Difference between passive and active noise cancellation in headphones

Bose QC 15 Cross Section and Noise Cancellation technology

For those of you out there not versed in the ways of headphones, the difference between passive and active noise cancellation is very simple.  Passive noise cancellation simply means that the headphone has been designed by the manufacturer to block sound without the use of internal electronics, thus it is blocking sound "passively".  Active noise cancellation is when technology has been built into the headphone itself to cancel out noise.  For example, my Sony MDR-7506 headphones are passive noise canceling, they are a tightly sealed closed construction and that is how they block outside noise.  on the other hand we have the Bose Quiet Comfort 15, which has active noise cancellation.  The Quiet Comfort 15 actively blocks outside ambient noise by using microphones on the headset that effectively "listens" for outside ambient noise to block.  The Quiet Comfort 15 then creates its own sound waves that effectively blocks the noise by mimicking the ambient noise coming in.  The only difference is that the mimicked sound is 180 degrees out of phase with the intruding ambient noise.  Typically, passive noise cancellation headphones are far less complicated, they lack microphones and electronics inside that actively block noise, thus they generally cost less.  Active noise cancellation headphones generally always require batteries to power to noise cancellation feature.

Sony MDR-ZX700 driver (example of a passive noise cancellation headphone)

There are some issues with active noise cancellation headphones however.  The first of these issues is the batteries required to power active noise cancellation headsets.  These batteries add weight, and limit the amount of time you are able to use your headphones.  Additionally, many active noise cancellation headphones will not work once the batteries have died, or the sound quality decreases significantly once the batteries do die (unless you are using the Logitech UE 6000's or 9000's which are the only active noise cancellation headphones I have used that still sound great when out of battery power).  Also, from experience and the dominating opinion among audio enthusiasts, active noise cancellation technology tends to degrade sound quality.  In the case of Beats by Dre and Bose headphones, I find the active noise cancellation to degrade sound quality and fidelity a substantial amount.  With headphones like the Logitech UE 6000 and 9000, there is not as much of a sound quality impact.  However I can still perceive a difference with every single active noise cancellation headset I have every used.  I will admit that the Bose QC15's perform admirably on a plane or a similar noisy environment, but for pure and pristine headphone audio quality, over ear passive noise cancellation headphones are the way to go for someone simply looking for the most rewarding headphone audio experience.