i can still hear something when generating a 30.000hz tone or 100.000hz tone and higher. actually that can't be true so i guess my soundcard does something wrong. how do i test my hearing properly? do you think on a normal cd player this will work better (diskwriter => burn). why does it not work with my soundcard? (creative extigy)

Regular Extigy works internally at 48 KHz, so a 24 KHz tone is the maximum you could play in theory. I don't know how the tone generator works, but if you hear something when playing a tone over 22 KHz, it has to be due to poor resampling creating bad distortion at audible frequencies.

Better generate some tones with an external program, such as CoolEdit, or GoldWave (free) at 48 KHz 16 bit. You can play them from the same audio editor, or save them as wavs, and play them with FB2K. I think that in this case there won't be much difference compared with a standalone cd player. Still, the cd player probably will have slightly better quality. If you use a cd player, be sure of generating the tones at 44.1 KHz 16 bit.

Take care of not blowing the tweeters of your speakers, because high level, high frequency, continuous tones quickly heat them.