Help with my Bose QC15 headphone

"Active noise canceling" only functions with continuous, steady noise. Quick noises or music will not be canceled because the circuitry has inherent latency.

Hard to say, but keep in mind anecdotes aren't exactly the bottom line. The technology just doesn't work that way. Higher frequency sounds such as consonants/cymbals in speech and music are difficult to cancel because the wavelength is considerably shorter than low frequency noise such as airplane engine rumble (or in your case, an air conditioner). When the circuit attempts to "cancel" the wave by producing a negative amplitude identical wave of a low frequency, a small amount of latency will still allow the peaks of the wave to line up enough to effectively cancel them to a certain degree. With a high frequency wave, the signal is changing much faster, so a high frequency "canceling" control effort of the same time shift will not line up enough to effectively cancel the wave when summed.

If you are looking to get rid of that higher frequency noise, noise "isolating" headphones such as the Sennheiser HD-25-1-II are particularly effective as dampening material more readily absorbs high frequencies.

Ok. I'm in somewhat the same situation. I realize that completely canceling all noise is impossible but I am looking to find headphones for studying indoors that do the best job of canceling out the tv and normal to a little bit louder than normal conversation of folks in the same room as me. I'm also looking to not spend more than 300 or so.

Does such an animal exist?