Three commonly used noise reduction measures, namely noise reduction at sound source, noise reduction during propagation and noise reduction at human ear, are all passive. In order to actively eliminate noise, people invented the "active noise elimination" technology. Its principle is that all sounds are composed of a certain frequency spectrum. If a sound can be found, its spectrum is exactly the same as the noise to be eliminated, but its phase is just opposite (phase difference 180), and this noise can be completely cancelled. The key is how to get the noise-eliminated sound. The practical method is to start with the noise source itself and try to reverse the phase of the original noise through electronic circuits. From this point of view, active noise elimination technology is actually "fighting poison with poison".

Sound is produced by the vibration of objects, and the propagation form of vibration in elastic media is sound wave. Sound waves in a certain frequency range (20 ~ 20~20000Hz) act on people's ears, resulting in the feeling of sound. Noise is usually defined as "unwanted sound" and is an environmental phenomenon. People are exposed to noise all their lives, and noise is also an environmental pollutant produced by various human activities. Vibration is the reciprocating periodic motion of an object (or a part of an object) through an equilibrium position along a straight line or curve. It exists widely in nature and engineering.

Because vibration is the source of noise, in the practice of reducing vibration and noise, the noise problem can be effectively solved by solving vibration. In common noise control, metal plate vibration, such as the pipe wall of pneumatic machinery, the shell of the machine, the car body and the hull, is generally made of thin metal plate. When the equipment is running, these sheets will vibrate, thus radiating noise. This kind of noise caused by the structural vibration of the metal plate is called structural noise. For this kind of effective control method of metal plate radiation noise, one is to reduce the noise radiation area as much as possible in design and remove unnecessary metal plate surface; The second is to coat a layer of damping paint on the metal structure, and use damping materials to suppress structural vibration and reduce noise. This method is called vibration reduction and is an active noise reduction technology.