Active noise reduction is a kind of noise reduction technology, which is to generate a reverse sound wave equal to the external noise through the noise reduction system, and neutralize the noise to achieve the effect of noise reduction. Its principle is that all sounds are composed of a certain frequency spectrum. If a sound can be found, the frequency spectrum is exactly the same as the noise to be eliminated, but the phase can be completely cancelled out if the phase is exactly opposite.
Sound is generated by the vibration of an object, and the propagation form of vibration in an elastic medium is sound waves. Sound waves in a certain frequency range (20 to 20000 Hz) act on the human ear to produce a sound feeling. Noise (noise) is usually defined as "unwanted sound" (unwanted sound), is an environmental phenomenon. People are exposed to noisy environments throughout their lives. Noise is also an environmental pollutant produced by various human activities. Vibration (vibraTIon) is the reciprocating periodic motion of an object (or part of an object) along a straight line or a curve and passing through an equilibrium position. It exists widely in nature and engineering.
Since vibration is the source of noise, in the practice of vibration and noise reduction, the problem of noise can be effectively solved by solving vibration. In the common noise control, the vibration of the metal thin plate such as the pipe wall of the aerodynamic machinery, the shell of the machine, the car body and the ship hull, etc. are generally made of thin metal plates. When the equipment is running, these thin plates will generate vibrations, which will radiate noise The noise caused by the vibration of the metal plate structure is called structural noise. For the effective control method of this kind of metal plate radiated noise, one is to reduce its noise radiating area as far as possible in design and remove unnecessary metal plate surface; the other is to apply a layer of damping paint on the metal structure and use the damping material to suppress Structural vibration and noise reduction, this method we call vibration damping (vibraTIon damping), is an active noise reduction technology.
The three noise reduction measures commonly used to reduce noise, that is, noise reduction at the sound source, noise reduction during propagation, and noise reduction at the human ear, are passive. But it is worth noting that this noise reduction method can effectively isolate the external high-frequency noise, but the blocking effect of low-frequency noise is not obvious. In order to actively eliminate low-frequency noise, people invented the technology of "active noise cancellation". Its principle is: all sounds are composed of a certain frequency spectrum. If a sound can be found, its frequency spectrum is exactly the same as the noise to be eliminated, but the phase is exactly opposite (with a difference of 180 °), and this noise can be completely cancelled out. . The key is how to get the sound that cancels the noise. The actual method is to start from the noise source itself and try to reverse the phase of the original noise through the electronic circuit. From this point of view, the technology of active noise reduction is actually "to attack poison with poison".
Shortcomings of active noise reduction
1. Excessive noise reduction effect may cause users to hear no danger warning and bring danger.
2. Since active noise reduction requires battery power supply, the battery power will be used up. And basically all noise-cancelling headphones will not work properly after the battery is exhausted.
3. Active noise reduction solves the problem of low-frequency noise, but it is not easy to deal with high-frequency noise, noise from scenes such as vocals, airplanes, or trains. It is difficult for active noise reduction headphones to cope with it completely.
4. If the active noise reduction equipment is not done well, it may cause loss of sound quality.
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