Speaker break-in is no myth and something significant really does happen. All speakers are built to meet certain specifications, and we work diligently through QC efforts during and after production to ensure that happens. Every component used in a speaker has tolerances, which can relate to small variances in initial performance. The mechanical properties of a speaker are slightly modified once a speaker is put into service, and the tone is affected by these changes. Speaker break-in is a natural process that is influenced by how much you use the speaker and how loud you play it. Think of a new pair of shoes. They are not most comfortable right out of the box. They feel best after you have worn them for a while, softened up, and formed to your feet. Much like your new pair of shoes, new speakers need time to break in and will not sound best until they do.
The components making up the speaker"s suspension are primarily what changes during break-in. These components are the spider (lower suspension) and the cone surround (upper suspension). As the speaker is used, the spider and cone surround begin losing some of their initial stiffness. The sonic results you will hear are an increase in overall warmth, slightly deeper/fatter lows, and warmer/smoother highs. Subtle changes will continue throughout the life cycle of the speaker, but the most noticeable amount occurs in the early stages of use.
The duration of time required to achieve break-in will vary between speakers. Your environment can affect speaker break-in as well. It may take longer in a cold, dry climate versus a hotter, more humid environment. Again, your usage and volume will also affect break-in time. There are several methods people use to speed up the process, but these methods can be damaging to the speaker and are not recommended. The best method is to simply play your new speaker at normal to high volume as frequently as possible. You may even find it is fun and enlightening to experience the changes in your speaker as it breaks in!