Pneumatic Piston (Impacting)
Piston model are either non-impacting or impacting models, with the former being preferred for most applications. In both types, a piston reciprocates inside a cylinder with internal ports giving the back & forth motion without the need for special valve arrangements.
Impacting models have metal-to-metal contact at each end of the piston stroke, which can be noisy and increas stress on the hopper wall. Continuous impact models are also available with a small air cushion to limit the noise to some degree but this also limits the force output. These models can be adjusted for frequency but by their nature cannot be adjusted for amplitude.
Single impact models use a piston held against a spring by air pressure. On release of this pressure the spring impacts against a strike plate.