These ear muffs will fit most hard hats and helmets including Kask Plasma, and the Petzl Vertex. Developed for the ultimate protection in the most demanding noise environments. The OPTIME 105 features added mass and volume, plus a unique “double-shell” ear cup design (two cups connected via a foam inner layer to reduce structural resonances) to provide the maximum in noise reduction throughout the full range of low and high frequencies. NRR 27 db.