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. In order to control the resonant properties in a reactive manner and maximize the sound attenuation effect of that resonant system, we have experimentally studied the properties of modeled resonant systems attaching a plate (or plates) of arbitrary shape in the resonant chamber. In this study, we separate the chamber by a partial shutting plate at the end of the inner cylinder modeling a fan unit into two spaces that are approximately expected to behave as a Helmholtz resonator of two degrees of freedom. We show the variations of the resonant properties experimentally obtained by modulating the shapes and dimensions of the plate, and verify the resonant frequencies by a simple analytical approximation. In this model, the second resonant frequency can be widely changed depending on the clearance between two spaces without attenuation of resonant effect.
