Studies on a new type of silencer for air ducts

Abstract

The research work deals with a study of a specific type of silencer previously designed and tested by Shenoda. It consists of a rectangular duct, which is treated at its side walls with a gradually variable flow resistance. It was realized by increasing the perforated area along the duct wall combined with an homogeneous sound absorbing material. Based on the assumptions that only plane waves propagate in the duct, ignoring the effect of the mean flow velocity (very low) and applying linear acoustic equations, the sound propagation in such a silencer was studied. Therefore the wave impedance, the sound reflection coefficient at the input of the silencer, and the transmission loss introduced by it were calculated, measured, and compared with a silencer of comparable dimensions which is treated with a constant flow resistance along its length. To optimize the design different configurations were theoretically studied, computed, and compared. An important result: The silencer performance depends on a unique design parameter, namely, the silencer characteristic area. The methods of realizing a variable wall flow resistance were introduced. The effect of using different kinds of sound absorbing materials and changing the graduality of the variable flow resistance were measured.