I saw that in the help file and for me 256K sweep lengths plots don't look right.The difference is due to small differences between the clock rate of your DAC and the UMIK. Here are the impulse responses with 21 parts per million clock rate adjustment applied:
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There is a note in the help about this, added following a similar issue that a user spotted in April:
If the input and output are on the same device and so share a common clock longer sweeps will provide higher signal-to-noise ratio (S/N) in the measurements. Long sweeps are not recommended when the input and output are on different devices, such as when using a USB mic, as their sample clock rates will differ. Over a long sweep the difference in clock rates can cause severe distortions in the shape of the impulse response and affect the phase response.
Thanks for the update. That will be a very useful addition.Clock rate mismatches are likely with different input and output devices, all clocks have a tolerance. To eliminate them the input and output need to share the same clock.
The effect is equivalent to an all pass filter. Magnitude response is essentially unchanged and the phase response effects aren't visible until higher frequencies. The change in the shape of the impulse response has much less effect than one might imagine. I'll add a feature in V5.21 to have a second timing reference after the sweep and compensate for clock mismatch automatically, but it is too big a change to go in 5.20. In the meantime I have added a manual rate adjustment feature for the next beta build.