Why not? Others do!
The NRR is short for Noise Reduction Rating and is the standard reference for hearing protection devices in North America. The objective of the rating is to guarantee the minimum protection that can be expected with a hearing protector. It is not as fair as a PAR.
Complex maths is used to calculate the weighted average over seven octave bands. In a nutshell the software uses use the average attenuation achieved in the sound booth for 20 test subjects minus two standard deviations to arrive at the number. The weighting by octave band is intended to reflect the increased risks of damage to the ear by the lower frequencies.
Statistical theory suggests that in a normal distribution the value of the mean minus two standard deviations should give you a lower threshold that ensures that 97.5 % of the population has a level of protection that is greater than the number that appears on the product.
In practice the NRR is heavily biased by the manufacturers in two ways:
The biased practice has led NIOSH to recommend a de-rating of the NRR by suggesting that the actual protection value should be obtained by subtracting 7 dB from the published number and then dividing the result by two. As an example, a popular foam hearing protector has an NRR of 33. The NIOSH de-rating methodology suggests that the actual protection for this hearing protector is more likely closer to 13 dB ((33-7)/2). The NRR is a rigged game and everybody in the industry knows it. The only people that don’t are the general public.
PAR is short for Personal Attenuation Rating. It is different since it is an objective measurement for each earpiece in the individual subject’s ear. The PAR should therefore not be subject to de-rating because you don’t need to predict the performance of the earpiece in the subject’s ear since you have its actual performance.
We have demonstrated in the past that the attenuation performance of the earpiece does not degrade over time (we measured attenuation over a three- hour period on numerous subjects and found that the PAR actually tends to increase slightly). Also we have demonstrated that over time (a matter of days) the PAR for an individual tends to increase slightly over a short period as the wearer becomes more familiar with the insertion. This suggests that the actual PAR for an individual is more than likely to be slightly above the PAR measured at initial fitting.