The invention concerns a volume flow regulator for ventilation systems with a conduit section (1) in which a regulating part (3) is pivotably mounted about an axle (2) which bisects the conduit cross section, whereby the regulating part (3) has a basically even surface on its leading side and a length which corresponds to the conduit height.In order to reduce the noises produced by the volume flow regulator itself, the regulating part (3) has a semicircular section in a plane vertical to its pivot axle (2), whereby the section radius corresponds to one half the conduit height. In a rectangular conduit section the regulating part (3) is a semicylinder whose curved contour fits against a conduit wall with a slight amount of play in all regulating positions. The regulating part can be balanced in various ways. In addition, a diffuser (24, 27) can be connected in after it which also functions as a sound absorber.