This article applies to centrifugal and axial compressors. Reciprocating, screw, and scroll compressors and ejectors have different considerations. For “dynamic” radial or axial gas compressors the predominant noise source is rotating blade passing frequency. There is also noise due to flow turbulence and vortex shedding from stationary vanes and rotating blades. The blade passing frequency is due to interaction of non-uniform flow. Noise levels increase as power increases, but especially as equivalent tip speed goes higher.