Return-air systems are generally one of two types: the ducted return-air system or the open plenum return-air system (“sandwich space”). See Fig. 15-24. The ducted return-air system duct is lined with insulation, which greatly reduces noise.
The open-plenum system eliminates the cost of return-air returns or ducts and is extremely flexible. In a building with relocatable interior walls, it is much easier to change the location of a ceiling grille than reroute a ducted return system.