# THD measurements



## SHNEE (Jul 6, 2014)

In my setup (Umik-1 miniDSP Mic) the measured distortion figures sometimes are consistently high, like >20% other times consistently low like <5% or even <1%. Lately it has been consistently low. Any suggestions as to what might be causing this? My graphs seems fine as they are very repeatable/consistent in successive trials.


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## Steve1533 (May 12, 2013)

I sheepishly admit :heehee: that I had a similar problem the very first time I used the THD measurement graphs in REW...

In my case, the cause and explanation were simple and resulted from my unfamiliarity with the graph/cursor behavior for the THD tab in REW.

The distortion readings at the bottom of the graph are a function of the plotted frequencies, and if one moves the graph/screen cursor for frequency, the THD values change accordingly with frequency. If one is unaware of this fact, the cursor is accidently moved from time to time while otherwise using REW, causing the THD values to appear to randomly change to various odd values, even though the graph is unchanged. So, perhaps try locating and moving the frequency cursor around on the THD graph to see what I mean.

End of story for me, but maybe this is not your problem.


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## JohnM (Apr 11, 2006)

Can you attach a couple of example measurements that show the problem?


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## SHNEE (Jul 6, 2014)

JohnM said:


> Can you attach a couple of example measurements that show the problem?


I did not save the ones with very high THD numbers, the high numbers could have been due to the cursor placed at the very low frequency such as 10 to 12 Hz. I do have a few that shows THDs being different in successive runs, but to much lesser extent, and again only at the very low frequencies such as between 10 and 20 Hz. Also, as Steve pointed out, those huge discrepancies between sweeps could have been due to my cursor not in the same place, though I really don't remember moving my cursors between sweeps.


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## SHNEE (Jul 6, 2014)

Steve1533 said:


> I sheepishly admit :heehee: that I had a similar problem the very first time I used the THD measurement graphs in REW...
> 
> In my case, the cause and explanation were simple and resulted from my unfamiliarity with the graph/cursor behavior for the THD tab in REW.
> 
> ...


Thank you Steve, that could well be my problem and I probably had the cursor placed in the extreme low end. However, I still have saved files of successive runs that show THDs being different though not by as much as I had seen before.


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## AudiocRaver (Jun 6, 2012)

Those high low-frequency numbers, with variation from plot to plot, are often a result of noise floor problems with the measurement environment - noise picked up from traffic outside, equipment, footsteps, etc.


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## Sleiven (May 20, 2013)

I cannot figure out how to g+et the THD plot to look like yours, with THD and level plotted against frequency on the X axis. How do I do that?, I can use the RTA plut but then I only get THD values for a single tone/frequency. How do I make this "frequency sweep plot" with THD? 

Thanks for advice

Sleiven


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## AudiocRaver (Jun 6, 2012)

Sleiven said:


> I cannot figure out how to g+et the THD plot to look like yours, with THD and level plotted against frequency on the X axis. How do I do that?, I can use the RTA plut but then I only get THD values for a single tone/frequency. How do I make this "frequency sweep plot" with THD?
> 
> Thanks for advice
> 
> Sleiven


It has to be based on a frequency sweep measurement, not an RTA measurement.

Once the frequency sweep has been taken, simply click on the _Distortion_ button along the top row of buttons. You might have to adjust the vertical scale (_zoom out_ at upper-left corner of plot area, or _Limits_ button at upper right) to see all of the individual distortion plot lines.


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## Sleiven (May 20, 2013)

I cannot find any THD button in REW, (except on the RTA display) ??


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## Sleiven (May 20, 2013)

after upgrading REW 5.0 til 5.01Beta the TDH button appared,,,


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## AudiocRaver (Jun 6, 2012)

Sorry, was holidaying and missed that last question. Glad you figured it out. We always recommend keeping up with the latest beta level of REW, as John is continually adding features, as well as staying on top of bug fixes.


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