Some headphones offer passive noise cancellation (also known as noise isolation) rather than active noise cancellation (ANC). Instead of using microphones to filter out undesired sounds, these headphones rely on soundproofing by physically blocking outside sounds. It's less effective than using a pair of headphones that use active noise cancellation, but it does mean there's less of a strain on battery life.  Some people also prefer passive noise cancellation, as active noise cancellation has been known to make people feel pressure in their ears. However, when searching for the best headphones for travel, this is less of an issue and something that frequent flyers will learn to live with.