I don't have that much theorethical knowledge, but I know that negative frequencies don't have representation in real world, althought they're part of it. If you think about electrical stuff alternating current: it has its Real and Imaginary part. Only te Real part is useful, but they both are vital to describe the current.
Similar situation is with FFT. Normally FFT algorithms does output array of Complex Numbers (this conversion I shown in 3 posts above is shortcut: Real->Complex conversion and Complex.Abs() conversion put together). You can do IFFT (Inverted FFT) only when giving output array of Complex Numbers containing both positive and negative frequencies.
To put it simply: negative frequencies aren't useful for you, but when trying to get FFT^-1 function, you need them.
[quote:30aw7lup]I just began to wonder if it is possible to combine the frequencies afterwards and input them back to XAudio2
. I was thinking some kind of controllable equalizer...[/quote:30aw7lup]
IFFT would theorethically do that (available in FFTW as well). Anyway I haven't seen anybody implementing such functionality.
...Well actually Winamp's Eqaualizer does something like this ^^.