Hi @Gruesome,
Let’s say the harmonics generally are 2,3,4,5,6,7,…N. So, with a 1 kHz tone 2 would be 2 kHz, 3 would be 3 kHz, etc.
There’s no way to just use, for example, 3 and 4th harmonics for computing THD if that is what you are wondering.
Now, if you wanted to limit the harmonics to just 2, 3 and 4 you could do that by setting your Measurement Stop Frequency (right click on THD) to 4.5 kHz.
And then, only 2, 3 and 4 will be included in the calculation.
You can also use the THD visualizer if you want to see the harmonic levels at each freq. For example, we have this spectrum:
And we have this THD Visualizer. Note H3 is -114.7 and H4 is -109.4.
If we did want to know the THD due to just H3 and H4, the math here is straightforward: Convert each to linear, square each, add, then sqrt the sum, convert back to dB. GPT does a good job with the math (see image below), but it’s quick fast on a calculator too. And a gut feel works well here too: If the two tones are 10 dB apart, it’s dominated purely by the larger tone. If 5 dB apart, add another dB. If equal, add 3 dB. Using this rule, you can usually look at a few harmonics and quickly get very close what the sum would. And using that rule we can look at the harmonics in the bar graph, mentally toss all but H3 and H4 (because they are >10 dB below H3 and H4), see H3 and H4 are within 5 dB, add a dB and we arrive at an answer of -108.4 dB (And note displayed in the spectrum with THD reported at -108.26…so just by eyeballing you can get very close very quickly).
) ! It’s super fun to be able to turn the thing on by just plugging one cable into my tablet, and not having to worry about calibrations etc.