Reduction of Discrete-Frequency Fan Noise Using Slitlike Expansion Chambers

As is generally known, discrete-frequency noises are radiated from fans due to rotor-stator interaction. Their fundamental frequency is the blade-passage frequency, which is determined by the number of rotor blades and their rotating speeds. To reduce such noises, several types of silencers have been designed. Among them, the authors noted a slitlike expansion chamber (hereafter referred to asslit, for simplicity) and have studied its performance. A slit is a simple expansion chamber with a very short axial length that is placed in a duct. A slit with a circular cross-section that is concentric with a circular duct may be studied using the same interpretation as is used for a side-branch resonator muffler (closed-end tube connected to a duct); that is, the resonant frequency of a slit depends on its depth (with an open-end correction). It is expected, hence, that a slit might be applicable as a simple and axially compact silencer that is effective on discrete-frequency noises. In this article, the properties of a slit are introduced, and the applicability of a slit to actual rotating machinery is described using experimental data.