Anti-Noise consists in cancelling the vibration by generating a force opposite to the incoming perturbation
Whenever the sensitive area cannot be isolated from the vibration excitation forces and whenever the vibrations are not amplified by the mechanical structure but mainly transmitted, the only remaining method to reduce the vibration (without a complete re-design of the system) is the anti-noise where a force opposite to the vibration excitation (same frequency, same amplitude but opposite phase) is generated resulting in a destructive interference process at the location where the control force are applied. It is important to note that anti-noise is a local vibration reduction method that lower the modal shape; applying damping locally will only reduce the vibration at this point and may lead to a vibration amplification elsewhere.
Passive anti-noise systems are mainly based on the use of Tuned Mass Absorbers or TMA where an oscillating spring-mass system is mounted on the target structure, i.e. at the location where the vibration is to be cancelled. This TMA will absorb the vibration energy at the resonance frequency of the oscillating appendice. It is well suited for stationary harmonic excitations.
Active anti-noise systems are open-loop devices, relying on actuators, sensors and a feedforward control algorithm. Two sensors are necessary in this control scheme: a reference signal correlated to the incoming perturbation (e.g. the engine speed) and an error signal located where the vibration is to be attenuated. The control algorithm (usually a x-filtered LMS) will filter the reference signal and apply the resulting signal to the actuator; the filter parameters being continuously adapted in such a way that the error signal is minimized. Because of the computation and adaptation, this control scheme is well suited for slowly varying harmonic excitations.