This old Amazon commercial bugs the heck out of me. In it, a young boy jumps on a bed, raucously playing an electric guitar while another kid plays drums. Meanwhile, a father relaxes blissfully undisturbed because he’s wearing noise-cancelling headphones. This scene drives me bonkers because, as a headphones expert, I know the truth: That’s not how noise-cancelling headphones work.It’s a popular misconception that noise-cancelling headphones can block out any sound around you. People purchase them in the hopes of dimming the din of kids at play, loud-talking officemates, the barking dog next door, and airplane engines. The trouble is, active noise cancellation is really effective on only one of those things. (Spoiler: It’s the airplane engine.) The reason has to do with the physics of sound and how noise-cancelling headphones work.