Wilgolf, some additional info that will be helpful in understanding how an Ethernet cable can impact audio quality. This is some basic, foundational info:
Remember, the signal is an
electromagnetic wave that propogates down the cable (it is
not electrons flowing through the cable like water in a hose or marbles in a tube). The
Vp (propogation velocity) of the signal is
frequency-dependent, and an electromagnetic "signal" wave can reach the end of the cable and "reflect back", just like water in a tub or pool. This means different frequencies propagate down and reach the end of the cable
at different times. And, because the
Vp is different for different frequencies, there will be a number of "back reflections" occuring at...different times†.
This is just some foundational info to understand, that particularly with respect to digital sources, our brains are very, very sensitive to the impact of
timing. For digital audio, for example, we can actually hear the influence of timing errors in the picosecond range, which is why we use
femtoclocks for components like DACs, streamers, network switches, network bridges, etc. The Sonore EtherREGEN uses a Crystek CCHD-575 clock, and this crystal oscillator was chosen specifically for it's audio quality. This is why components like the Cybershaft master clocks, etc., have such a positive impact, and why EtherREGEN also has an external 50 Ohm clock input*.
The 1.5 M minimal length that Shunyata uses for Ethernet cables and USB cables, for example, was empirically determined as the shortest length that can be used to minimize the impact of signal wave "reflections" on audio quality.
It's physics, so yeah, it's a real thing*
*–Here is a figure showing the impact of signal/timing for Iconoclast speaker cables from Galen Garies, EE and audio cable lead engineer at Belden.
†-these timing differences can be measured using a time-domain reflectometer.
*-An EtherREGEN's audible performance can be signifcantly enhanced by use of an external master clock, e.g. the AfterDark series of master clocks.