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From a purely scientific standpoint...

...it makes little sense.

My credentials: Ph.D. in materials physics, and a Ph.D. mentor who earned his reputation working on the quenching of metals.

Here's the problem: While it's true, as the article states, that people use cryogenic treatment to make metals harder, that process works in a way that's just the opposite of what's described here: the goal is to quench as fast as possible, not to relax the bonds between the molecules but to essentially "freeze" the material into an unstable state...which happens to be mechanically harder. My memory is a bit vague (it's been about 20 years since I studied this stuff) but I believe the quenching is done from a high temperature--well above room temperature--and I know that it's done as quickly as possible...precisely to freeze in an unstable state...not to relax the bonds.

Now in principle if you cool the disc very, very slowly from room temperature and then warm it back up again, you accomplish precisely nothing, in a homogeneous material. This is what's known in thermodynamics as a "reversible" process--reversible here has a technical definition, but it means pretty much the same thing it means in the layman's sense.

But here's the key point: a CD is NOT a homogeneous material. It has layers, and the layers are made of vastly different materials, with different rates of thermal expansion (and contraction). So no matter how fast you cool something (or heat it) the layers will contract/expand at different rates and there will be (probably destructive) changes in the lamination. In other words, the metal (or whatever the reflective material; it depends on the particular CD I think) will come unstuck from the plastic. That is an IRREVERSIBLE process. If you cool it down--I don't care how slowly--and heat it back up again to room temperature the changes in the disk will remain. On a microscopic level, you'll be see flaws ("extended defects" as the materials scientists would say) in the lamination. The only way to fix it would be to heat it up to close to the melting point of the metal, at which point the plastic would have long ago melted away. In other words, the damage is permanent.

As most thinking people with experience in harsh climates would expect, cryoing a disc damages it. I've never tried, but you coudl probably see the effect under a good microscope. Will this have an effect on the sound? Quite possibly. I would not expect it to be a positive effect, but who knows? Since there really are only two variables in a digital storage medium--the data (bits is bits) and its readability (absence or presence of errors; jitter), both of which we're quite good at measuring--the effect ought to be easily measurable.

A couple of other points: if you want to "relax" the chemical bonds between molecules you would need to heat it up, not cool it down. The idea is to give the molecules just enough energy so that they can rearrange, then let them settle back to their lowest energy state. Here's an analogy that's easily understood: if you take a bunch of marbles and dump them on (eg) a piece of plastic with a bunch of indentations of various depth, some of them will get stuck in the higher spots. But if you shake it up (the equivalent of heating), giving the marbles enough energy to get over the hills between the valleys, you'll find that after a few seconds more of the marbles are in the deeper valleys. That process is equivalent to the relaxation of molecular bonds to a lower energy state.

Conclusions:
* Harley's account--and probably Mietner's ideas--don't make any sense. But they're merely trying (ineptly) to provide a scientific rationale for a phenomenological observation. Just because they got the explanation wrong doesn't mean that it doesn't change the sound.

* There will almost certainly be a change in the disk; but far from relaxing it to a lower energy state, cryoing the disk will have a descructive effect on the structure. That doesn't mean, however, that it cannot change the sound.

* If there is a change, it ought to be measurable. A disc damaged by cryo treatment would presumably be harder to read, so it would result (at least at fast reading rates with minimal error correction) in more disc-reading errors. The effect on jitter (to make it worse or better) is less certain, but it ought to be measurable using standard techniques.

* Key to this--and the reason I've used the word "damage"--is the laminar structure of a CD. For more homogeneous materials there would be less damage, but things like solder joints could take a real beating, since no matter how slowly you cool something down or heat it up, unlike materials will contract and expand at different rates. Solder joints will be weakened by any cryogenic process.

Hope this helps.

Jim


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