The idea behind this project is that by picking up extraneous noise with a microphone on a pair of headphones, inverting that noise signal with a circuit, and playing the inverted signal back through the headphones' speakers, the two signals will add together to zero and cancel the noise.
The circuit's input is the noise received by the microphones which are mounted on the headphones. The circuit's output is the inverted noise, and this output go to the headphones' speakers along with a "line-in" that could be the music you would usually be playing through your headphones.
Noise cancellation is an important concept to the improvement of our hearing as electronic technology (and the hum it makes) becomes more wide-spread. As we found, perfecting a sound-cancelling technique is difficult because of the precision and sensitivity required-- but the benefits of a simply built system like ours are also important.
The electronics consist of three op-amp circuits: a non-inverting pre-amp, a phase-inverter, and another inverting amplifier. The noise we hear is in stereo, as are the headphones, so the sound-cancelling headphones have two microphones and two circuits in parallel.