I'm sorry, but this is not correct.
... summing -50dB sinusoidal signal with two signals at -120dB, with the same frequency and same phase, will give you a new signal at -49.995 dB.
That's 0.005 dB difference.In the worst case scenario.
...
Thanks for doing the math. I did not. Guess you are both right and I'll update my initial "very roughly 0.5Db" estimate.
However, that number does not matter much, its only point was to show that it's "minuscule". Otherwise, the THD number/diff is ~useless: -50Db of pure-H2 will sound diff than -50Db of pure-H7 (that's approx what you hear on the linked wiki demo page).
What actually matters is the THD spectrum (i.e. the level and phase of each HD spike). Very hard to test because there are ~infinite variations. Even a simpler test using "pure Hx" is very hard because the HD spectra of your electronics can easily mask the test HDs.
IIRC, I saw a pure-Hx test somewhere and the results were like: H2 becomes audible around -20Db, H3 around -30Db and so on...
Here's a somewhat similar graph (second one from top).
Anyway, absolutely impossible to hear.
...
A strong hypothesis at -120Db and seems to be the majority opinion on ASR. Actually, the "ASR educated guess" seems to say that HDs become inaudible/useless much earlier (-80-90Db). But still, it's just a hypothesis. Not fully tested. And close to un-testable with current tech.
Personally, I won't bet on this hypothesis. It's not like I have any better proof, just different "educated guesses" (at best).
Here are a bunch of people who do not seem to buy it either. Won't pretend that I fully understand what is the H3 doing in that article but it's just the latest in a long series of HD/IMD improvements that they did in a supposedly inaudible area (i.e. Hypex UCD had "inaudible" HDs at -110DB, nCore at -115DB, Purifi at -120Db). Is that just because they can? Or is it marketing/sales only? Or maybe those minuscule THD/IMD numbers are not so 'inaudible'?! Just saying...
P.S.
sorry for the somewhat long & offtopic posts. And not sure if it's worth continuing. There is not conclusive test/proof and we can all write 1000 opinion-replies without any useful info/conclusion