Even though your measurement system might be doing specific time windowing, the IR still has both loudspeaker phase and high delay which manifests as high phase slope when exporting as freq/mag/phase TXT. (I’m guessing this high delay is the time-of-flight across your chamber, plus soundcard buffer delays.)
In FIR Designer there is no error to give. The TXT file HAS phase and so the loading process is doing the best it can to interpolate the TXT file to the finer resolution FFT spacing. The high phase slope is the problem, and in this case is causing spurious distortion in the calculated IR. (You can see this on the FIR Designer Import tab if you expand the time axis and zoom the vertical axis.) If you truely want to ignore phase, then you can set all the phase values to 0 deg in the TXT file. (FWIW SMAART spectrum measurements come across to FIR Designer with 0 deg phase.)
In this situation it’s always best to avoid using log-frequency-spaced freq/mag/phase data – no matter how fine in resolution – for transfer between programs. Conversion to this format looses information which then causes problems when trying to reconstructed the impulse response – which is what FIR Designer is doing. It’s always best to transfer using an impulse response or time format, where information doesn’t get lost. (E.g. AFMG ETM files, or the time impulse as TXT or WAV.
>> And since Auto Mag should be trying to work on matching measured magnitude to target only, I do not see the relation to phase.
Auto Mag works with complex input and calculates the magnitude using either power averaging or complex averaging across frequency. You’re correct that power averaging should avoid issues of high phase slope but the additional distorting (added by the import interpolation process due to the high phase) is also causing problems in the Auto Mag.
I guess we could add an option to completely ignore phase on import, but it’s not something people normally want. When doing any filtering on a loudspeaker, most folks want to see both the phase of the filtering and the effect – both magnitude and phase – of the filter on the loudspeaker measurement.