|Inspect the result of FFT|
|Choose "Scale 10x" in View menu.
The scale is expanded and check the peak is 6000Hz. And note the magnitude of imaginary part, the green, is about 500.
The following is the definition of FFT. x[n] is the input wave, exp (-j 2PI nk/N) is a complex vector which real-part is cosine and imaginary-part is sine.
The following is a case in which N is 1024 and k is 63. The spectrum becomes maximum in this case.
The magnitude of input wave was 1.0 (see this). So it naturally becomes as big as 500 because it's made of 1024 accumulations.