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Facts About Noise #5

What’s wrong with Naïve Subject Fit and Experimenter Supervised Fit

You are really getting involved now! You know the difference between Naïve Subjects and Supervised Subjects. When hearing protectors are tested in a laboratory 10 subjects are tested three times at nine different frequencies. All this date is translated into mean attenuation values and standard deviations for the nine frequencies. This accumulated data is then compared to test population data collected without the use of hearing protection. The next step is to calculate the NRR from these numbers.

All test reports in use today for the calculation of NRR values are based on Experimenter Supervised Fit, meaning that the individual performing the tests is supervising the test subjects, in order to make sure that they are following the manufacturers usage instructions. This method results in NRR values that reflect a hearing protector’s performance, when it is used as directed. Disadvantages: hearing protectors are not used correctly in the field, and the NRR values are therefore optimistic. Advantages: We are actually testing the hearing protectors performance, without influence of inaccurate use.
We will in a few years start to see NRR values based on Naïve Subject Fit, meaning that the test subjects have no prior experience with hearing protection, and they will get no other instructions than the printed usage instructions. It is known that the results will be lower NRR values. Disadvantages: We will be measuring the difference between different test populations as much as we will be measuring hearing protectors. Test results will vary by three or four dB up or down, from one test to another. Test the same protector at two different laboratories or twice at the same lab and you will get great differences in NRR. Larger manufacturers will have the ability to test the product many times until they get a good (lucky) test. Advantages: Lower NRR’s will be achieved, giving end users values that are deflated and closer to real world performance.
In our modest opinion the Naïve Subject Test Protocol makes very little sense. We will take one step forward and two steps backwards. In the 1970’s several studies were performed, comparing the results of different laboratories testing the same product. There were almost no correlation between the results, although the laboratories were attempting to follow the same standard. As the procedures were reviewed in the testing community, and three laboratories have become certified, these differences have been tremendously reduced. We at Elvex have for instance tested 10 new hearing protectors during the last three years at a certified laboratory in the U.S. and at a certified CE laboratory in Europe. The consistency between the results is just amazing, considering the situation just a decade ago. Our CE reports have with one exception reported +4 dB, as compared to the U.S. certified report (example: US NRR=29, CE SNR=33). The exception reported +3 dB.

If you have an opinion about the test population question, please let us know. We will periodically publish the results of our informal poll on this page. Will lower NRR's help your company's hearing conservation program?

Facts About Noise #5

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