Silencer Design for the Control of Low Frequency Noise in Ventilation Ducts

Abstract

The control of noise propagating along ventilation system ducts has always been an important issue in the building and vehicle sectors. This problem is generally tackled by selecting noise-reducing components with a suitable transmission loss, possibly verifying their effectiveness at a later time. The aim of this article is to characterize the nature of the problem and propose a design approach focusing directly on the perceived effect, that is, on the sound pressure level downstream of the outlet. Because the nature of the noise emission depends on various generation mechanisms, different methods can be applied. Usually, it is more difficult to realize good attenuations at low frequencies because of the limits of sound absorbing materials in such frequency range. For this reason, the ability of reactive components to attenuate the noise below the cut-on frequency will be investigated. This goal is reached by applying the transfer matrix approach to a duct system, with the implementation of the transfer matrices of each single element, and then assembling a system capable of acoustically describing the source and the duct structure. The coupling between the duct system with source and receiver impedances allows one to predict the sound pressure level at a given distance from the outlet. The proposed methodology is implemented in a user-friendly calculation tool with possible academic and professional application. Predictive capability, usability, and intuitiveness of the proposed design procedure are validated against experimental results by real potential users, who express positive feedback.