If you've shopped for ear plugs, you've probably come across the letters 'NRR.' This stands for noise reduction rating and refers to how much any given ear plug or muff blocks out noise in real world circumstances.
Measured in decibels, the NRR is often included in the marketing materials of any given ear protection device. A higher NRR blocks more sound, a lower NRR blocks less. Kind of like a ruler for your ears.
Screenings are typically conducted by OSHA, the American body overseeing occupational health and safety. They run hearing products through controlled laboratory experiments during which devices are tested in order to determine real world capabilities.
NRR is particularly important for those working with loud machinery and their employers, as a certain NRR might be required for ear safety.
NRR is also affected by their ear plugs or ear muffs to communicate. When hearing protectors are not worn 100% of the time protection drops to less than half the stated NRR protection level after only 30 minutes during eight hours of noise exposure.