Abstract: A self-modulating diffuser for regulating the volume flow of conditioned air from a duct to a selected space, affording a high level of air mixing and sound performance combined with ready control of the space temperature comprises an adjustable square cone plaque-type ceiling diffuser with an outer cone for developing horizontal air flow, a square plaque for attachment to the outer cone and an inner dampening cone positioned intermediate the outer cone and square plaque. Separate first and second sensor means measure the temperature in the duct and in the room space, respectively, and direct digital control means respond to signals from the sensors to produce a control signal which operates an actuator mechanism for adjusting the position of the inner dampening cone.