ANSI S12.6-2016 is the most recent US standard that describes a method of evaluating hearing protective devices. 20 human subjects are required for insert-type hearing protectors, cap-mounted muffs and semi-aural devices. 10 human subjects are required for other types of muff-type devices and for helmets. 7 test frequencies are used. The Noise Reduction Statistic (NRS) is then typically calculated from these data. ANSI S12.68-2007 describes a rating system that requires ANSI S12.6-2016 data for rating calculations.