The Environmental Protection Agency requires all noise-reducing products sold in the United States to have a Noise Reduction Rating or NRR. The NRR estimates a wearer's noise exposure and is measured in decibels; the higher the number, the more effective the product. The NRR formula is complicated, and you can find out more about it here.

Typically, ear plugs are tested in controlled laboratory conditions. The devices are placed in a tube with a microphone at the end, and then a technician measures the amount of noise that the plugs reduced. According to the Ear Plug Super Store, the maximum known NRR is 36, and that requires the user to wear both high-quality foam ear plugs and ear muffs.