Noise generated by a fan consists of the rotating noise and the turbulent noise. These two kinds of noise have entirely different characters with respect to decay in an axisymmetric pipe. In this paper the generation and decay of these two kinds of noise are experimentally examined by changing various factors, such as the suction pipe geometry, the location and the thickness of struts, and the combination of the number of impeller blades and the number of struts. The rate of decay of the rotating noise is predictable and in most cases the noise quickly decreases in proportion to the distance from the rotor, providing that the number of impeller blades and the number of struts are properly chosen. Therefore, the noise observeda way from a low pressure axial flow fan is mostly the turbulent noise.