In certain situations, an existing condition may exist that needs silencing that is already constructed. For instance, a noisy generator may already have a housing with large enough intake and exhaust ducts to accommodate silencers. In this situation it would be quicker, cheaper, and easier to insert some sort of silencer into the ductwork instead of chopping off the existing housing and adding on a silencer. Another case would be installed ductwork that needs silencing after construction. It would not be cost effective to try and get to the ductwork and remove a section of it in order to accommodate a silencer.