The results of an investigation of acoustical effectiveness of different dissipative duct silencers packed with lightweight porous materials are discussed. The comparison of experimental results with that given by existing theories and empirical equations are also available.
The noise in ducts of air-conditioning and ventilation systems is generally reduced with the help of dissipative silencers, i.e. silencers made of absorbing material that damps the sound over a wide frequency range.
Sound attenuation in dissipative silencers depends on their design, size, sound frequency, nature of sound absorbing material, type of protective cover as well as on the velocity and direction of air flow.
The acoustical effectiveness of different splitter silencers has been investigated in the USSR by Belov, Leizer, Malyuzhinets, Razumov, Yudin; and in other countries by Dyer, 1 Doelling, 2 Kerka, 3 King, 4 Kurze, 5 Lippert 6 and others. However, the published data are inadequate and the number of types and constructions investigated are not great in number. Besides, the available data are difficult to use since there are no uniform techniques for the investigation of silencers.
When the air flow is large, which is characteristic of central air-conditioning and ventilation systems, splitters, egg-grates and chamber silencers, are used. An analysis of conventional silencers showed that the splitter type silencers are the most common, for they offer weaker air flow resistance and produce less noise than any other sound dampers. The efficiency of splitter type silencers is not inferior to that of egg-grate type silencers but is even better in certain cases.
The main source of noise in ventilation systems is the fan. Fan noise has a wide frequency spectrum in which low frequencies are the most dominant. We measured 47 Applied Acoustics (3) (1970)--O Elsevier Publishing Company Ltd, England--Printad in Great Britain