Active noise control (ANC), also known as noise cancellation, or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first.
Feedforward active noise canceling
Feedforward ANC is, arguably the simplest type of active noise cancellation. With feedforward technology, the noise capturing microphone(s) is placed on the outside of the headphones. This is pretty handy for ANC earbuds, where there’s limited surface area for a mic inside the wearer’s ear.
Feedforward ANC uses a digital signal processor (DSP) or other dedicated ANC processing hardware to map the noise signal to the frequency response the user will actually hear on the inside of the headphones. Product testing is a requirement to ensure this mapping applies correctly for maximum noise cancellation. However, this is not as accurate as placing a mic inside the ear cup. Also, noise-canceling properties change a bit between wearers. A loose fit, for example, may allow extra high-frequency noise to bleed through which the processing doesn’t account for.
Placing the microphone outside the headphones has its share of pros and cons. The external microphone has the best noise sensitivity, making it good for mid-frequency noise canceling. As such, it can be used to isolate specific sounds, such as speech or traffic, for more advanced ANC and ambient sound control technologies. However, feedforward ANC is more sensitive to wind howl and other forms of short burst high-frequency background noise. These may end up being amplified as they are not picked up inside the earcup to be canceled out.
Feedback ANC
Feedback ANC is the opposite of feedforward, with the microphone placed inside the ear cup or inside the wearer’s ear with earbuds. Although picking the right place on this side of the headphones presents a new set of difficulties. The major benefit is that noise captured by the microphone more accurately reflects noise the wearer hears, regardless of the exact positioning and fit of the headphones. You can think of it as a little bit self-correcting in that sense. This also makes the headphones more resistant to wind howl but devices can lose high-frequency noise-canceling sensitivity, as less of this noise is likely to pass through the headphones from the outside.
This technology type still requires a processor to handle noise filtering. For instance, feedback noise canceling has to contend with the fact that the user’s audio is also likely to be captured by the internal microphone. This needs to be filtered out and also corrected for the frequency profile of the headphones when being worn.
As with all feedback systems, there’s a risk of runaway amplification. There’s a small risk of the system picking up its own anti-noise signal and increasing the level of amplification in a bid to canceling it out, but actually increasing the amount of noise or even producing a ringing feedback sound. This is very rare but can happen in models that don’t take adequate precautions. There’s also less processing time with the feedback design, as it’s working on audio already very close to the ear. As such, feedback ANC is most effective at low frequencies which have longer wavelengths.
Hybrid ANC
Hybrid active noise cancelation offers the best of both worlds. As you may have guessed, it combines both feedforward and feedback microphones and processing to cover all the bases.
You’ll receive the best noise attenuation frequency coverage and the lowest chances of feedback issues with hybrid technology. Furthermore, hybrid ANC can still be used for ambient noise and sound isolation features, while retaining the benefits of accurate, tailored ANC.
The drawback is that hybrid ANC is more expensive. Not only are there two microphones but these microphones need to be of high quality to avoid introducing extra noise. Headphones also require more powerful dedicated processing hardware to handle the extra math. Developers also double up on all the frequency and performance testing to maximize noise-canceling performance. These products make up the most expensive headphones on the market, but they offer the best quality ANC around.