The 7N part is a reference to the purity percentage expressed as the number of nines of purity being claimed. “Two nines” (2N) would be 99% pure, “three nines” (3N) would be 99.9% pure, and so “7N” would be 99.99999% pure.
The standard for industrial copper wiring is 3N, and that’s generally what you’ll get if you go buy a bulk reel of non-specific copper wire somewhere. And, at least at an industrial level, I believe 4N is still the highest grade that’s actually certifiable (it may have changed).
Purity affects the overall metal density, which affects conductivity. Higher purity is ore conductive and a wave front will have a higher velocity factor across the charge carriers. The difference in conductivity between 2N and 6N copper is on the order of about 1%, where as 2N silver would be 6%.
In real terms, this matters more for a) very long runs of wire (e.g. transformers) and b) for cryogenic applications. The tiny change in impedance and velocity factor won’t even show up without very specialist, and expensive, test equipment, and maybe not even then.
Most of the best cables use what claims to be 6N copper (with silver it’s more commonly 4N). Whether that’s verifiable or not is questionable. But regardless that’s what tends to be used … as much as anything because successful marketing has made it something that is “expected” among audiophile cable buyers.
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OCC is another thing. That’s about reducing/eliminating the internal crystalline structure of a conductor. You can see the results/differences in material cast this way under an optical microscope (though for small boundaries in “normal” castings it’s easier with an electron microscope, STM or field-ion scope).
Why is this interesting?
In any conductor, any boundary or junction causes reflections … i.e. a wave gets reflected back down the cable. That wave will interact with the primary signal causing additive and subtraction effects to the voltage on the line (it’s a complex deal to assess, with multiple factors … this will give you an idea).
This is measurable, and in fact there are commercial tools that do just that on a gross-level (e.g. to find breaks or iffy junctions) using something called “Time Domain Reflectometry”.
So, the vastly smaller number of crystals in the metal means fewer boundaries and fewer internal reflections. A true single-crystal conductor would only have reflections at the termination points of the cable. The audiophile interpretation of this is typically that all these internal reflections cause distortion, loss of detail, and so on. And, at a raw level, that’d be true. Whether the effect is audible or not is another matter entirely, but it’s certainly calculable and is a concern in the design of certain types of network and other signalling systems.
So, I use raw 6N OCC wire to build my cables.
And then I assemble multiple runs of that to give me the geometry (responsible for noise/EMI rejection) I want and to attain the effective wire gauge I want (which has a MUCH bigger affect on resistance than the purity of the conductor). Then I add shielding to it. Then the whole assembly is cryogenically processed (the theory is that this helps with the crystal structures in the non-OCC parts of the cable, such as the connectors). And finally I have it electron-beam irradiated, which improves the flexbility of the cable (which is beneficial for microphonics).
The cryogenic treatment is a batch process and adds almost nothing to the cost of the cable itself, and while I don’t necessarily think it makes an audible difference, its so cheap to do, and there are enough people that want it done, that there’s not a good reason not to.
