Costescu M., Platon S.

By principle of operation, there are two main categories of silencers: the active silencers and the reactive silencers. In this paper, the category of interest is that of active silencers, usually made as a duct lined with sound absorbing material, which is the main element for noise reduction. When the sound waves are spread in the sound absorbing layer, there are losses depending on the viscosity friction when the air is flowing through the material pores, the internal friction during the alteration of material structure, as well as on the heat exchange between the pores and the material structure. As the sound wave reaches the surface of sound absorbing material, the air flow is divided between different pores, being reduced to their sizes. Thus, the wave speed rises comparatively with the free wave speed, and the sound energy is effectively absorbed. The active silencers may be used to reduce the noise of ventilation equipment, air conditioning device or other aero dynamical equipment as well as to occlude the holes in sound insulating enclosures or cabins, to allow the air intake without impair the sound insulation performances. The authors considered this category, because the great importance of such a silencer efficiency. A sound insulating cabin or enclosure without a silencer or with a silencer of low efficiency mounted in the ventilation holes (those that allow the air flow to obtain optimum exchanges for workers or to cool the machines enclosed) becomes on ineffective solution for noise reduction, however the structures of cabin or enclosure walls are efficient from de point of view of their sound insulating properties. The authors present three types of active silencers, sized to be used in the cases presented above, namely to be mounted in the ventilation holes of the sound insulating cabins or enclosures.