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.
