The argument regarding the issue of equipment and cable break-in (and, for that matter, the whole subject of cables in general) has raged for years among subjectivists and objectivists. That debate will not soon (if ever) be resolved.
There is absolutely no question in my own mind that the sound of a component or cable changes during the first few hundred hours of use. As audiophiles we know this process as "break-in." It has been proven to me hundreds of times, though my own experiences and those of my clients and others in the industry.
Most every manufacturer in the (audiophile) industry believes that the sound of their product changes (improves) after a break-in process, and almost always address state this their manuals. I honestly do not know if any of them have measurements that document break-in, though I doubt it, and it certainly doesn't make send to fund such research. At least as far as the audio industry is concerned, having numbers to validate what listeners have been hearing for years is meaningless.
But is there a technical explanation that clarifies what's? The answer depends on the item (cables, electronics, etc.) and who you ask. I believe we're still learning what is transpiring, but here are a number of theories.
Cables: Most all cable manufacturers agree that break-in is a result of changes in the conductor and/or dielectric. According to one manufacturer: "The insulation (or dielectric) will absorb energy from the conductor when a current is flowing (i.e. when music is playing). This energy-absorption causes the dielectric's molecules to re-arrange themselves from a random order into a uniform order. When the molecules have been rearranged, the dielectric will absorb less energy & consequently cause less distortion."
Cardas has, for years, included a Cable Break-in Guide with their cables. In it they state: "All cables need a break-in and warm-up period. Better cables require longer break-in. With all cables, the more you play them and the less you move them, the better they will sound." The unique geometry of Cardas cables require that "...the strands be of equal tension... Current flowing through the cable during break-in, and each warm-up period, will relax the structure of the strands."
Another prominent cable designer believes that during the break-in process electrons are establishing new micro pathways through the conductor material. changes in the conductor is the primary reason for the improvement realized through break-in. They believe that most of the signal travels across the surface of the conductor. Viewed under a microscope the surface that appears smooth to the naked eye is really a series of peaks and valleys. The irregular surface forces electrons along a circuitous path to their destination. When a cable is bent or twisted, new tears and fissures form, disrupting existing pathways and requiring new ones to be formed. This explanation lends credence to reports that cables need to be re-conditioned and being handled. I've seen this in a very real way.
Years ago, when break-in first became realized, we decided to complete the process before shipping our cables and advertised the service as a value added benefit. It didn't take before we started hearing back from customers who questioned whether we'd actually done what we promised since their cables indeed improved after a week or two of use. We soon learned that the act of coiling the cables for shipment, and them being straightened after arrival, was the culprit.
While there is no hard data (that I am aware of) to prove to the engineers among you that break-in in exists, I'm sure 99.9% of audiophiles have heard the benefits of the process and believe strongly in its importance.
The articles I've written on the topic are not intended to convert engineers looking for measurements or other empirical proof, but rather to explain to listeners why they hear changes in the sound of a new component or cable. I often get calls from people new to audio, who express surprise that the sound of their new system has changed and are eager to find out why.
So, as I stated previously, I doubt this debate will ever see resolution. We all have our precepts, and they will determine what type of proof we are willing to accept. But those who refuse to waiver without hard data might consider the following quote from a fellow scientist: