The Noise Reduction Rating (NRR) measures how much a hearing protection device (HPD) reduces sound levels in a lab test. However, it doesn't consider the loss of protection when HPDs aren't fitted properly or worn for the entire exposure time. For many users, the NRR printed on the label overestimates the actual protection in real-world situations.
The NRR on the label is based on a lab test where the HPD is fitted on 10 different people by the tester. This method often results in high ratings, up to 33 dB. Research shows that these ratings are higher than what many users actually experience. Therefore, it's recommended to adjust the NRR before estimating the HPD's effectiveness.
To adjust the NRR:
- Subtract 7 dB if noise is measured on the A-weighted decibel scale (dBA).
- Divide the result by 2. This is called "derating".
In the future, a new method called the NRR (SF), or subject fit, will be used. In this method, subjects fit their own HPD according to the manufacturer's instructions. The NRR (SF) is usually a lower rating than the current NRR, and it may be closer to the actual performance of the HPD.
