In a crowded subway, the roar of engines and the sound of station announcements come one after another; in an open office, the sound of keyboard tapping and the conversation of colleagues interweave into background noise; on long-distance flights, the low-frequency vibration of the engine makes it difficult to fall asleep... These noise problems that plague humans are being quietly resolved by a technology called ANC (active noise reduction). It is not only a standard feature of high-end headphones, but also penetrates into the fields of automobiles, homes and even the medical field, becoming an "invisible shield" for modern people to fight against noise pollution.
ANC is a technology that uses electronic technology to generate sound waves with opposite phases to ambient noise and reduces noise through interference cancellation. Unlike passive noise reduction (relying on physical materials to block noise, such as earplugs and soundproofing cotton), ANC belongs to "dynamic noise reduction" and can actively fight against noise of specific frequencies.
This physical phenomenon based on destructive interference makes ANC particularly effective for low-frequency noise of 50-1000Hz (such as subways and aircraft engines), and the noise reduction depth can reach 35-45 decibels - equivalent to reducing the roar to whisper level.
Although ANC performs amazingly, it is helpless against high-frequency noise (such as human voices and door slamming). The reasons are: ① The period of high-frequency sound waves is extremely short (the period of sound waves >2000Hz is <0.5 milliseconds), and existing technology is difficult to accurately generate reverse waves; ② Physical limitations: The passive sound insulation effect of the headphone structure on high frequencies is more direct, and ANC has become a "supporting role". This also explains why high-end headphones always emphasize "hybrid noise reduction" - using ANC to process low-frequency noise + earmuffs to physically isolate high frequencies to achieve full-band silence.

1. Active noise reduction vs. passive noise reduction: The essential difference between the two types of "silence":

(1) Many people confuse the concepts of "active noise reduction" and "passive noise reduction" when they first come into contact with noise reduction headphones. Simply put, passive noise reduction is a "physical defense" - through the sealing of materials such as earplugs and earmuffs, it blocks external noise like a wall. Typical representatives are 3M noise-isolating earplugs or sponge earmuffs of traditional headphones. This method has a certain effect on mid- and high-frequency noise (such as human voices and keyboard sounds), but when facing low-frequency noise such as aircraft engines and car roars, sound waves will be directly transmitted to the eardrum through air vibrations, and the noise reduction effect will be greatly reduced.
(2) Active noise reduction is a "precise counterattack". It relies on electronic technology to generate sound waves with opposite phases to the ambient noise, and achieves energy cancellation through sound wave interference. Just like the "anti-magic shield" in the game, the microphone of the active noise reduction headset first "monitors" the external noise, and the chip then generates "anti-phase sound waves" in real time, which are finally superimposed with the original noise in the ear canal, making the sound you hear close to "silence". This technology is particularly good at processing low-frequency noise of 20-200Hz, which is the weak link of passive noise reduction.

2. Technology Application

(1) Consumer electronics: such as noise-canceling headphones, smart home electronic products, etc.
(2) Automotive field: The noise of the engine, tire noise, wind noise, etc. is collected through the microphone in the car, and then the sound system emits the reverse sound wave, which can reduce the noise in the car by 5-8 decibels.
(3) Industrial and medical: noise-canceling earmuffs/helmets, hearing aids, etc.
From the initial military aviation field, ANC technology has gradually penetrated into all aspects of consumer electronics, reshaping people's relationship with noise through "physical cancellation + algorithm optimization". Although it is still limited by high-frequency processing and power consumption, with the advancement of AI and chip technology, ANC is expected to achieve a full-band, personalized, low-power noise reduction experience in the future, and even become the core solution for noise management in smart home environments.