A theory is presented for the discrete-frequency noise generated from axial-flow fans and compressors. The theory describes the noise generation from the fluctuating forces on either rotor or stator blade rows due to the interactions with the adjacent upstream and/or downstream blade rows. The present aim is to give a qualitative description of the total acoustic power for the noise without obtaining the powers for each discrete noise component. For this purpose, a far-field equation is derived from the Ffowcs Williams-Hawkings equation on the assumption that the rotor tip Mach number is small. The equation is utilized in deriving an expression for the total acoustic power. It is shown that the acoustic power includes the important parameters in the study of noise reduction such as the direction of force, the number of blades, and the angular velocity.