In Active Noise Cancellation (ANC) technology, the term "active" refers to achieving noise reduction through active components, naturally including modern electronic elements like microphones, ADC (Analog-to-Digital Converters), DSP (Digital Signal Processors), and DAC (Digital-to-Analog Converters). Because of this, ANC is often considered a very "novel" technology. However, the first ANC patent was filed back in 1936 by German physicist Paul Lueg. He was the first to propose the concept of active noise control, based on the fundamental principle of using sound wave cancellation to reduce noise.
In 1953, Dr. Fogel obtained a patent for helicopter noise reduction and developed the first noise-canceling headphones to provide active noise cancellation for pilots, ensuring effective communication. Around the same time, Willard Meeker, project leader at the Air Force Research Laboratory (AFRL), decided to use earmuffs as hearing protectors, creating the first active noise cancellation model. This model had an effective frequency range of 50–500 Hz and a maximum noise reduction capability of up to 20 dB. In 1978, Dr. Amar G. Bose, troubled by the noise of airplane engines during travel, began his research on noise cancellation. However, it wasn't until 1989 that Bose introduced the first active noise-canceling headphones for pilots on commercial flights, followed by similar products for U.S. Army combat vehicle crews. The technology truly entered the consumer market in the 2000s with the release of Bose's QC active noise-canceling headphones.
Active noise cancellation technology is divided into analog and digital types. Analog ANC has zero latency, but it involves many components, requires high precision, lacks consistency, and offers limited adjustment options, leading to its gradual phase-out. Digital ANC, on the other hand, enhances noise reduction by adjusting gain and EQ coefficients, and can further improve performance through firmware updates. With the emergence of highly integrated single-chip solutions, costs have significantly decreased, and device sizes have also shrunk. As a result, digital ANC has become mainstream, appearing not only in over-ear headphones but also in TWS (True Wireless Stereo) earbuds. For example, the TS-BT06 and TS-BT07 headphones are equipped with ANC technology and support a "Transparency Mode," allowing users to easily switch modes to hear external sounds when communication with the outside world is needed.
Mutual Cancellation of In-Phase and Out-of-Phase Sound Waves
Take the noise generated by an airplane engine as an example. This noise is typically a sound wave with a constant amplitude, meaning the peaks and troughs of the wave are generally continuous. If we simultaneously generate a sound wave with the same amplitude but an opposite phase-known as an "out-of-phase wave"-the two waves will cancel each other out when combined, achieving the effect of "noise reduction."
When you're on a plane or high-speed train, the microphones in active noise-canceling headphones pick up noise generated by engines or the friction of train tracks (along with other ambient noises). Theoretically, their detection frequency range spans from 100 to 1000 Hz, with some reaching as high as 3000 Hz (Step 1: Noise Collection). The collected sound waves are then processed by the control circuit (ADC-DSP calculation-DAC, Step 2: Noise Analysis). The headphones' speakers emit an out-of-phase wave with the same amplitude (Step 3: Generation of Out-of-Phase Sound Waves), which combines with the original noise to achieve cancellation and reduce noise.
FeedForward, Feedback, and Hybrid Noise Cancellation
Active noise cancellation in headphones can be classified into FeedForward ANC, Feedback ANC, and Hybrid ANC.
- FeedForward ANC features a microphone located on the outside of the headphones, primarily capturing external environmental noise. It is especially effective at canceling mid-frequency sounds, such as specific noises like voices or the sound of vehicles passing by. However, its performance is limited when it comes to reducing wind noise and sudden, intermittent sounds.
- Feedback ANC has a microphone positioned inside the ear cup or ear canal (in earbuds). This microphone captures noise that more accurately reflects what the wearer actually hears, making it a form of self-correction. Feedback ANC is mainly used to reduce low-frequency noises, like the hum of airplanes or trains, and is more effective against wind noise.
- Hybrid ANC combines both FeedForward and Feedback systems to deliver more comprehensive noise cancellation across a broader frequency range.
Hybrid ANC integrates the first two noise-canceling methods, combining their advantages to provide effective noise reduction across both mid and low frequencies. This results in superior noise attenuation characteristics for users. The main drawback, however, is its higher cost, as it requires a greater number of microphones and a more powerful DSP processing chip. Fortunately, more manufacturers are now able to offer highly integrated solutions, such as those from Cirrus Logic, Qualcomm, Bose, Harman, and Apple .
