Phase dependance on measurement length

[DPendavingh]

Hi All, I'm quite new to REW and i've encountered some strange results and hope that someone could explain what is happening.

I noticed that I get different phase responses (the phase for the highs goes up for longer measurements) for different measurement lengths (and I'm pretty sure that I get similar results when I change the sample rate).
I made 3 measurements for you all to show this, measurements are done with my miniDSP umik-1 and Behringer audio interface connected to one of my JBL 305p mkii's.
I did not move the mic in between measurements.

measurement 1 is 256K samples long
measurement 2 is 512K samples long
measurement 3 is 1M samples long

I also attached the settings.

Cheers

 

[jtalden]

Try this:

1. Select "Wrap phase" for all measurements.
2. "set t=0 at IR start" for each measurement. It is found in the REW impulse chart controls.
3. Unwrap the phase after placing the cursor someplace in the higher frequencies - say10 kHz or so.

These steps will place the reference time for the 3 measurement to the same place for each impulse. The phase will then be aligned, or nearly so. If there is still a small difference in the high frequency phase t=0 can be adjusted manually using the graph control "Offset t=0" for each measurement impulse as needed.

 

[DPendavingh]

I tried the steps you suggested now the results defer even more... Am I doing something wrong?

I attached the measurements if someone wants to look at them.

All help is highly appreciated.

 

[John Mulcahy]

The phase changes at different sweep lengths are caused by small clock rate differences between the UMC202HD and the UMIK. To correct them select "Adjust clock with acoustic ref" in the Analysis preferences.

 

[DPendavingh]

Ah thanks John! I'm slightly diapointed that I didn't think of this myself.