Passive noise cancelling refers to the ability of certain materials and designs to block out external sounds without the use of electronic components. This is typically achieved through the use of thick padding or insulation in products like headphones and earplugs, which physically obstruct sound waves from entering the ears. Unlike active noise cancelling, which uses microphones and speakers to counteract noise, passive noise cancelling relies solely on the physical properties of the materials. This method is effective in reducing higher frequency sounds, making it a popular choice for environments like airplanes and busy offices.