108 Property of a resonant chamber as a model of a ceiling-mounted ventilator : Attenuation of radiated sound using a chamber of two-degrees of freedom located at one end of a duct
Details
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 separated the chamber by a partial shutting plate at the end of the inner cylinder into two spaces that are approximately expected to behave as a Helmholtz resonator of two degrees of freedom. In our former research, a chamber having a simplified symmetrical shape modeling the real chamber concerned was located in a duct of infinite length. In this research, we attach the chamber at the end of a duct and examine the properties of the sound radiated from the duct end to a free space experimentally and numerically (BEM). Every effective frequency for sound attenuation is the same between two cases : (i) the chamber is located at a duct end ; (ii) the chamber is located in an infinite duct. However, some differences between two cases are also revealed. The situations of the differences are precisely studied through examining the sound fields in and around the chamber. The most notable difference between two cases is that the sound is amplified by means of the chamber in a low frequency range in case (i). Fortunately, this is not serious for a real ventilator because the corresponding frequency range is not dominant for sound radiation of a real ventilator.
