The two clear 7mil plies are glued and sealed only at the edges and tame unwanted overtones while focusing on the low frequencies. These heads are designed for loose tuning and deliver that low, fat, punchy tom-tom sound that was a big part of classic rock and roll records. Frequency control that adds focused power is built right into this head.  The PFII is a great head to use to upgrade and immediately improve the sound of low-priced kits.