Active noise Reduction Technology

Do you notice the noise pollution of the surrounding environment when you take the transportation every day? In the face of annoying noise in the ear, in addition to blocking the ear of this passive method, we can now take the initiative, using sound to eliminate the noise.

The principle of active noise reduction is summed up in a sentence: phase cancellation is achieved by emitting sound waves and noise interferences with the same frequency and amplitude as the opposite of the noise phase.

To understand this sentence, first understand what the voice is. What the phase and amplitude are. How the interference happens.

The sound we hear at ordinary times is essentially the phenomenon of air being squeezed and constantly passing.

When the air is squeezed, there will naturally be compression and sparse parts. The two sections are interlaced, alternating once as a cycle, and the number of cycles in a second is called frequency. The sound we hear is often composed of many voices with different frequencies, and they are not only different in frequency, but also in varying degrees of compression/stretching, that is, the amplitude is different, and the size of the sound is not the same.

The meaning of the phase is also very simple, the position of the sound wave in a period of a certain time is the phase. If we stipulate that the most powerful part of the compressed air is the 0 point of the phase, then after half a cycle, where the air becomes the most sparse, the phase becomes 180 °. By the time the air is compressed again, it passes through the entire cycle, changing the phase 360 ° to 0°.

If we have two frequencies approaching or the same sound superimposed together to synthesize a sound, their amplitude will become larger or smaller or constant, this phenomenon is called interference. If the two sound phases are the same: when you have the greatest amplitude, I also have the smallest, you are the youngest, then the resultant sound amplitude will become larger, it sounds changed. If the two sound frequencies are the same, the amplitude is the same, but the phase is the opposite: two sound waves are reversed, and the sound is eliminated.

Now active noise cancelling headphones sell well, specifically how to realize the process of canceling the noise.

To generate sound waves to counteract noise, the first thing to do is to get the noise information. Active Noise cancelling headphones feature a feedback microphone for capturing noise signals in the environment. There is a sequential problem: the noise is collected first, but the noise can be reduced by generating an offset sound at the same time. So the processor predicts the noise at the next moment and produces a corresponding offset sound.

In order to guarantee the quality of noise reduction, a feedback microphone is also needed to detect if the resultant noise is really smaller. The processor then adjusts the process to further reduce the resultant noise volume, based on the results measured by the feedback microphone, which is called the adaptive process. As the processor becomes smart, it can adjust itself according to the effect of noise elimination, in order to achieve the best noise reduction effect.

Finally, it is also necessary to know that the process of concrete implementation has many complicated and fine links, from the principle to the reality always. Now you know the active noise reduction technology.



This article is for the following: Key laboratory of noise and vibration of CAs Institute of Acoustics "Magic Active noise Reduction Technology"
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