Active noise cancellation of equipment fan noise on aircraft

An active noise cancellation system for forced air aircraft equipment heating or cooling systems of the type having at least one motor-driven fan unit which produces an airflow stream to add or remove heat, the active noise cancellation system comprising:

• a duct having a proximal end coupled to the fan unit to entrain the airflow stream in the direction of a distal end of the duct, the duct having a mouth at the distal end;
• a reference sensor positioned within the proximal end of the duct;
• an airflow diverter extending axially within the duct and positioned to direct the airflow stream away from the center of the duct and to produce a sheltered region of reduced velocity airflow separation proximate the diverter in the distal end of the duct;
• a control transducer coupled to the airflow diverter and operable when energized to transfer mechanical vibrations to the airflow diverter and thereby introduce an audio frequency control signal into the airflow stream;
• an error sensor positioned at the distal end of the duct where it is responsive to sounds carried by the airflow stream and to the audio frequency control signal introduced by the control transducer;
• and
• an electronic circuit coupled to the reference sensor and to the error sensor, and configured to supply a noise abating control signal to energize the control transducer and thereby substantially reduce at least one noise harmonic of the fan unit through destructive interference.