The human ear is an amazing instrument, but when we are talking variations of single digit ppb, or ppt, I think we have arrived at the point of diminishing returns, don’t you think? ; )
Separately, and just as a matter of intellectual curiosity, wouldn’t it take a (very) long time to test and verify ppt accuracy?
Actually no.
For example, the human nose has been able to pick up changes gas chromatographs struggled with.
Then there are canine noses.
Yes, absolutely. I believe there are scientific papers which suggest the human hearing threshold for jitter is around 15 picoseconds, no less. So, any Clocking system that improves jitter any less than that is likely not useful.
(That said, my involvement with Clocks, specifically precision time synchronization, distribution, and measurement, is actually work related, not just the Audio hobby).
As for how long it would take to measure them, even for Cesium Clocks, which are a 1,000 times more stable than Rubidium Clocks, there are ways to measure both accuracy and stability in much shorter order, like in days, albeit with very specialised expensive test equipment, and mathematics.
Interesting and thank you @Anupc.
From this thread and a little research I have learned that the time standard in the United States is based on a Cesium clock:
“The uncertainty of NIST-F1 is continually improving. In 2000 the uncertainty was about 1 x 10-15, but as of January 2013, the uncertainty has been reduced to about 3 x 10-16, which means it would neither gain nor lose a second in more than 100 million years” (!)