noise measurements, and so the first dosimeters that were developed used 90 dBA both as the threshold and criterion levels. Paragraph 29 CFR 1910.95(c) of the 1983 Hearing Conservation Amendment to the Occupational Noise Exposure Standard requires employers to administer a continuing, effective hearing conservation program for all employees whose noise exposures equal or exceed an 8-hour time-weighted average (TWA8) of 85 dBA or, equivalently, a noise dose that is equal to 50 percent of the PEL. The standard requires that all continuous, intermittent, and impulsive sound levels from 80 dB to 130 dB be included in the measurement of dose.
Dosimeter Readout

The hypothetical exposure situations shown in the table below illustrate the relationship between criterion level, threshold, and exchange rate and show the importance of using a dosimeter with an 80 dBA threshold to characterize an employee's noise exposure. An instrument with a 90 dBA threshold will not capture any noise below that level, and will thus give a readout of 0 percent even if the employee being measured is actually being exposed to 89 dBA for eight hours (equivalent to 87 percent of the allowable noise dose over any eight hour period).


DOSIMETER READOUT, IN PERCENT OF MEASURED DOSE
Exposure conditions Dosimeter with threshold
set at 90 dBA
Dosimeter with threshold set at 80 dBA
90 dBA for 8 hours
 
100.0% 100.0%
89 dBA for 8 hours
 
0.0% 87.0%
85 dBA for 8 hours
 
0.0% 50.0%
80 dBA for 8 hours
 
0.0% 25.0%
79 dBA for 8 hours
 
0.0% 0.0%
90 dBA for 4 hours plus 80 dBA for 4 hours
 
50.0% 62.5%
90 dBA for 7 hours plus 89 dBA for 1 hour
 
87.5% 98.4%
100 dBA for 2 hours plus 89 dBA for 6 hours 100.0% 165.3%
* Assumes 5 dB exchange rate, 90-dBA PEL, ideal threshold activation, and continuous sound levels.

Some dosimeters indicate when a 115 dBA (A-weighted decibel) sound level has been exceeded. Do not use this indication for compliance determination.

Using the Noise Dose Reading

The noise dose provided by dosimeters can be used to calculate both the continuous equivalent A-weighted sound level (LA) and the eight hour TWA for the time period sampled.


Equation One:
A-weighted Sound Level Calculation

 
LA = 16.61 log10  D
12.5t
 + 90

Equation Two:
Eight Hour TWA Sound Calculation
TWA = 16.61 log10  D
100
 + 90


  LA   =   the continuous equivalent A-weighted sound level in decibels for the time period sampled
D   =   dosimeter readout in percent noise dose
t   =   the sampling time in hours
TWA   =   the eight hour time-weighted average in decibels (dBA)

Equation Two is used for enforcement purposes and Equation One can be used to assist in evaluating hearing protectors and engineering controls. Note: Most dosimeters perform the above calculation and automatically provide data for LA and TWA.

NOTE: In general, for enforcement purposes a 90-dBA threshold dosimeter is necessary to establish noncompliance with Table G-16 or Table D-2. However, in unusual situations, results obtained with an 80-dBA threshold dosimeter may be discussed with the ARA for Technical Support to determine whether they sufficiently demonstrate noncompliance with Table G-16 or Table D-2.

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