Same noise, different results

February 9, 2009
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In addition to meeting OSHA regulations, a growing number of companies must report noise exposures to a second standard. Organizations whose head office or parent company is based in Europe, for example, may need to calculate employee noise exposure for the EU Occupational Noise Directive (or ISO standard). This article looks at how information reported to two different standards for the same noise exposure can vary significantly.

Channel 1: OSHA Regulations
OSHA regs specify the following when configuring a noise dosimeter to assess noise exposure:
  • A TWA action level of 90dB (equivalent to the EU “Upper Action Level”)
  • An exchange rate of 5dB (for every doubling of the noise energy, the noise level increases by 5dB)
  • A threshold of 80dB (noise levels below this value are not included in noise data)
  • Use of Slow Time Weighting (which reduces the effects of fast, impulsive noises)
  • A Criterion Level of 90dB and a Criterion Time of 8 hours
Recorded data would typically be the LAVG value with a Time Weighted Average (TWA) value calculated from this and the measurement duration. Dose or % Dose value calculated from the TWA value is typically reported along with the TWA.

Channel 2: EU Occupational Noise Directive
The EU Physical Agents (Noise) Directive 2003/10/ EC defines the levels and measurement methodology used to assess exposure in EU member states:
  • An upper action level of 85dB (equivalent to the OSHA 90dB TWA level)
  • An exchange rate of 3dB (for every doubling of the noise energy, the noise level will increase by 3dB rather than the 5dB used by OSHA regs)
  • A Criterion Level of 85dB and a Criterion Time of 8 hours
  • No threshold is used so all noise energy is included in calculations, along with no Time Weighting
Measurement function used to calculate noise exposure is Leq ,or LAeq,t. This is equivalent to the LAVG function used in OSHA regs. Noise exposure limits are expressed in terms of an LEP,d or LEX,8h which is an 8 hour value. Again, this is equivalent to the TWA function used in OSHA regs. Dose or % Dose values may be reported along with the LEX,8h values, although this is less common than in the U.S.

Here’s a summary of the differences between the two regulations:

Function OSHA EU (ISO)
Exchange Rate 5dB 3dB
Threshold 80dB No Threshold
Time Weighting Slow No Time Weighting
Criterion Level 90dB 80dB
Criterion Time 8 Hours 8 Hours

Real-world comparison
In this example, a noise dosimeter — worn for 7 hours, 8 minutes and 22 seconds —measured exposure using two independent channels but a single microphone to ensure noise levels calculated for each channel were based on the same information. Channel 1 was configured to meet OSHA Regulations and Channel 2 was configured to meet the EU Directive. Results are listed below.

Channel 1 OSHA Results
LAVG 88.9dB(A)
TWA 88.1dB(A)
% Dose 75%
Estimated % Dose 83%

Channel 2 EU Results
LAeq,t 92.6 dB(A)
LEP,d 92.1 dB(A)
% Dose 511%
Estimated % Dose 573%

Under OSHA Regulations, this noise exposure would be reported as 75% of the daily limit, whereas under EU Regulations, the same noise would be reported as 511% of the daily limit. Differences stem from the fact that the methodology used to calculate the % Dose values varies significantly between the OSHA and EU regulations.

Both measurements sets are correct when reported against their respective regulations. But this highlights the importance of understanding different configurations available from modern noise measurement instruments and how they can significantly affect final reported values.

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