A diffuser includes a plastics baffle which has an air inlet end, an air outlet end and an outwardly tapering wall extending between the air inlet end and the air outlet end, and which defines a diffusing air chamber. A plurality of circumferentially spaced weld ribs is upstanding from the air outlet end. A plastics grille has a plurality of air outlets and an annular shelf to which the weld ribs of the baffle are ultrasonically welded.