We can always do with a little bit of silence. To be able to shut out the noise during a noisy commute or a long flight is one of the simple joys of life. You can do that with active noise cancellation (ANC) earbuds, but do they live up to the hype?
No technology can create silence. Even the best rated ANC ear buds will merely reduce noise, such that the roar on a plane sounds quieter; instead of a loud hum, you hear a quieter hum. ANC earbuds address some sounds better than others. They work best for constant sounds like hum and engine roar, but do very little for high-frequency sounds such as the human voice. Crying children, slamming of a door, clapping and other fast and transient sounds are not blocked out effectively.
Passive noise 'blocking' is also a thing. If your earbuds fit properly and snugly they can reduce all external noises because they act like earplugs.
Earbuds with ANC use a digital signal processor (DSP), essentially a miniature synthesizer, in conjunction with a microphone. They analyze the ambient sound then generate an opposite wave to counteract the amplitude of the incoming sound waves. All this electronic voodoo uses power, so battery life can be a problem. A power compromise keeps them running for acceptable periods with less power devoted to making them sound good.
The DSP electronics actually introduce new noise in the form of a slight hiss. You can hear this in the background once you stop playing your music or any other audio. To improve the audio quality turn ANC off when you don't actually need it. Especially when crossing the street.
Famous audio companies like Bose have built a large following for their ANC products based on their brand reputation for great sound. But regardless of the brand, ANC buds will never sound as good as a fine pair of non-ANC buds like the Grado GT220. The latter being an example of passive noise reduction through their fit and seal.