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Calculating THD from FFT data

I'm trying to calculate the THD of a 8" speaker driver from the FFT of the current waveform in the driver under some test conditions. I understand the basic concept of adding up the power level of the harmonics in relation to the fundamental to get THD. I've been playing around with the method described on this page. My confusion is that in the FFT data from the oscilloscope the fundamental is not that close to 0dB at low levels or high levels. At a low levels of ~6W the fundamental is -10dB. At high a levels of ~180W the fundamental is +5dB. Should I be normalizing the FFT data so the fundamental is at 0dB before computing THD or not?

For example, in the ~180W test case normalizing the data so the fundamental is at 0dB gives me a THD of 20.05%. Leaving the data alone gives me a THD of 36.5%.

Thanks!

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After thinking this over a bit more I think the fundamental needs to be at 0dB since THD is a ratio of the power in the higher harmonics vs. the fundamental.
REW's new beta version does THD for you, as well as each harmonic if called upon.

JSS
Quote:
Originally Posted by maxmercy

REW's new beta version does THD for you, as well as each harmonic if called upon.
JSS
That doesn't do me any good when I'm using an oscilloscope and doing the FFT (and THD) on the waveform of the current through the driver though as opposed to acoustic THD with a microphone.
Quote:
Originally Posted by Stereodude

That doesn't do me any good when I'm using an oscilloscope and doing the FFT (and THD) on the waveform of the current through the driver though as opposed to acoustic THD with a microphone.
If you're not measuring the acoustic THD you're not getting a useful result. It is, after all, acoustic THD that you hear.
Quote:
REW's new beta version does THD for you
HolmImpulse does also.
Normalizing shouldn't change the results. It's just a scaling factor that would be cancelled out.

Convert the dB values to power then THD = (Sum of power of harmonics / power of fundamental). Much easier than going to voltage like your link says to do for some reason...

Power = 10^(dB/10)
Quote:
Originally Posted by Bushi

Normalizing shouldn't change the results. It's just a scaling factor that would be cancelled out.
Convert the dB values to power then THD = (Sum of power of harmonics / power of fundamental). Much easier than going to voltage like your link says to do for some reason...
Power = 10^(dB/10)
Yeah, it looks like the spreadsheet off that link assumes the fundamental is 0dB which is why the data is changing instead of using the fundamental. I'm going to change it later to use power and consider the fundamental.
Quote:
Originally Posted by Bill Fitzmaurice

If you're not measuring the acoustic THD you're not getting a useful result. It is, after all, acoustic THD that you hear.
Yes, I realize that. I was curious to compare the two. See how much THD is from the motor vs. the rest of the driver.
Quote:
Originally Posted by Stereodude

Yes, I realize that. I was curious to compare the two. See how much THD is from the motor vs. the rest of the driver.
That depends on the signal level. That from the motor is relatively constant, that from the cone is proportional to excursion; most THD at normal listening levels is cone sourced. Once you hit xmax that from the motor becomes significant again, as both the motor and cone are clipping the fundamental, causing THD to skyrocket.
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