Noise attenuation in air-conditioning ducts

Abstract

Air-conditioning systems have introduced in our homes, offices, departments etc. the commodity of a controllable temperature/humidity. This however necessitates having closed windows which in turn often eliminates the sounds of traffic and other outside activities. This factor brings more to attention the noise made by the ventillating system. Noise control in a building should begin at the primary planning stages. Some acoustical problems could easily be avoided eg. those due to location of offices near the machine shop or the air-conditioning plant. In Malta most buildings are rarely planned with a specific air-conditioning installation in mind. Thus when it comes to designing silencers which can handle the required air flow and produce the desired attenuation, there is usually insufficient space. If the higher noise levels are unacceptable, then more efficient devices must be introduced to control the noise generation. Noise in a ventilation system can be produced by bends, diffusers, reducers and grilles but the fundamental noise source is the air circulation plant which is generally an arrangement of axial or centrifugal fans. The fan noise can be reduced to the desired level by applying sound attenuation devices between the fan and the supply duct. The most basic sound attenuator is the lined duct and this is the type of attenuator generally used. The lined duct silences the noise in proportion to the amount of acoustic absorbent material it contains. Thus the longer the lined duct the better the attenuation. This silencer though cannot be defined as efficient or compact when a high level of attenuation is required. Additions to the lined duct such as porous material splitters and resonators were analysed and the results tabulated and plotted. All the attenuators constructed were tested at different air-flows. The experiments were carried out as much as possible according to British Standards such as BS848 Fans for General Purpose and BS4718 Silencers for Distribution Systems. The experiments were carried out at the TIP factory.