SISOM 2010 and Session of the Commission of Acoustics, Bucharest 27-28 May NOISE REDUCTION IN INDUSTRIAL INSTALLATIONS USING A SILENCER Ovidiu VASILE, Nicolae ENESCU POLITEHNICA University of Bucharest ovidiu_vasile2002@yahoo.co.uk, nae_enescu@yahoo.com This paper investigates the acoustic performance of a reactive silencer (muffler) for three special cases using numerical and experimental techniques. The plane waves based models such as the transfer matrix method (TMM) can offer fast initial prototype solutions for silencer designers. In this paper, the principles of TMM for predicting the transmission loss (TL) of a side-branch muffler are briefly presented. The method is applied for each silencer configuration and the numerical predictions are compared with the results obtained by means of an experimental setup. Only stationary, non- dissipative silencers are considered. Keywords: silencer, transmission loss, transfer matrix method. 1. INTRODUCTION A silencer is an important noise control element for reduction of machinery exhaust noise, fan noise, and other noise sources involving flow of a gas. In general, a silencer may be defined as an element in the flow duct that acts to reduce the sound transmitted along the duct while allowing free flow of the gas through the flow passage. A silencer may be passive, where the sound is attenuated by reflection and absorption of the acoustic energy within the element. An active silencer is one where the noise is canceled by electronic feed forward and feedback techniques. In this paper, we will examine three cases of reactive (passive) silencers, also called mufflers, using numerical and experimental techniques. The detailed design procedures for mufflers are available in the literature (Munjal, 1987). [1] The multi-chamber muffler is one type of silencer used to reduce noise emission in a restricted frequency range from a mechanical system. This kind of muffler consists of a Helmholtz resonator connected to the main tube through which the sound is transmitted. The silencer acts to reduce sound transmission primarily by reflecting acoustic energy back to the source, so it is classed as a reactive silencer; however, some energy is dissipated within the acoustic resistance element of the silencer. This paper investigates the acoustic performance of three configurations for the side-branch muffler with are shown in Figure 1, cases a, b, c. The expansion chamber muffler consists of one or more chambers or expansion volumes which act as resonators to provide an acoustic mismatch for the acoustic energy being transmitted along the main tube. Reactive silencers consist typically of several pipe segments that interconnect a number of larger diameter chambers. These silencers reduce the radiated acoustical power primarily counting on to the impedance mismatch, that is, by allowing the acoustical impedance discontinuities to reflect sound back toward the source. Essentially, the most relevant discontinuities are commonly achieved by: (a) sudden cross-sectional change (expansions or contractions), also by (b) wall property changes (transition from a rigid wall pipe to an equal-diameter absorbing wall pipe), or by (c) any combination thereof. The use of a silencer is prompted by the need to reduce the noise radiated from a source but in most applications the final selection is based on some trade-offs among the predicted acoustical performance, the mechanical performance, the volume/weight ratio, and the cost of the resulting system. The impact of the silencer upon the mechanical performance of the source is determined considering the change in the silencer back pressure. For a continuous-flow source, such as a fan or a gas turbine, the impact is determined from the increase in the average back pressure; by contrast, for an intermittent-flow