THD, no, it's worthless. The individual orders as a graph, not a single number, yes, that's very useful. Granted it's probably not that useful to you, but it's invaluable to engineers designing a headphone.
this post was submitted on 18 Nov 2023
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You can think about it in terms of dB. 0.15% is about -56dB, so that means the distortion products they measured were 56dB below the signal. That isn't likely to be audible, especially if the distortion is in the harmonics we find "pleasing" like the 2 and 3.
below 1% is usually the standard for headphones/iems, and idk about the amp
Generally speaking, THD is just the start of a larger discussion. It's not a useful metric for listener preference.
Psychoacoustics is wildly subjective compared to the THD measurements.
Dr. Earl Gedees wrote about this in Premium Home Theater, 4.1.b (pg 58) which claims:
There is virtually no correlation between Total Harmonic Distortion (THD) or Intermodulation Distortion (IMD) measurements of a system and the subjective impression of the sound quality of that system. The correlations were weak, but most shockingly they were negative—according to these tests people liked THD distortion. This is actually somewhat true in general that people prefer some forms of distortion to no distortion.
What The [THD] Specs Don’t Tell You… And Why is a great video from RMAF 2015 from Audio Precision. The presenter demonstrates different types of distortion and shows just how inaudible or audible it can be.
Take this pre-amplifier comparison for example. The OP could reliably ABX the Conrad Johnson tube from the Benchmark solid state and preferred the one with arguably higher distortion.
This second vs third harmonic distortion test concluded that tracks with second harmonic distortion were preferred in about 75% of cases; 25% those of the third; surprisingly never the original tracks.
Have a look at this blind test of distortion. A second harmonic of 0.02% THD is weakly preferred over 0.0000002% THD in nearly every case.
Bob Katz also built a harmonic generator to test his preference as well - read about it here. Their preference? Added second harmonic distortion between -60dBFS (0.1%THD) and -66dBFS (0.050% THD).
Some people like tube amps - they have lots of harmonic distortions. It's the same with vinyl or R2R DAC - all these outdated tech are not accurate and measure terribly but some people like them.
I think it’s a misnomer calling this tech “outdated”. Calling something outdated assumes that there’s a newer and better version of said tech. While yes, something like a CD will be more technically accurate than vinyl, it’s not an upgrade to vinyl; it is its own technology which is completely unrelated to vinyl (aside from both needing to spin in order to read data/vibrations)
Assuming it is low enough (<1%) it will never matter more than pretty much anything else in your listening environment like ambient noise, psychoacoustics, mood, volume level, recording quality, the precise position of the headphones on your head, a thousand other things. It doesn't really make sense to compare such tiny metrics when you're talking about something so subjective like audio. I can bet you'll never hear the difference between 1% and 0.1% THD