Aerodynamic and acoustical effects caused by placing two prefabricated duct silencers in series

It once was not uncommon to find ductborne noise control designs and recommendations that would attempt to provide high sound attenuation values, especially for problematic lower frequencies, by using a pair of prefabricated duct silencers in series with one another, sometimes immediately adjacent but typically separated by some distance. Similarly, heating, ventilating, and air-conditioning (HVAC) duct silencers are occasionally required to accommodate fire dampers or access sections that effectively break up the silencer along its length, creating an empty gap between the noise-attenuating internal elements. Typical published performance characteristics of prefabricated duct silencers do not include effects from the use of additional silencers nearby and little information is available in common duct design and application literature. This paper will present information from a series of tests of various silencers in different configurations and spacings in an aero-acoustic test facility that will help describe the effects on insertion loss, generated noise, and pressure drop.