The difference between the batter head and the resonant head
The batter head is located on the top of the drum. It is the one on which you tap with your drumstick. The resonant head (or reso' head for the initiated) is the one that is located on the lower part of the shell, opposite to the batter head.
The role of the resonant head
As its name indicates, the resonant head is there to resonate, while the batter head is there to transform the energy you give to your drumstick stroke into sound. The roles of the two drumheads being very different, the settings and the nature of the resonant drumhead are to be considered independently of the batter head. The resonant head is there to amplify the sound caused by the batter head and the vibrations of the shell. The setting of the resonant head will therefore play a major role in the sound of your kit.
Thin drumhead, thick drumhead, what are the differences?
The thinner the resonant head, the clearer the sound and the more immediate the response. The thicker the drumhead, the deeper the sound. This rule is valid for all drumheads, whether for snare drums, toms, bass drums, in striking or in resonant drumheads (or timbre skin).
How many plies for a resonant head?
Unlike the batter head, resonant heads and snare drum resonant heads are always single-ply heads. The snare drum resonant head is even thinner than the toms and bass drum, to respond even more directly to the impact of the drumsticks and to accurately reproduce the sound of the timbre in contact with it.