Pre echo in impulse response?

[koldby]

Hi
I am rather new to REW, although I have some time ago made some measurments and used DRC to make room correction as a tryout.
Now I seriously want to use the REW measurements for DRC to make my Line Arrays really shine.
BUT...
When I take a sweep of one speaker I get a pre-echo in the impulse response:

No matter what I do, change soundcard , drivers reinstall REW and asio drivers, even changing PC , the result is the same.
Any suggestions?

 

[John Mulcahy]

Those aren't pre-echoes, they are distortion harmonics. The nature of log swept sine measurement is that distortion harmonics end up at negative time in harmonic order. There is more info on that in the Distortion graph help.