Noise cancelling headphones
A noise-cancelling headphone reduces unwanted ambient sounds by using Active noise control. This is distinct from passive headphones which, if they reduce ambient sounds at all, use techniques such as soundproofing. To cancel the lower-frequency portions of the noise, noise-cancelling headphones use active noise control. They incorporate a microphone that measures ambient sound, generate a waveform that is the exact negative of the ambient sound, and mix it with any audio signal the listener desires.
Noise-cancelling headphones specify the amount of noise they can cancel in terms of decibels. This number may be useful to compare products, but does not tell the whole story, as it does not specify noise reduction at various frequencies.
To prevent higher-frequency noise from reaching the ear, most noise-cancelling headphones depend on soundproofing. Higher-frequency sound has a shorter wavelength, and cancelling this sound would require locating devices to detect and counteract it closer to the listener's eardrum than is currently technically feasible, or would require digital algorithms that would complicate the headphone's electronics. An in-ear headphone uses the natural insulation to cancel the unwanted noise. The music is not affected by digital masking software and it's played as it is sent by the source.