How Passive Noise Canceling Headphones Work
Passive noise canceling headphones basically imitate what you do whenever you cover your ears with your hands, or pop in a pair of ear plugs.
Any headphones in reality can be said to use passive Noise cancellation, though certain headphones are designed for it more than others.
Supra-aural headphones, which are headphones that are placed ON the ear (instead of around or in it) are probably the worst at sound cancellation. They're typically smaller, light, and low density, consequently there is not really much substance to block any outside, ambient sound from coming into your ear. Moreover, because they only rest on your ear, they leave a great deal of space for external noise to travel around them and find its way into ear canal.
Very rarely are you going to ever come across pro, top quality supra-aural headphones, because they just are unable to give you the noise canceling advantages that circumaural or in-ear headphones can. Supra-aural headphones are light-weight, rather simple and inexpensive to manufacture, which means that you will often notice them being aimed at consumers who aren't exceedingly interested in headphone quality, but are just looking for base functionality.
For example, a great number of desktop or video game headsets are usually supra-aural. Since they are light they can be comfortable for the extended use that computer users or gamers need them for. Moreover, these types of people are likely to be not exceedingly concerned about the audio quality-they're quite often employed for spoken communication, where clarity is king and nothing else actually matters.
Circumaural (around ear) Headphones generally are significantly better for noise cancellation. They are much bigger, and consequently they often include more noise reducing material that creates a better buffer to protect against outside noise. Furthermore, since they totally block off the ear, they are able to block it off from any outside noise that could possibly slide in. Though, this is assuming that a person's headphones are built well so they fit well. If they're shoddily designed, ambient sound could be able to still slide in.
Shure's SRH 840 headphones are extremely well made for passive noise cancellation. Shure manufactured the ear pads from thick memory foam, which in addition to delivering enhanced sound isolation also makes these headphones feel like you could take a nap in them.
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