Things applied to the head alter its sound

Coating is the most common application. It is a fine material sprayed onto the head to simulate (somewhat) the feel of calf for brushes to have something to grab onto. Coating warms the sound up a bit, and takes away some of the high end harshness and overtones. Most players use coated snare drum batter (top, beating side) heads. Many also use coated heads as batters on toms and bass drums. They can be used as resonant (bottom, non-played side) heads as well, but not as frequently. White coating is most common, but at least one company makes black coating (Aquarian’s DeJohnette Series)

Dots are also frequently used. They consist of an extra piece of Mylar stuck in a circle (4"-10" are most common) on the center of the head. They mute some overtones, and make a more fundamental-heavy sound. However, they also can make toms sound "boingy" due to the thickness of the head with the dot. Many players use a head with dot on the underside on their snare drum. This so that they can enjoy the benefits of the dotted head, without their brushes getting stuck on the added piece of Mylar.

Underlays are strips of Mylar bonded at the collar on the underside of the head, usually about 1-2" thick, which are much like a second ply with the center cut out. They muffle high overtones, and control some resonance. One company (Evans EQ series) makes a bass drum head with an underlay, and a second removable ring which sits between the head and the underlay. This allows the player to have more control over the amount of muffling s/he wants. (Author’s note: I often wonder why companies don’t make head with underlays on top…overlays?? With the extra Mylar on the bottom, the actual head does not touch the drum’s bearing edge, which I do not appreciate -DR)