Basic principles of active noise cancellation (ANC) headphones

Active noise cancelling headphones have become a common audio device in commuting, office, and travel. To understand its working mechanism, one can start with the basic logic of noise control. Similar to the three elements of interference source, interference path, and interfered object in electromagnetic compatibility, noise reduction can also be approached from three directions: sound source, propagation path, and receiver.

Passive noise reduction, also known as passive noise reduction, mainly relies on sound insulation, sound absorption, damping and shock absorption methods to achieve. Its mechanism is to consume sound energy through the interaction between noise waves and acoustic materials or structures, thereby reducing noise. Acoustic sponge, sound barrier, and silicone ear cap are common passive noise reduction applications.

Active noise reduction, also known as active noise reduction, is based on the destructive interference of sound waves. Sound is a type of mechanical wave. After environmental noise is collected by a microphone, the system processes it to generate reverse sound waves that are 180 degrees out of phase with the noise. These waves are then transmitted to the human ear through headphones and speakers, thereby canceling out the noise that the human ear would otherwise hear. ANC headphones and car noise reduction are typical applications of active noise reduction technology.

From the perspective of advantages and disadvantages, the advantages of passive noise reduction are simple materials, no need for power supply, low cost, high reliability, and good suppression effect on high-frequency noise. Its shortcomings are limited suppression of low-frequency noise, often large size and weight, and inability to dynamically adjust the noise reduction effect according to the environment. Active noise reduction can dynamically adjust the noise reduction effect according to the environment, and has good suppression of low-frequency noise. However, it has limited suppression of high-frequency noise, higher cost, and also requires power consumption. In actual products, the best noise reduction effect usually comes from the combination of passive noise reduction and active noise reduction, which can cover a more complete frequency band and provide a quieter and more comfortable environment.

According to the microphone position and working mode, active noise cancelling headphones can be divided into feedforward, feedback, and hybrid types. The feedforward active noise cancelling microphone is located on the outside of the headphones and mainly collects external environmental noise. Therefore, the noise cancelling frequency width can be made wider, but the noise cancelling depth is generally average. The feedback type active noise reduction microphone is located inside the coupling cavity between the earphone and the human ear, collecting the sound inside the cavity, which includes both normal music signals and residual noise from environmental noise entering the cavity. Its noise reduction bandwidth is not easy to widen, but the active noise reduction depth can be made deeper. Hybrid active noise reduction is a combination of feedforward active noise reduction and feedback active noise reduction, which can balance the advantages of both and provide better noise reduction effects.

In practical headphone products, the optional ANC mode is a common design, which allows users to choose by dividing the noise reduction level. Taking Huawei FreeBuds Pro 3 as an example, it offers four optional modes: Smart Dynamic, Mild, Balanced, and Deep. Mild, Balanced, and Deep correspond to quiet environments such as offices, noisy environments, and very noisy environments, respectively. The options available on the Sennheiser App are maximum, wind resistance, and pressure resistance. Bowers&Wilkins Px8 has options for Office, City, and Flight, corresponding to office, city, and flight, respectively.

Adaptive ANC allows the headphone's noise reduction system to detect ambient sounds on its own and automatically select the appropriate noise reduction algorithm. It should be noted that adaptive ANC also needs to be coordinated with other configurations to complete. Nowadays, almost all mid to high end active noise cancelling headphones require fit testing, which means making targeted presets based on the condition of each user's ear. FreeBuds Pro 3 has intelligent wear detection before its official launch; Denon, a subsidiary of Masimo Group, has implanted Masimo AAT adaptive acoustic technology into its PerL series true wireless earphones. By emitting test sounds, it detects the high, medium, and low frequency sensitivity of the ear and customizes sound parameters.

At the simulation level, the controller of the simulated ANC system is mainly composed of analog circuits, which have the advantages of low cost and simple structure. However, the system is non time-varying, so only simple transfer functions can be used, and it is generally only applicable to single channel ANC systems. Digital systems have adaptive time-varying characteristics, and their controllers are composed of digital circuits with good stability. They can be used for both single channel ANC systems and multi-channel ANC systems.

Many people may ask, can active noise cancelling headphones suppress all noise? If the noise frequency is low and continuous sound is produced, the noise reduction effect is usually the best. Sudden noises, such as horn sounds and human screams, can still be heard because even the best ANC technology cannot create real-time inverted sound waves.

Why do active noise cancelling headphones generate ear pressure? Sound propagates through air vibrations, and the vibrating air carries energy, which neither disappears nor generates out of thin air. When noise is interfered and cancelled out, the original energy will be converted into thermal energy of air. According to the principle of thermal expansion and contraction, the conversion of sound wave energy will increase the air pressure inside the ear canal. Although this part of energy may seem insignificant, even not enough to raise the temperature by 0.1 degrees Celsius, the human ear has a very high perceptual sensitivity, and such small pressure changes can be perceived. This is why active noise cancelling headphones usually have high ear pressure.

Will active noise reduction technology reduce sound quality? To some extent, it will. Active noise reduction itself requires the sound unit of the headphones to generate vibrations opposite to the noise, thereby canceling out the noise. Therefore, it is inevitable that some audio details will be lost. Moreover, the noise reduction circuit itself will increase the interference source of the headphones, thereby increasing the interference floor noise. Especially in the early days of active noise cancelling headphones, when not playing music, there was a noticeable rustling of base current noise. However, with technological advancements, this issue has been greatly improved.

What is the difference between ANC and ENC? ENC stands for Environmental Noise Cancellation, which is a technology that focuses on reducing noise during calls and is mainly used in voice communication scenarios. ANC focuses more on providing tranquility during listening, while ENC focuses on improving the clarity and quality of calls.