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2 - 3 dB difference between two Umik-2 in identical setup 1 week 3 days ago #65520

  • gruesome
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I'm comparing two Umik-2, both (I assume) recent with serial number above 4000, and obtain very different results measuring the same setup (KEF R3 speaker at 1m distance, same amplifier & Windows & REW settings).

The two microphones show 2dB difference below 200Hz, and 3 dB above about 2kHz (in opposite directions, i.e. one has a tilted response with respect to the other). I was hoping to use one of these microphones for dB level precision measurements.

I verified that when I switch back and forth between the microphones, with the same microphone I get the same result within a fraction of a dB.
I verified that changing the REW (and windows) sampling from 48kHz to 192kHz does not make a difference, ditto for explicitly making sure that audio effects for both input and output device are off.
I verified that I applied the correct calibrations. (Applying no calibrations, or reversing/swapping calibrations, makes the difference larger.)

Is such a large difference expected?

I'm attaching one plot, showing four measurements (108 - 111), done in the sequence mic1 - mic2 - mic2 - mic1.

I did submit a support ticket, but wanted to see whether anybody else has seen something similar. There is a thread about Umik-1 to Umik-2 differences that also seem to be larger than expected. 
 
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Last edit: by gruesome. Reason: clarification of 'reversing' calibration: swapping between the mics

2 - 3 dB difference between two Umik-2 in identical setup 1 week 3 days ago #65521

  • Kpjamro
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You didn't say, but are the microphones being compared located in exactly the same spot?
In a real room, with real acoustics, the only way to get identical readings is to basically strap the two microphones together.

 

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2 - 3 dB difference between two Umik-2 in identical setup 1 week 3 days ago #65522

  • gruesome
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Kpjamro, yes, I am leaving the microphone stand in place, unplug the cable at the microphone and just swap out the microphones. The microphone is pointed from 3.5" [edit: I also checked at 1m distance, but the graph above is a comparison at 3.5" = ~9cm] distance straight on the tweeter axis at the center of the tweeter/midrange speaker combination (mounted coaxially on the KEF R3).

I also looked at the sensitivity to placement and room variations, by moving the speaker (and microphone stand) a foot further into the room, and by opening a door. The effects were all below 100Hz, and limited to changing the depth of dips and moving a low dip frequency by a few Hertz. There were no effects on the general level at 200Hz, or in the high frequency range. 

As you can see from the plot, I can reproduce the setup when replacing microphones to within fractions of a dB.

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Last edit: by gruesome.

2 - 3 dB difference between two Umik-2 in identical setup 1 week 2 days ago #65527

  • Ultrasonic
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Not a difference I'd be happy with myself. I'd suggest you contact miniDSP support via the link at the top of the page and see what they have to say. 

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2 - 3 dB difference between two Umik-2 in identical setup 1 week 2 days ago #65528

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You are like the man with two watches. 
A man with one watch is sure what time it is. 
A man with two watches is never sure.

You need a third one  ;-)

Or have the company take back both of yours and sort things out. 

 

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2 - 3 dB difference between two Umik-2 in identical setup 1 week 2 days ago #65532

  • gruesome
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Thanks for the replies. The miniDSP dev team is replying to my service ticket. I sent them the REW .mdat files for the four measurements in the graph I posted. Hopefully they can figure out what is going on with the calibration data. I hope this is not an instability or drift issue with the mics themselves. A 3dB systematic deviation is not something one would want in a room equalization curve.

Apparently early on after introduction of the Umik-2 there was a problem with the intercalibration; maybe something similar crept into the calibration chain again? This miniDSP dev team post has some details. I notice that the two mics I have agree at 1kHz, which is where the calibration curves are anchored, so maybe not all is lost. But the slope of the response is equally important: you can't do much with just one calibration point at 1kHz.

Does anybody else have access to more than one mic and could do a quick comparison? I would really appreciate that. If you don't touch the gain settings and the microphone stand, and just swap out the mic on the stand, the SPL curves are quite reproducible.

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Last edit: by gruesome.

2 - 3 dB difference between two Umik-2 in identical setup 1 week 2 days ago #65533

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Of course all this could be user error; I'm no REW expert. I was wondering whether letting REW automatically set the measurement time windows could have a significant effect on the response (e.g. by allowing more room reflections in for one mic vs the other, for some reason), so I changed the analysis preference settings and reanalyzed the four data files. No visible effect on the FR spectra. Room decay time is below 300ms for most of the range, rising to 500ms at the very low end. Placing the mic close to the speaker should somewhat minimize room effects anyway.

Also no visible effect when deselecting the 'limit calibration effect to 20dB' switch. 

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2 - 3 dB difference between two Umik-2 in identical setup 5 days 17 hours ago #65586

  • gruesome
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[...]
Does anybody else have access to more than one mic and could do a quick comparison? I would really appreciate that. If you don't touch the gain settings and the microphone stand, and just swap out the mic on the stand, the SPL curves are quite reproducible.

Anybody with more than one Umik-2 that could run a quick comparison? I have not heard back from miniDSP since Friday. 

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Last edit: by gruesome.

2 - 3 dB difference between two Umik-2 in identical setup 4 days 2 hours ago #65624

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Still nothing from miniDSP.
Update: miniDSP responded today. They were puzzled, and suggested some more tests.
I did those, and now both microphones behave similar, and both match the old data for one of them! I can not reproduce the old sloping shape of the frequency response in REW measurements with the other one at all. The mics are within about 1 dB, the main difference is which calibration curve gets applied.

I tried restoring the default analysis settings in REW, but I must have changed something, since the old data are not reproduceable for one of the two microphones (and none of the REW analysis settings seems to make a difference for the shape of the response curve).

So, I'm still not 100% happy, since it would be nice to know what went wrong, but I can now get reproducible data, and I hope I will be able to use my measured KEF R3 near field response as a 'calibrated source' for future measurements.

Thanks to everybody and miniDSP for their efforts to help me sort this out! 

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Last edit: by gruesome.
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