That would defeat the definition of 'active noise canceling' then. I do believe there to be some electronic component .. unless Bose and everyone else is performing a big scam.
FWIW:
http://en.wikipedia.org/wiki/Noise-cancelling_headphones
Noise-cancelling headphones are headphones that reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control (ANC). This involves using one or more microphones placed near the ear, and electronic circuitry which uses the microphone signal to generate an "antinoise" signal. When the antinoise signal is produced by the speaker driver in the headphone, destructive interference cancels out the ambient noise as heard within the enclosed volume of the headphone. They should not be confused with noise isolating headphones.
Noise cancellation makes it possible to enjoy music without raising the volume excessively. It can also help a passenger sleep in a noisy vehicle such as an airliner. Research examining the benefits of noise cancelling headphones in the aviation environment has found that compared to passive noise attenuating headphones or no headphones, noise cancelling headphones significantly increases the signal to noise ratio, making hearing important information such as safety announcements easier.[1] The benefits of noise cancelling headphones also extend to situations involving concurrent task (dual task) performance, particularly mind-games (such as Sudoku) as well as listening tasks.[2]
Noise-cancelling headphones typically use ANC to cancel the lower-frequency portions of the noise; they depend on more traditional methods such as soundproofing to prevent higher-frequency noise from reaching the ear. This approach is preferred because it reduces the demand for complicated electronic circuitry that would be required for noise cancellation at higher frequencies, where active cancellation is less effective.[3] To truly cancel high frequency components (coming at the ear from all directions), the sensor and emitter for the cancelling waveform would have to be adjacent to the user's eardrum, which is not currently technically feasible.
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http://electronics.howstuffworks.com/gadgets/audio-music/noise-canceling-headphone3.htm
Noise-canceling Headphones
Unfortunately for music lovers, many types of ambient sounds can interfere with or even block the sounds coming through their headphones. If you have ever tried to listen to a CD or MP3 player on a plane, then you know the problem well: The roar of the engines makes it difficult to hear what's being piped through the speakers -- even when those speakers are situated in or on your ear. Fortunately, noise-canceling headphones can provide a more enjoyable listening experience.
Noise-canceling headphones come in either active or passive types. Technically speaking, any type of headphone can provide some passive noise reduction. That's because the materials of the headphones themselves block out some sound waves, especially those at higher frequencies. The best passive noise-canceling headphones, however, are circum-aural types that are specially constructed to maximize noise-filtering properties. That means they are packed with layers of high-density foam or other sound-absorbing material, which makes them heavier than normal headphones. The tradeoff of all that extra weight is a reduction in noise of about 15 to 20 decibels (dB). But considering jet engines create 75 to 80 dB of noise inside the aircraft cabin, passive models have some serious limitations. That's where active noise-canceling headphones come in.
Active noise-canceling headphones can do everything that passive ones can do -- their very structure creates a barrier that blocks high-frequency sound waves. They also add an extra level of noise reduction by actively erasing lower-frequency sound waves. How do noise-canceling headphones accomplish this? They actually create their own sound waves that mimic the incoming noise in every respect except one: the headphone's sound waves are 180 degrees out of phase with the intruding waves.
If you look at the illustration below, you can see how this works. Notice that the two waves -- the one coming from the noise-canceling headphone and the one associated with the ambient noise -- have the same amplitude and frequency, but their crests and troughs (compressions and rarefactions) are arranged so that the crests (compressions) of one wave line up with the troughs (rarefactions) of the other wave and vice versa. In essence, the two waves cancel each other out, a phenomenon known as destructive interference. The result: the listener can focus on the sounds he wants to hear.