Attenuation of Blast Waves in a Duct

Abstract

With recent increase of cars, the noise problem is caused by the exhaust sounds generated from exhaust pipes, which consist of weak pressure waves called blast waves. To diminish the noise, silencer is installed in front of the exhaust pipe. In the present study reflectors were installed in the high pressure portion of the shock tube to generate blast waves, and as silencer models, three basic types of expansion regions are combined with four types of perforated panels and an acoustic material (glass wool). The pressure decay was investigated by transmitting factor and reflection factor to the incident blast wave, together with pressure sensors and high-speed movies by the shadow graph method. As results, it was confirmed that the perforated panel contributes to weaken the blast wave to some extent, while the acoustic material does greatly: the one-stage expansion model with perforated panel and glass wool shows the highest decay of the peak over pressure for transmission and the two-stage expansion model other models with those shows the second highest. The acoustic material also contributes to decay of reflected shock waves propagating toward an upstream duct.