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Subject THD+N internal/external

Date Wed Nov 24 2004 12:45
Author Lars Morset (

You solved this yourself, good. Anyway, here is a description and some comments:

In the wizard (to start the wizard if it is not started automatically, go to 'Measurement->Wizard->Start') for this setup you are mentioning, there is a text saying:
IMPORTANT: Click the 'Help' button below for information on how to use this setup.

When clicking this help button you will se a text that explains how you can change the mode from 'external sinusoid' to 'internal sinusoid'.
You want it to be 'internal sinusoid', 'external sinusoid' will not work well for such low frequencies.

In principle it should not be any problem to plot continuous THD graphs in parallel with Frequency Response versus SPL graphs. Please let me know what you find out.

Here is another issue
>On the swept version, if I leave the default upper and lower >frequncy limits, the plot works fine. However, if I restrict >the range to 10Hz to 200Hz which is what I would like to do >for subwoofer testing, I get a blank plot or nonsensical >THD% values. Am I doing something wrong? I would really like >this to work as it will save me a lot of time.

I checked this and you are right, it does not work.
I also got this problem with 100,000+ that you are mentioning. This has to do with the time window start that is not set to zero. This you can monitor from the time window toolbar. We will try to fix this very soon.
But all this should work if you go to 'Measurement->Settings...' and in step 5 set the range from 1 Hz to 200 Hz it will work. Remember that if you want to display THD at 10 Hz, the sine sweep must start before 10 Hz. If it starts at 1 Hz, the 10th order harmonic is at 10 Hz so you can measure up to the 10th harmonic with this setting.

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