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

摘要

The quietness of a ceiling-mounted ventilator consisting of a rectangular outer cover and a cylindrical fan unit is investigated. In order to confirm the sound-attenuation effect using a space between the cover and the fan unit, several properties of a chamber having a simplified symmetrical shape, which models an actual ventilator, have been investigated. In a previous study, in which the chamber was located in an infinite duct, the chamber was separated into two domains by a partially shutting plate to control the chamber performance. In the present study, this chamber is attached to one end of a duct and the properties of the sound radiated from the duct end into free space are investigated experimentally and numerically using the boundary element method. The effect of the chamber is similar when located at a duct end and in an infinite duct. In a certain low-frequency range, however, the chamber has an adverse effect in the duct-end case. Fortunately, this is not serious for an actual ventilator because the corresponding frequency range is not dominant for the sound radiated from an actual ventilator. Although the sound pressure in the chamber is quite different between the two chamber locations, the sound pressure ratio of the two domains is the same. Although the chamber performance depends on the chamber location, the physical aspect of the chamber as a resonator can be said to be independent of its location. When the opening connecting the two domains is moderately narrow, the chamber behaves approximately as a Helmholtz resonator having two degrees of freedom.