What is Passive Noise Cancellation?
Passive Noise Cancellation is the noise that headphones block out based on the physical design of the earcups. Based on the shape of the headphone earcups and how it fits over the head determines to a large degree how much noise the headphones can block out. This comes in handy when a user is listening to music or whatever desired sounds he wants to listen to. When passive noise cancellation is strong, the user hears less outside unwanted ambient noises and can focus more on the desired sound at hand. This is in comparison to active noise cancellation, which works differently; active noise cancellation is noise cancellation that works through powered electronic circuitry to produce noise cancellation. Passive noise cancellation is all about the physical, or you can say mechanical, design of the earcups.
The type of headphones that produce the best passive noise cancellation and that is used most heavily in the headphones market are circumaural headphones. Being that circumaural headphones are large and fit entirely over and seal the ears, they allow for the least amount of ambient noise to seep into the headphones. Thus, they allow for the most reduction of outside noise. Neither supra-aural headphones nor earbud headphones offer the passive noise reduction or cancellation that circumaural headphones do, because they don't completely seal over the ears and isolate them.
Passive noise cancellation accounts for 99% of the noise reduction the headphones produce on the market. This is because most headphones or headsets on the market only offer passive noise cancellation. In order to receive also active noise cancellation, the headphones must specifically be active noise-cancelling headphones. These headphones are usually a good deal more expensive because they take more sophisticated engineering design to produce. These headphones also need a battery source to power them to produce active noise cancellation. So passive and active noise-cancelling headphones aren't really at all in the same arena in comparison. Passive noise-cancelling headphones are far more common and less sophisticated, while active noise-cancelling headphones are more rare and harder to produce.