The 7dB reduction is a correction to be used with the employee's A-scale noise measurements. Most people are surprised to learn that the current NRR system was developed in the 1980s for use with C-scale workplace noise readings (the C-scale is a broad-spectrum frequency setting on sound level meters and dosimeters). Employee time-weighted-average noise exposures, however, must be calculated with an A-scale frequency setting on the sound measurement equipment (the A-scale is a weighted frequency setting more closely matched with human damage-risk criteria). The bottom line: if the employer has A-scale noise information, but not C-scale, then a 7 dB correction factor is required when using the NRR to assess hearing protection for the workplace.

For more information, check out our SoundBytes newsletter article, The NRR: What's in a Number?