The shell is the beating heart of your drums. The number of plies affects how readily energy is transferred from the heads to the shell. This single factor profoundly affects the tonal characteristics and projection of the drum.
Thin shells (4 ply, 5mm and 6-ply, 5.4mm) enable relatively easy energy transfer from the heads to the shells, causing the shells to vibrate. This vibration imparts a very rich "wood" tone to the overall sound that can be most appreciated in near-field applications, especially in recording.
Medium shells (6 ply, 7.5mm) have greater stiffness and resist energy transference from the heads. With less shell vibration, a trade-off is achieved: the sound is slightly "cooler" than thinner shells, but the projection is greater. Drums made to this thickness are ideal for general-purpose applications and situations requiring more "horsepower."
Thick shells (8-ply, 10mm, and 10-ply, 12.5mm) are extremely "efficient" and allow most of the player's energy to be focused towards the audience. These drums are ideal for coliseum-type venues and other applications requiring high sound pressure levels. Snare drums made in this thickness rival metal snare drums in intensity and projection.