Excellent! Now I think I understand this better! As you say the...

Excellent! Now I think I understand this better! As you say the graph is of instantaneous pressure, not pixel displacement. It is only when the pixels move, and thus move air, that they are generating pressure. When they reach their maximum displacement they stop creating pressure, so if they stopped at that point the air pressure immediately adjacent to the piston face would dissipate via air decompression in all directions, and this would follow a decay curve depending on how easily ambient pressure dissipates. So the pressure profile of a piston movement's pressure decay rate may not be a mirror image of its pressure rise rate. The pressure pulse of the pistons is also not going to be sinusoidal in form, but closer to a saw tooth wave with a fast and steep front edge rise and a curved decay profile at the end. In that sense the graphic may be a bit misleading, because we don't get to see any of that. It is also likely that the pulse profile will go above the smoother sound wave as it is the "average" pressure of the resultant pulses that recreates the sound wave profile.

In practice I think what is happening is that for every clock tick another group of pistons moves "up" and adds more pressure, so the air immediately adjacent to the piston (and the array face) is re-pressurised by the next group of moving pistons. This is what builds the increasing pressure zone at the face of the array. This continues until the largest group of pistons moves to their "up" position, and the pressure is at its highest. After this point smaller groups of pixels actuate in each clock tick, so while more pressure is being exerted the total pressure at that instant is less than that of the previous step. When the last and smallest group of pixels has moved, all of the pistons are at their "up" position, and there is a moment when there is no positive pressure at the face of the array. Then as the groups begin to reverse their movements in sequence, the pressure around the array becomes increasingly negative as the array "pulls' the air "down" towards it. When it is moving down in the negative phase the pressure profile as a whole will be the mirror image of the pressure profile of the forward phase. This is the familiar sinusoidal pressure wave that is sound. However it is being produced it is ultimately the same sound wave whether it comes from a physical speaker membrane or a DSR array of pixel pistons.