a cylinder;

a piston configured to axially reciprocate in the cylinder;

a back cover disposed at a rear of the piston, the back cover defining an opening at a central area thereof in a radial direction;

a first muffler unit coupled to the piston, the first muffler unit comprising (i) an inner guide disposed in the piston and (ii) a first intake muffler disposed between the inner guide and the back cover; and

a second muffler unit comprising (i) a second intake muffler that is in fluid communication with the first intake muffler and is coupled to the opening, (ii) a muffler body surrounding the second intake muffler, and (iii) a muffler cover disposed between the muffler body and the back cover,

wherein an outer circumferential surface of the second intake muffler defines a first communication hole that fluidly communicates an inside of the second intake muffler with a first space that is defined between the second intake muffler and the muffler body, and

wherein the muffler body defines a resonance communication hole that fluidly communicates the first space with a second space that is defined between the muffler body and the muffler cover.