They are rigidly constructed in steel, with a highly absorbent sound attenuating lining between outer casing and inner perforated steel lining. The silencer end faces have a series of threaded holes for direct mounting to the L.C, or S.C fan mounting flange. The EP (podded) versions provide increased attenuation. The pod, constructed from perforated sheet steel, has a sound absorbent infill. Two alternaltive silencer lengths are available at one, or two times the nominal fan diameter. eg. ENP - 1D and ENP - 2D, or EP - 1D and EP - 2D.