Active noise control

Active noise control is a method of reducing unwanted sound in the environment by using destructive interference between sound elds generated by primary sources of the original noise and secondary sources which can be controlled and are usually moving coil loudspeakers controlled by an electrical signal. It works best at low frequencies of sound, where the distance between these sources is small compared to acoustical wavelength. There are two basic control strategies called feedforward and feedback control, the basic dierence being that feedback control only measures resulting error eld in the so called zone of quiet around monitor microphone and feedforward control uses an reference signal of the noise before it reaches this zone. Some examples of system where active noise has proven to be eective are used in reducing noise in ducts, engine exhausts, mains transformers, protective headsets, car and aircraft interiors.