We experimentally study a high-speed synthetic jet actuator consisting of a cylinder with an orifice plate at the top and a piston driven by a motor. The experiments are performed for four orifice plates with a single hole or multiple holes with different diameters. The characteristics of the synthetic jets are investigated with cylinder pressure measurement and flow visualization based on the shadowgraph method, where the Mach disks in the shadowgraph images are used for estimating the Mach number of supersonic synthetic jets. A total orifice area on the plate affects cylinder pressure and jet Mach number. The number of the orifices has a direct influence on the downstream region where the jets from different orifices interact with each other. A smaller total orifice area results in a higher jet Mach number, a larger spatial extent of Mach disks, and an earlier formation of the Mach disks in one cycle. The influences of total orifice area are well explained by larger/smaller values of maximum/minimum values of the cylinder pressure for a smaller total orifice area. It is also found that the maximum jet Mach number is represented as a function of the maximum cylinder pressure even for different orifice configurations. Furthermore, for a given operating frequency, the maximum cylinder pressure increases almost linearly with the stroke length divided by an effective diameter of the total orifice area independently of the number of the orifices.