Identifies the capabilities and limitations of active noise control (ANC) - creating sound to cancel another, undesirable sound. States that ANC can be used in ducted HVAC systems to cancel ductborne, low-frequency fan noise by injecting sound waves of equal amplitude and opposite phase into an air duct, as close as possible to the source of the unwanted noise. The noise problems which can be solved are typically described as 'rumble', 'roar' or 'throb'. Also contrasts the use of active against passive noise control techniques. Presents the fundamentals and basic physics of ANC and describes its application to real HVAC systems.