Passive noise-cancelling refers to the ability of certain materials or designs to block out external sounds without using electronic components. This is typically achieved through the use of thick padding or sound-absorbing materials in products like headphones or earplugs. These physical barriers help reduce noise by creating a seal around the ears, minimizing the amount of sound that can enter. Unlike active noise-cancelling technology, which uses microphones and sound waves to counteract noise, passive noise-cancelling relies solely on the design and materials used. This makes it a simpler and often more affordable option for those looking to reduce background noise in various environments, such as airplanes or busy offices.