When you take the vehicle every day, have you noticed the noise pollution of the surrounding environment? In the face of the annoying noise running in the ear, in addition to the passive method of blocking the ears, we can now take the initiative to destroy the noise with sound.
In one sentence, the principle of active noise reduction is summarized: phase cancellation is achieved by emitting sound waves with the same frequency and amplitude as the noise phase. To understand the principle of noise reduction, you first need to understand what the sound is, what the phase and amplitude are, and how the interference occurs.
The sound we usually hear is essentially the phenomenon that the air is squeezed and transmitted continuously. When the air is squeezed, there will naturally be a compressed portion and a sparse portion. These two parts appear alternately with each other, alternating once as a cycle, and how many cycles are changed in one second is called frequency. The sound we hear is often composed of many sounds with different frequencies. They are not only different in frequency, but also different in compression/stretching. That is, the amplitude is different, and the sound sounds differently.
At a certain moment, the position where the sound wave is in one cycle is the phase. If we specify that the most powerful part of the air is compressed by the zero point of the phase, then after half a cycle, where the air becomes the most sparse, the phase becomes 180°. When the air is again compressed the most densely, it takes a full cycle, and the phase changes by 360° and becomes 0° again.
If we have two sounds that are close to each other or the same sound are superimposed to form a sound, their amplitude will become larger or smaller or unchanged. This phenomenon is called interference. If the two sounds are in phase: I am the biggest when you have the largest amplitude, and I am the smallest when you are the smallest, then the amplitude of the synthesized sound becomes larger and it sounds louder. If the two sound frequencies are the same, the amplitude is the same, but the phase is reversed: the two sound waves are opposite, cancel each other, and the sound is eliminated.
There is an active noise canceling headset on the market that sells very well. How does it achieve noise cancellation?
In order to generate sound waves for canceling noise, it is first necessary to obtain information on the noise. The active noise canceling earphone is provided with a feedback microphone for collecting noise signals in the environment. There is a sequence of problems here: the noise is collected first, but the cancellation noise is generated simultaneously with the noise to reduce the noise. Therefore, the processor will predict based on noise, predict the noise at the next moment, and generate corresponding offset sound waves.
In order to ensure the quality of noise reduction, a feedback microphone is also needed to detect whether the synthesized noise is really small. At this time, the processor adjusts the processing according to the result measured by the feedback microphone to further reduce the volume of the synthesized noise. This is called an adaptive process. For example, the processor becomes smarter and can adjust itself according to the effect of noise cancellation to achieve the best noise reduction effect.
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