Deep earplugs or earmuffs like those worn by factory workers typically reduce the racket by 15 to 25 decibels, but they are uncomfortable and do not allow wearers to hear the audio from an airplane movie, music channel or their own music player. Noise-reduction headphones can help. The most advanced models, priced around $300, are made from structural materials that passively block higher-frequency noise (above about 200 hertz). They employ electronics and a speaker to actively cancel lower-frequency sounds, which are otherwise difficult to stop. A microphone inside each muff senses sound waves that make it through the outer ear cup, and a speaker creates pressure waves that cancel them. If desired, music can then be piped in at a comfortable level.