Active Noise Cancellation (ANC) technology is a staple of the modern headphone industry. While still a higher-end feature, it’s no longer reserved for the most expensive headphones. In fact, you’ll find a selection of earbuds and even true wireless products sporting some form of noise-canceling technology. However, as our reviews so often show, not all ANC implementations are equal.
In fact, there are quite a few different ways to design and implement ANC technology. Each of these has implications for the quality and type of noise that headphones are good at canceling out. We’re going to explain the differences between these types and what they mean for your next headphone purchase.
Active noise-canceling in a nutshell
We’ve already covered how noise-canceling headphones work in-depth, so be sure to check that article for the complete ins and outs.
In a nutshell, noise-cancellation is based on the principle of wave phase cancelation. Waves, such as sound, that are 180 degrees out of phase, or inverted from one another cancel out when added together. Think of it as adding -1 to 1, and you end up with zero. The idea with noise cancellation is to record the background noise, invert the noise signal to create “anti-noise”, and then add it to your output signal which includes your music. The anti-noise signal cancels out the actual background noise by the time it reaches your ear. The idea is surprisingly simple and dates back to the 1930s, but it’s easier said than done.
The biggest issue with noise cancellation is capturing background sounds accurately enough to provide the maximum amount of attenuation. Microphones have imperfect frequency responses, electronic conversion introduces noise of its own, and the phase of the cancellation waveform leaving the speaker might not perfectly line-up with the phase of the noise once it reaches your ear. These systems need to be finely tuned, but even then you won’t ever see 100 percent cancellation. Instead, between -20 and -30 dB of noise reduction is quite common, which cuts the background volume anywhere from 1/16th to 1/32nd. A considerable amount.
Another key point to consider is that the noise you hear on the inside and outside of headphones is very different. Compare the passive noise attenuation of closed-back headphones versus earphones, for example. This difference in sound capture substantially changes the quality and capabilities of active noise cancellation headphones.
This begs the question, where do you best position the microphone in order to capture and cancel out noise? Outside the headphones, inside, or perhaps a little bit of both works best?
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 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 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.
Why does it matter?
While manufacturers seldom talk about the ins-and-outs of their noise-canceling technology, knowing more about the three main types can help inform your purchasing decisions. If you’re having problems with feedback or not enough high-frequency cancellation, you may want to switch from the feedback to the feedforward type. Alternatively, noise cancellation that seems a bit temperamental could be a sign to switch from feedforward to something else. While not an automatic guarantee of quality, keeping an eye out for hybrid ANC should ensure a nice quiet listening environment free from issues.
Finally, keep an eye out for our noise-canceling performance charts in our reviews. These give you the best picture of all about how headphones perform at canceling noise and can help us to infer a bit about the technology onboard.