Earphone help - how does "noise isolation/cancellation" work?

[The Stig]

I'm looking to replace my earphones with something more luxurious. Not super expensive but something average/above average. I have looked into a variety of ones but the noise isolation/cancellation ones interest me the most. Having never known anyone that has them and reading about it doesn't seem to help, I was wondering if anyone owned them here and can tell me how they work?

What I am thinking is that these earphones will completely block out external noise, ambient/background/people talking. I only know of one that's completely overpriced, by Shure (in the £300/$500~ range ; the Shure E500PTH specifically) that has a function called "PTH" which is push to hear, which is where I got the assumption that these earphones have such capability, even the cheaper ones.

I got a good handful of midranged ones I looked at which seems decent but not as outrageously expensive as exampled above, but I just need to know how the tech works if anyone has experienced them!

EDIT:

To quote, the review I read for the E500PTH mentions:

The problem with in-ear, noise isolation headphones is that if you want to talk to someone, stopping the music isn’t enough, since you’ll still barely hear the person in front of you unless you physically remove the ear buds. However, with the PTH module, you simply flick the switch and the volume of the music is dropped back, while a microphone in the PTH lets you hear everything around you without removing the headphones. In practice, the PTH works brilliantly, and saves you the hassle of removing your headphones whenever anyone talks to you.

Which is what really gets me excited that I can literally escape the horrors of public transport. I'm hoping the cheaper alternatives on a reasonable price has similar functions which trap noise that good (I'm assuming this is also done electronically, instead of having the plugs wedged into your ear like an earplug -- if someone can confirm)

[Furtwangler]

Noise canceling headphones come in two varieties, passive and active (the latter being more expensive.)

Passive ones just use different materials that are made specifically to filter out certain frequencies of outside sounds. That and just being dense, and filling your ear full of the material.

Active ones do the previous, while also they have a microphone, which picks up surrounding noise, and then emits the same sounds from outside, however 180 degrees opposite phase. This creates what they call destructive interference, and when the opposite phase noise is played into your ears along with your music, it cancels out the rest of the noise.

The headphones you're looking at that let you hear outside noises when muted, probably just have a slightly changed microphone in them that just allows everything to be passed through.

See http://electronics.howstuffworks.com...headphone3.htm if you're interested more. Pretty neat stuff.

[The Stig]

Thanks for that link, very interesting read. I will continue to do a bit more hunting around.