Up to 30% of hearing loss in adults may be due to the effects of noise. One of the major effects of hearing loss is the inability to selectively hear content in a noisy background. Noise-cancelling headphones can help save our hearing and allow us to enjoy clearer phone calls, quieter and more peaceful flights, and an overall higher standard of living.
  "Passive" noise reduction uses earplugs or earmuffs made of plastic, foam, silicone rubber, or other materials to mechanically isolate the ear from the environment. Depending on the various materials used, passive noise reduction can only have an impact on sounds above 500Hz. Active noise cancellation (ANC) technology is effective for frequencies as low as 20Hz and as high as thousands of Hz. "Active" noise reduction solutions fill a gap that standard passive noise reduction solutions cannot fill.
  Active noise cancellation uses the sound produced by the speaker to reduce other sounds that reach the ear. There are two design options for detecting unwanted sound: expose the microphone to the noise and isolate it from the speaker; Or place the microphone as close to the speaker as possible (Figure 1).

  Figure 2: At a distance of 0.7cm from microphone to speaker, the propagation delay of the signal chain does not exceed 20μs.
      Typical power consumption for standard 150MIPS, 24-bit DSPS, analog-to-digital converters (ADCs), and digital-to-analog converters (Dacs) that meet timing requirements is about 113mW. In contrast, analog applications with similar (if not better) performance, such as the AS3400 from Austrian Microelectronics (AMS), consume less than 10mW. Due to battery life considerations, the industry generally prefers analog solutions.
      On the other hand, the development of digital solutions has become more precise and software-oriented. And designers will be able to more easily implement features like equalization, bass enhancement, and surround sound - at the cost of extra computing and more power consumption, of course.
      While simulation solutions are more experiential and rely on a lot of existing electro-acoustic engineering expertise, this will give designers who can apply that expertise another important advantage that goes beyond battery life, size, and cost: immediate response.

Noise reduction level
  Above 1 or 2kHz, each headset exhibits some form of passive noise reduction because the insulation blocks ambient noise from entering the ear. Similarly, noise isolation of 20 to 30dB is usually higher than thousands of Hertz. A good noise-canceling headset has a passive noise filtering function, which can filter the noise to a very low frequency and achieve noise isolation of 5~30dB. Headphones bring peace and quiet even without power.
      However, there is a downside to good passive noise reduction: a phenomenon known as occlusion. The best example is sticking your fingers in your ears and talking. Because sound is transmitted through the bone structure of the chin and nose, the experiencer will hear their own voice dull and distorted.
      This phenomenon often causes users to raise their voice (or even yell) during a conversation, which is both indiscreet and unpleasant. In addition to removing headphones or earbuds during a conversation, techniques such as sidetone can be employed - the sound is picked up by a separate microphone and played back slightly dimmed.
      An unexpected advantage of active noise reduction is its ability to replace the need for high passive filtering at lower frequencies, such as voice bands. Since the user can turn active noise cancellation on or off by controlling the switch, it will be a great convenience when a seat mate on the plane passes the service car in the hope that it will stop the flight attendant.
      The feedforward scheme has a sweet spot: a narrow frequency band that maximizes noise reduction performance (Figure 3a). In addition, feedforward schemes can achieve noise reduction in a wide spectrum, typically up to 4 or 5kHz.
      Feedback active noise reduction tends to be more uniform across its frequency band (Figure 3b). However, the frequency bands under consideration are usually limited to about 1kHz. This limitation depends on the need for stability to ensure that different people or different headphone pressures do not change enough of the transfer function into positive feedback to cause oscillations. Noise reduction up to 20dB and 1kHz bandwidths is common for feedback designs.
     The hybrid design combines the advantages of both topologies while minimizing their disadvantages (Figure 3c).

  Figure 4: Feedforward schemes (a) typically have a narrow frequency band where noise reduction performance can be maximized. Sweet spots over 25dB and bandwidths up to 4kHz are common.
      Feedback active noise reduction (b) is flatter. Hybrid design (c) combines the best of both designs.
      Next in the prototyping phase, the headset developers turn the 3D model into an electroacoustic solution. In this phase, OEM engineers make reasonable modifications to the sound of the headset, and chip suppliers can use simulation tools and development kits to participate in the development of the first active noise reduction filter circuit.
      The output at this stage is a fully functional headset, a first prototype that exhibits the best active noise cancellation performance while including a separate frequency response graph. Most suppliers offer comprehensive circuit reference schematics and suggestions for further improvements to the mechanical design of the headset to improve active noise reduction performance. The prototype phase is usually completed after 1 or 2 repetitions.
      The final step is to integrate these approved modifications and design and layout the printed circuit board (PCB). Once the production confirms that the sample is complete, the active noise reduction transfer function can be fine-tuned.