Active noise reduction

Noise reduction method
The three noise reduction measures usually used to reduce noise, namely noise reduction at the sound source, noise reduction during propagation, and noise reduction at the human ear, are all passive. In order to actively eliminate noise, people invented the "active noise reduction" technology. Its principle is: all sounds are composed of a certain frequency spectrum. If you can find a sound whose frequency spectrum is exactly the same as the noise to be eliminated, but the phase is exactly opposite (a difference of 180°), the noise can be completely cancelled out. . The key is how to get the sound that cancels the noise. The actual method used is to start with 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 "fighting poison with poison."
Sound is produced by the vibration of an object, and the propagation form of vibration in an elastic medium is a sound wave. Sound waves within a certain frequency range (20 to 20000 Hz) act on the human ear to produce the sensation of sound. Noise is usually defined as "unwanted sound", which is an environmental phenomenon. People are exposed to noisy environments throughout their lives, and noise is also an environmental pollutant produced by various human activities. Vibration is the reciprocating periodic movement of an object (or part of an object) along a straight line or a curve and passing through a balanced position. It exists widely in nature and engineering.
Since vibration is the source of noise, in the practice of reducing vibration and noise, the problem of noise can be effectively solved by solving vibration. In common noise control, the vibration of thin metal plates, such as the tube wall of aerodynamic machinery, the outer shell of the machine, the car body and the hull, etc., are generally made of thin metal plates. When the equipment is running, these thin plates will vibrate and radiate noise. The noise caused by the vibration of the metal plate structure 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 the design, and to remove unnecessary metal plate surface; the other is to coat a layer of damping paint on the metal structure and use damping materials to suppress Structure vibration and noise reduction. This method is called vibration damping (vibration damping), which is an active noise reduction technology.

Noise-canceling headphones
Noise-canceling headphones use active noise control, which is different from the passive sound insulation of general headphones. The principle is:
1. First detect the low-frequency noise (100 ~ 1000Hz) in the environment that the ear can hear by the signal microphone installed in the earphone (currently it can reach 3000Hz)
2. Then the noise signal is transmitted to the control circuit, and the control circuit performs real-time calculations
3. The Hi-Fi speaker emits sound waves with the opposite phase and the same amplitude as the noise to cancel the noise
4. So the noise disappears and is inaudible

Active noise-canceling headphones are expensive, but they are generally effective and comfortable to wear. However, a separate battery is required, and most passive noise reduction headphones can be used without power consumption (nor active noise reduction).

Related applications
The application of active noise reduction in cars:
Cadillac is the first brand to apply active noise reduction technology to the automotive sector. In the first quarter of 2013, the Cadillac XTS luxury sedan was launched in China. Its BOSE 5.1 surround sound premium audio system is equipped with an active noise reduction and muting system (ANC). Noise samples are collected by the microphone in the car. After the system is analyzed and processed, the speakers of the car audio Play the opposite audio to offset the noise wave and create a quieter atmosphere in the car.
The application of active noise reduction in indoor noise reduction:
As people’s living requirements increase, the demand for indoor noise reduction has become more prominent. Aiming at noise sources such as outside windows, neighboring activities, and the operation of household appliances, three-dimensional open sound field active noise reduction technology has been developed and applied to indoor noise reduction to improve sound Environment is a feasible solution for indoor noise control in the future.
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