Here's an interesting article by Rick Turner (of Compass Rose ukuleles) on your question:
https://www.premierguitar.com/artic...dboard-the-sonic-effect-of-time-and-vibration.
In short, every experienced musician and luthier knows it and believes it, but there is very few scientific proof or explanation. Some hypotheses:
- any playing enhances an instrument: wood fibers loosen up along modal nodes and lines, like metal fatigue - hence the fibers move more, and produce more volume and harmonies.
- good playing (in tune) enhances an instrument: change in frequencies makes the right modal nodes and lines are 'imprinted' in the loosening wood fibers.
- time in itself enhances an instrument: finishes and natural oils harden, moisture evaporates (even with well-dried tonewoods, and in climate controlled environments), cell structures crystalyze even, making the wood harder and lighter (better weight-to-stiffness ratio) and more resonant.
- any vibration (even non-musical) enhances an instrument: the parts become more attuned to each other by movement - there are stories about resonator guitars becoming better after having been dropped.
- lots of playing makes the player become more attuned to the instrument, which in itself doesn't change a lot: the player automatically develops an adapted technique to suit the instrument through simple repetitive playing, and starts to recognize more tonal qualities he can coerce from the instrument. This one is more or less a debunking hypothesis.
There is not a whole lot of scientific experimental work to support any of the assumptions above. Wood instruments do become lighter with age, and good players tend to quickly adapt their playing style to their instruments, but it could be a mix.
I do believe instruments change over time, especially the ones that are lightly built out of solid woods and that have been played a lot in tune or out of tune; and that the change is almost exclusively for the better. I'm not sure if intonation and playing in tune is required.
Some luthiers have used 'time machines' for drying or 'playing machines' for either strumming in guitars or playing recorded music close to the finished instrument, but it's not a standard practice.
Bob Taylor also noted that either ageing or playing in (instruments becoming better with the passing of time or lots of hours being played) "is not a myth. But nobody really knows why."
