signals before peak?

Probably a distortion image.

zangaidaige said:

impulse is ifft of frequency response so I think harmonics should merge with peak?

I use window separate the peak from harmonics, however the distortion plot still show the harmonics?

Click to expand...

No, the frequency response is the FFT of the windowed IR. Log sweep deconvolution places distortion images at defined times before the linear response, those images are individually windowed to generate the distortion data when the measurement is made.