Last week, when I published my look into the RME ADI-2 Pro FS ADC performance, I started using SpectraPLUS for measurements in high resolution for THD and THD+N. I know, it's an old measurement and everyone does it. I realized shortly after publication of the article that while I was using the same technique to measure the RME vs. Focusrite ADCs, I had the ADCs running at 192kHz and so issued an addendum that the results included the rising ultrasonic noise from the ADC and not just a reflection of the DAC. Not unreasonable as a comparison between the two ADCs I think, but this would not be fair to the DAC or other component measurements since much of the noise would be arising from the ADC stretching out to a 96kHz bandwidth. It's worth taking some time to think, going forward, how I could improve the usefulness of this test and in a standardized way here when focusing on whatever device is being tested...
If we look around, we see that the THD+N spec is probably the most used objective "number" for audio equipment as a quick snapshot of fidelity. The THD (page on calculation, how it's done) component tells us whether harmonics are being added to the sine wave at the integer multiples of the fundamental frequency (these are the results of "nonlinearities" in the equipment), and the +N piece adds the noise component found in the signal being tested which of course is also subject to noise limitations of the measurement device and the computational limits of the FFT technique used. In essence, the THD+N ratio is a representation of everything that's being added to a simple single-tone test which of course has its limitations as a test paradigm as well when real music is far from static.