[Arizhel]

Noise-canceling headphones do not cancel out conversations, so you must listen at high volume to drown out that other noise.

The only thing noise-canceling headphones are really good for is canceling out constant drone, for things such as fan noise and jet engine drone. That's why they're so popular for frequent airplane travelers. They do absolutely nothing if you're in an office surrounded by yapping coworkers (at least nothing more than a comparable set of non-noise-canceling headphones).

In fact, I'll add that if I'm surrounded by a bunch of computers with fan noise, using noise-canceling headphones lets me hear coworkers' conversations even better.

[cmdrfred]

Your right. When you consider it there has been a few million years of iteration of the human ear to attune it to the sound of the human voice. A difficult problem to overcome to say the least.

[Arizhel]

It's not just that, it has to do with the way noise cancellation circuits work. They work great on constant white-noise-like hums within a certain frequency band; they don't work on transients that cross a wide range of frequencies (as does human voice). They especially don't work well at higher frequencies because of the shorter wavelengths and the distance between the drivers and your eardrums (which can vary), and human voice has a lot of higher-frequency components. If you could stick the drivers right next to your eardrums with custom-fitted in-ear plugs, you could get far better performance, but that's neither economical nor desirable to most people for comfort reasons.