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

Abstract

Some ceiling-mounted ventilators have a sound-attenuating function due to the resonant effect that takes place within a chamber that consists of a rectangular outer cover and a cylindrical fan unit. In order to control the resonant properties in a reactive manner and maximize the sound-attenuation effect of this resonant system, the chamber is separated by a partial shutting plate, called a "separator," located at the end of an inner cylinder-modeling a fan unit-into two spaces that are expected to approximate a Helmholtz resonator with two degrees of freedom. Variations in resonant properties are shown, experimentally obtained by modulating the dimension of the separator. In this model, the second resonance frequency can be widely changed depending on the clearance between the two spaces, without attenuation of the resonant effect. This is meaningful because the second resonance frequency can be shifted into a frequency range that corresponds to the main component of the radiated sound from an actual ventilator. In addition, it is indicated that the resonance frequencies can be approximated by a simple analytical approximation. Furthermore, detailed considerations of sound fields of relatively delicate models concerned are provided using numerical calculation (BEM) and the effectiveness of the chamber is summarized.

Attenuation of Radiated Sound from a Duct End Attached to a Resonant Chamber Having Two Degrees of Freedom and Modeling a Ceiling-Mounted Ventilator

“…1), a space is formed as a combination of domains I and II. Based on a previous study (2), (3) , the shape of this space was simplified axisymmetrically. The outer cover is replaced by a rectangular open box with a square cross-section, and the fan casing is replaced by an inner cylinder .…”

“…In a previous study (3) , a chamber consisting of domains I and II was placed in an infinite duct. In the present study, the chamber is located at one end of a hemi-infinite duct.…”

“…The inner diameter of 124 mm and height of 54 mm of the cylinder correspond to the dimensions of the fan casing, and the side length of 130 mm of the square cross-section corresponds to the size of the outer cover. Based on the previous study (3) , the boundary between domains I and II is partially closed using a plate called a separator. The thickness of the separator is fixed at 2 mm, and the diameter d S is varied, i.e., the clearance b S is varied.…”

“…Based on the characteristic shape of the chamber, the chamber can be separated into two domains using a plate that partially closes off the intermediate area of the chamber. By investigating the property of this chamber experimentally, analytically, and numerically, its basic function was explained by a Helmholtz resonator having two degrees of freedom (3) .…”

“…The difference in the properties of the chamber located at one end of the duct and those of the chamber located in an infinite duct are revealed. In addition, the characteristics of the chamber, which basically acts as a Helmholtz resonator having two degrees of freedom, are described more precisely, as compared to a previous study (3) , by summarizing the frequency dependence of the sound pressure in the chamber.…”