Like every pair of noise cancelling headphones, the Mid ANC rely on microphones on the outside of the headsets to monitor noise. That signal is then compared against what you're actually hearing, using another two microphones within each cap, to block out external low-end noise, like the low hum of an airplane. The Mid ANC still let in some higher frequency sounds, so you're not completely oblivious to a car honking as you cross the street. That's not too different from other noise cancelling headphones, though. That technology is great at cancelling out low-end sound, but not so much at stopping high-pitched noise.
Marshall claims its implementation of ANC is better suited at urban environments, where you need to be more alert than if you were just sitting on an airplane. Based on a few days of testing, the Mid ANC did a solid job of blocking out plenty of city noise. It was particularly great at cutting out the random assortments of subway racket. But it didn't sound significantly different than other noise canceling headphones, like Plantronics' BackBeat Pro 2.
Outside of its ANC performance, the headphones also sound fantastic, with a well-defined (but not overpowering) low-end, and crisp mids and highs. The Mid ANC's 40mm drivers had no trouble tackling my eclectic library of tunes, from film soundtracks to classic soul. Given the way noise canceling works, you typically lose a bit of sound quality when it's activated, but I didn't notice any significant changes with the Mid ANC. The headphones also feature the AptX codec, which will give you a bump in wireless fidelity from supported devices. If quality is your main concern though, and you're not typically sitting in noisy environments, the Monitor Bluetooth might be a better option for you.