Abstract
Some ceiling-mounted ventilators have the function of sound attenuation by a resonant effect in a chamber consisting of a rectangular outer cover and a cylindrical fan unit. The properties of the chamber have been studied using several simple models of the chamber by measurements and numerical calculations. The chamber consisting of two specific regions basically behaves as a Helmholtz resonator of two degrees of freedom. In this research, the second region of the chamber is filled with an absorbing material (a fibrous material). The effect of it on the sound attenuation properties of the chamber is studied experimentally using an axisymmetric chamber located at the end of a hemi-infinite duct. The frequency response of the chamber changes from that of two degrees of freedom to that of single degree of freedom as the density of material increases. While the sound attenuation level falls around the resonant frequencies, it rises in the intermediate frequency range. The properties are also examined by numerical calculations using a FEM software that gives reasonable results to describe the measured results. In the present model, the flow resistivity of the material is the decisive valuable among the several valuables defining a porous material. As the effect of the absorbing material, the resonant character diminishes in the sound field of the chamber.
