Modern methods for attenuation in ductwork and silencers

Abstract

Silencers in power plant ductwork should find a good compromise between the (generally rather high) needed sound transmission loss and the static flow pressure loss induced by them. The annual costs of the electric power consumption created by a silencer may exceed the costs of investment. Modern design possibilities will be displayed. All of them can be computed with the instrument of mode analysis. On the classical background, that attenuation is concentrated in broadband silencers with straight, homogeneous linings, the following improvements will be illustrated with examples. First, because a broadband silencer needs thick linings for low frequencies and small free duct width for high frequencies, both increasing the static pressure loss, subdivide the silencer in sections for different frequency subbands, with stepping lining thicknesses and duct widths. It will be indicated how such steps can be computed Second, insert the necks of Helmholtz resonators between lining sections designed for medium and high frequencies. The resonator volumes are behind the lining sections. The increase in silencer length is very small; the increase in thickness is small, because the main extension of the resonator volumes is in the duct direction. Such resonators may be designed for an efficient low-frequency additional attenuation. Third, duct elbows with their static pressure losses exist anyhow in most ductworks. They can be designed as very efficient high-frequency silencers, thus permitting to avoid the narrow lining spacings in the silencer. Criteria for good corner silencers will be illustrated. Fourth, most silencers in a ductwork begin and end with a conical duct transition. If they are rigid, their combined effects are small. If, however, they are suitably lined, they can produce remarkable additional attenuation.