Reduction of Discrete‐Frequency Fan Noise Using Slitlike Expansion Chambers

Abstract

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.

Sound Attenuation by a Resonant Effect of a Rectangular Box Containing a Cylinder as a Model of a Ceiling-Mounted Ventilator

“…Since the purpose of the present study is to clarify the basic performance of the chamber, the chamber is set in a long circular duct having the same diameter of the fan inlet. This type of experimental configuration has been confirmed to be meaningful for examining the performance of a chamber facing an open space (4) . Although the duct has a finite length of 4 m, it can be considered to have an infinite length in an appropriate frequency range for the measurement by attaching absorbent cones at both ends of the duct.…”

Sound-Attenuation Effect of a Partially Shut Rectangular Chamber at the End of an Inner Cylinder, Modeling a Ceiling-Mounted Ventilator

“…As the purpose of this study is to clarify the change in the chamber's property caused by the separator, the chamber is positioned in a long circular duct. This type of experimental configuration has been confirmed to be useful in examining the performance of a chamber facing an open space (3) .…”

Modulation of Sound Attenuation Properties of a Short Expansion Chamber by Changing the Temperature in It

“…Although a shallow branch is preferable to control the temperature in it, its effect is limited only to a discrete frequency sound in a high frequency range. Nevertheless, this type of shallow short chamber can be utilized for, e.g., a fan noise radiated from a vacuum cleaner (6) .…”