A link to OSHA's Noise Regulations at 29CFR 1910.95 for General Industry and is law.

OSHA Technical Manual (OTM)
Section III: Chapter 5 on noise.

OSHA PDF - Hearing Protector Fit Testing: Ensuring Appropriate Noise Protection at Work

PDF - From the Canadian Centre for Occupational Safety. 

OSHA PDF - Hearing Conservation. It's a small summary guide about noise 

Online hearing protection calculator from Note: Use the NRR calculation per OSHA.