Hearing Protection and Noise Reduction Ratings
American Work Safety provides disposable and reusable earplugs, rated in Noise Reduction Rating or NRR. An NRR of 20 dB, for example, would reduce the sound of a typical chainsaw (around 100 dB) to approximately 80 dB (the volume of a ringing telephone) under ideal circumstances.
OSHA recommends that hearing protection be worn under any circumstances where you are exposed to 100 dB or more, and recommends a maximum expoure time of 8 hours for 90 dB, 4 hours for 95 dB, 2 hours for 100 dB, 1 hour for 105 dB, 30 minutes for 110 dB, and 15 minutes for 115 dB.
Each 3 dB represents an effective doubling in sound pressure (or protection); using NRR 20 dB earplugs with NRR 20 dB earmuffs would provide a total effective NRR of 23 dB.