What is the principle of active noise reduction earphone?

Noise reduction earphone can be divided into passive noise reduction and active noise reduction.

Passive noise reduction is the use of very thick earmuffs to block outside noise. This method is simple and crude, and the difficulty is to select sound-absorbing materials. However, they are relatively large, and the price must be related to the materials used. Another point is that the sound absorption material the size of an earphone is basically ineffective in the low frequency range (imagine that the length of the absorption wedge in the anechoic chamber is determined by the 1 / 4 wavelength theory).

Now let's talk about high-tech, that is, the principle of active noise reduction earphone, which is very popular on the market. Active noise reduction (ANC), active noise control (ANC), or active noise reduction (ANR) are also available in English. If you look up English literature, don't be confused by these different names. They all mean the same thing.

Before we talk about active noise reduction earphone, we have to classify it

Active noise reduction can be divided into feed forward ANC, feedback ANC and hybrid ANC. Each of these methods can be divided into adaptive and non adaptive ANC. Generally speaking, adaptive ANC can only be digital control. However, under non adaptive ANC, it can be divided into analog and digital ANC. A long time ago, because the DSP chip is not powerful and does not support real-time complex computing, there were only analog ANC earphones on the market, that is, the calculated ANC controller was realized with capacitors, inductors, resistors and other components. With the rapid development of DSP chip, ANC controller can be digitized. There are many advantages. For example, the strength of ANC can be adjusted. Some earphones, such as bose700, can be controlled by mobile app. At present, it is basically the world of digital ANC earphones in the market. Here is just to let you remember the name and classification, the details will be described slowly below.

For noise reduction earphones, the main acoustic hardware includes external microphone, headphone speaker, and internal microphone. Of course, digital signal processor (DSP) is used to process signals in real time. The microphone in Figure 1 is a little exaggerated. In fact, there are two micro microphones, one on the headset shell (Ref MIC), the other inside the headset, close to the headphone speaker (error MIC). In general, ref MIC is used to measure ambient noise, and error MIC is used to detect noise between the headset and the human ear after wearing headphones. The smaller the noise measured by error mic, the smaller the noise that people hear (of course, this statement is not very accurate, because the sound size perceived by people is different from that measured by instruments, so some ANC will consider the psychoacoustic part, which will be discussed later).