1. Passive noise reduction.
This refers to in-ear earplugs, which are inserted into a person's ear canal through a silicone sleeve. So it has the effect of isolating the outside sound better. And this noise reduction is full-frequency, that is, it works on sounds from 20 Hz to 20K Hz, and this passive noise reduction is lossless. So the general HIFi earplugs are in-ear for this reason.
2. Call noise reduction.
Call noise reduction is designed to make the other party can hear clearly when the Bluetooth headset is used for a call. Common examples are Qualcomm's CVC noise reduction. Its principle is to use the chip inside the Bluetooth headset to filter the signal received by the telephone microphone to reduce the outside wind noise. At the same time, you can also pass the other side of the voice echo weakened processing. CVC noise reduction because there is no extra microphone to collect external noise, so the noise reduction effect can not meet the practical application at present. So some Bluetooth headsets have extra microphones designed to pick up ambient noise, and then the internal processor will subtract the external noise picked up by the noise pickup microphone from the signal picked up by the talking microphone. So this kind of noise reduction effect is much better. This kind of noise reduction is generally called "ascending noise reduction".
3, active noise reduction.
Active noise reduction refers to collecting some regular noises (such as the roar of an aircraft engine) from the outside world by means of a noise pickup microphone in some specific occasions, such as high-speed rail/aircraft. Through the circuit or algorithm, the reverse noise signal is added to the music signal, so that the sound in the earphone heard by the human ear is directly cancelled out with the noise of the outside world, so as to achieve the effect of noise reduction.
The above 3 kinds of noise reduction for noise reduction principle, but as structural personnel, we want to do in the structure of noise reduction is no more than a few ways, the first sealed sound cavity, so that before and after the sound cavity do not interfere with each other, the second in the structure to do the reverse sound