The control of cavity and panel vibration resonance in a room with ventilation duct

Abstract

Buildings with double skin facade or using the solar stack ventilation design to achieve natural cooling are the latest sustainable building designs for energy saving. However, sound can enter buildings through the ventilation ducts or openings. It is, therefore, important to take into consideration satisfactory amounts of both noise reduction and daylight when considering natural cooling systems for buildings. The purpose of this investigation is to design: (1) a silencer with cavity and/or membrane absorbers to control noise in a ventilation duct; (2) a membrane and/or cavity absorber to control Helmholtz resonance in a ventilation space or room. The silencer is designed as a panel absorber system using divided panels with two layers, a transparent plastic membrane and a micro-perforated Plexiglas sheet. The two-layered panel absorber system extends the absorption to lower frequencies as well as maintaining good mid-frequency absorption, and hence becomes a desirable broadband absorber. If ventilation openings exist in a room, it is possible for air flow to generate an infrasound of a significant magnitude in the openings to irritate the people inside. Helmholtz resonance can be controlled using a membrane structure that is either cavity backed or not. There are significant improvements in low frequency sound reduction when the Helmholtz resonant frequency of the ventilation openings matched with the membrane vibrating resonant frequency. Formulae with satisfactory accuracy are derived for the panel absorber system with combined layers and the ventilation space with a membrane structure. The agreement between the experimental and theoretical results demonstrates the effectiveness of the formulae. The new silencer design and membrane absorber have been proved to achieve desirable broadband noise reduction, offer excellent aesthetical properties and at the same time provide a healthier indoor environment in sustainable buildings.