How does noise reduction headphones achieve noise reduction?

Generally speaking, there are two types of anti-noise: "active anti-noise" and "passive anti-noise". The former generally has IC processing equipped with noise reduction algorithms, while the latter reduces noise through the characteristics of the material. The core of the active noise reduction method is "phase cancellation". The sound is formed by sound waves generated by vibration, and then transmitted to the human ear through the medium (air). When two sound waves with equal amplitude and opposite phase are stacked together, the energy is Will offset. Active noise reduction uses this principle: the pickup receives external sound, and through IC processing, the speaker emits the opposite sound wave, which interferes with the external noise, and it will be much weaker when it finally reaches the ear. Active noise reduction is better for low-frequency noise, but high-frequency is not strong. Because the wavelengths of high and low frequencies are different, and the wavelength of high-frequency sound waves is shorter, it is difficult to achieve a complete anti-noise effect; in addition, because we usually use anti-noise The noise that products face is mostly concentrated in the low-frequency area, so research is also invested in the low-frequency area. Passive noise reduction performs better in the high frequency range. Passive noise reduction is to prevent sound waves from reaching the tympanic membrane through various materials. In construction materials, rigid, smooth, and strong materials can reflect sound waves without passing through walls, and have excellent "sound insulation" effects; soft, rough, and porous materials "trap" sound waves in In the material, it diffuses and reflects inside, and finally consumes all energy. This method is "sound absorption". This mechanism is very effective for high-frequency sound waves. High-frequency sound waves consume energy in sound-absorbing materials, while low-frequency sound waves consume energy. Part of the sound wave energy can pass through. For example, the soundproof wall on the side of the highway is a rigid material, 

and the sound insulation of a home usually uses two methods at the same time: isolating external sounds and absorbing internal sounds. The anechoic room is the most extreme example. The application in life is mainly concentrated on the two products of "active noise reduction headphones" and "passive noise reduction earplugs", and there are different designs according to different usage scenarios. Some active anti-noise earphones are designed to attenuate the ambient sound, but will not hear the people around you. The noise-reducing earplugs are aimed at high and low frequencies, retaining the conversation sound of the middle frequency; but in the dead of night, it is often the person next to the pillow The snoring, the buzzing of the refrigerator makes it hard to sleep. If you want to use active noise reduction earplugs before going to bed, obviously you must choose a style dedicated to sleep. Passive noise reduction method-earplugs have many styles. The density and nature of the material, as well as the design of the internal structure, will affect the effect of earplugs at different frequencies. Some earplugs that are mainly used for sleep also advertise that they specifically block the snoring, which is the design that requires the best effect on the frequency of the snoring. Ear comfort earplugs have the best effect in the range of 1k ~ 10k Hz. They have a great effect on the engine sound of large equipment and vehicles, mixed human voices and snoring sounds, etc. They can reduce 40dBSPL at most, and low-frequency sounds such as ordinary speech sounds , The sound of most musical instruments and the sound of refrigerator compressors can be blocked to 15dBSPL. Choose the noise-reduction product that best suits your usage situation to prescribe the right medicine. You still have to actually experience it. Bluetooth headsets that support active noise reduction are a very important category for professional headset brands. Since the release of Airpods Pro, mobile phone manufacturers, audio brands and even start-up brands have joined the true wireless headset market, and a hundred flowers bloom.