In Building services engineering
, a bell mouth
is a tapered expanding or reducing opening in the end of a ventilation
duct, so named because the taper can resemble that of a bell
shape. They are primarily designed and used for return air or extract air purposes within building ventilation systems, more commonly located within ceiling voids or other similar plenum
. The bellmouth cross sectional area is normally double that of the duct area, so that the air velocity entering the bellmouth is low (to reduce noise, turbulence and pressure drop), and gradually increases to the normal design velocity of the ductwork. The angle of the bellmouth is normally tapered at about 45° as a balance between keeping the bellmouth short without causing too much turbulence or excessive pressure drop. Bell mouths can be manufactured to suit either circular or rectangular ductwork sections.