Hi Murali,
I understand that you are trying to use hanning window on an FFT and want to know the cause of variation before and after hanning. Please go through the following points to understand the effects of hanning:
- Hanning smoothly reduces the ampiltude of the signal to zero at the edges.
- It reduces leakage of signal energy into adjacent frequency bins in Fourier Transform.
- Moreover, due to hanning side lobs i.e. additional peaks, are minimized giving more accurate representation of signal's frequency.
You can refer the following documentation about MATLAB function 'hann' which has more details about the underlying equation:
