The Noise Reduction Rating (or NRR) method is the easiest of the three common methods. This method should be acceptable unless the noise is tonal or is dominated by low frequencies. In this case use either the HML or Octave Band method.
In order to estimate the sound level at the ear using the NRR method you need the following:
In the USA, the use of the NRR value is recommended by both OSHA and NIOSH, but the method is slightly different and gives different results.
|A-weighted measurements:||Lprot = L - (NRR - 7) / 2|
|C-weighted measurements:||Lprot = L - NRR / 2|
|A-weighted measurements:||Lprot = L - (NRR - 7)|
|Foam Earplugs:||Lprot = L - (NRR * 0.5)|
|Other Earplugs:||Lprot = L - (NRR * 0.3)|
|Earmuffs:||Lprot = L - (NRR * 0.75)|
Lis the measured sound level or TWA
NRR is the Noise Reduction Rating provided by the hearing protector manufacturer
Lprot is the estimated sound level at the ear when wearing the hearing protection
You should aim to reduce the sound level at the ear to between 70 and 80 dB(A).
Under-protection - If the level at the ear is still above 80 dB(A) then the protectors are not providing adequate cover for longer exposure times.
Over-protection - If the level at the ear is below 70 dB(A) then the worker is being over-protected. This can result in difficult communication and the inability to hear warning alarms. The hearing protection is also likely to be heavier and more uncomfortable then necessary.
For both OSHA and NIOSH, the NRR calculation can be made using either "A" or "C" weighted sound level. For noise with noticeable low-frequency content, we recommend the use of "C" weighting where possible, or switch to the HML method.
You can use our online NRR hearing protection calculator to carry out this process and produce a simple report.
We have had calls in the past from customers having difficulty calculating the sound level at the ear using the NRR or HML method. The reason is usually that they are using the C-Weighted Peak measurement, rather then the C-weighted Lavg, TWA or Sound Level.
Most meters designed for occupational noise will clearly display the A-weighted Sound Level and the C-weighted or Z-weighted Peak together as these are the first measurements you use when making an assessment, so the mistake is very easy to make.