There are three
playing positions on a tympani
head.
The most commonly used one is about 4-7 inches from the rim, dependant on the size of the drum you're working with. This produces the characteristic deep, tonal booming sound. This is the position you use unless otherwise instructed.
In some advanced tympani literature, you may be instructed to use one of the other two positions. One is about 1 inch from the rim, which makes a higher pitched "ping" sound. The other is the center of the drum, which produces a dull "thud" sound.
A singular drum is a tympano. Tympani is actually the plural form, but is still used informally to refer to a single drum.