Bose Foundations found that just as opposing speakers cancel out each other’s sound waves, a source of sound could be utilized to lessen the engine noise. As we all know, sound is a pressure wave which comprises of a compression phase and a rarefaction phase. The noise – cancellation speaker emits a sound wave which has same amplitude but with inverted phase (also called anti – phase) to the original sound. The waves add to form a new wave, through superposition principle, in a phenomenon called interference, and effectively cancel out each other. This effect is called phase cancellation. Applications can either be 1 – dimensional or 3 – dimensional depending upon the type of zone to protect. Periodic sounds or even complex ones are easier to cancel rather the random sounds are difficult because of the repetition in the wave form. Noise control can be either an active or a passive means of reducing sound emissions, generally for personal comfort or for environmental considerations or legal restrictions. Reduction in sound using a power source comes under Active noise control whereas the Passive noise control is sound reduction by noise isolating materials such as a muffler, sound – absorbing tiles or insulation instead of a power source. For low frequencies, the Active noise canceling is better suited whereas for higher frequencies, the spacing requirements for free space and silence zone, the tactics become forbidden or we could say that passive systems become more effective at higher frequencies and generally, are a better solution without the requirement of active control. The number of modes grows quickly with increasing frequency in the acoustic cavity and duct based systems, these modes quickly make the active noise control techniques unstable.