This head has the napa layer on the inside! The head has 2 layers. The outside layer on top that you hit is made of plastic and the interior layer on the inside is napa. Napa is similar to Naugahyde, the material that they make fake leather couches out of. When you use these heads on your surdo you get a nice aggressive attack sound but without the overtones and thin sound of heads with just the plastic layer. The napa muffles it a little keeping the sound nice and warm.
Other napa heads have the napa layer on the outside, the side that you hit, while having the plastic layer on the inside. This works for a while but the napa is not as durable as the plastic and will eventually wear thin and breakdown from repeated beatings. This problem is accelerated by poor surdo playing technique.
They do great in wet conditions! These heads will not "melt" or stretch out if they get wet. These are great for the Pacific Northwest's rainy weather.
I am constantly impressed with the high quality of Timbra's instruments and accessories. They know what they are doing in their factory in São Paulo. All of their drum heads are made by hand and quality tested. Their heads are all tested for material thickness, tension, and pressure so that their heads are consistent.