Ductwork can come in many different sizes and configurations, but some manufacturers' silencers do not always match these configurations. Many manufacturers have a limited amount of sizes in which their silencers are made (18", 15", 12", etc.), and these sizes don't always match the ductwork size exactly. When this happens, a transition would have to be manufactured in order to accommodate the silencer. These transitions are costly to install and often result in extra construction time and money.

Also, by having a silencer that is not fit to the ductwork, a sudden expansion will be encountered by the air. This sudden expansion causes much higher than normal turbulence and will decrease silencer performance while increasing the static pressure. The transitions can often cause gaps in the ductwork where the fitting had to be made. These gaps have to be blanked off and that can cause increased velocity through the silencer and thus a higher backpressure.