An air duct comprises a duct wall bounding an airflow channel that extends through the air duct along a flow path. The airflow channel has a plurality of cross-sections that are bounded by the duct wall and that are perpendicular to the flow path. A first one of the cross-sections has an aspect ratio that is less than two. A second one of the cross-sections has an aspect ratio that is greater than four and a minimum dimension that is less than three-quarters of an inch. A third one of the cross-sections has an aspect ratio that is less than two. The second cross-section is positioned between the first and third cross-sections along the flow path. The air duct channels air beneath a door, thereby allowing a vacuum or blower tool to operate in one room while the vacuum or blower device operating the tool operates in a different room.