Speaker Break-In - What Physically Changes During Break-In To Enable Better Sound?

It all depends on design. Big motors and acoustic suspension will dominate response on high quality drivers - spider and surround compliance being negligible. On low quality drivers then you can fully expect the manufacturer to recommend a break in period - this is because the response of poor quality cheap drivers will be affected by minor things like spider and surround.

On high quality speakers the drivers should be ALREADY broken in by extreme stress testing like in these two test examples (especially watch the second test on a small driver which is totally extreme)

https://youtu.be/ZEBICv7QPDM

The following video shows how drivers are built and individually tested from the ground up.

https://youtu.be/HJHul8HgbPs


Many speaker manufacturers are simply using mass produced OEM parts often they don’t even run multiple tests except a go no go (it works or it doesn’t). Mass produced parts produced for a large market of speaker builders are designed to be easy to build and reliable with low reject rates. This means performance and tolerances are much looser with design considerations weighted towards reliability and ease of manufacture rather than pure performance. These speaker manufacturers may indeed recommend break in as parts may not have settled thermally and mechanically....having never been tested thoroughly until you receive your speaker.

What sort of a chimeric abomination is a "michiganbuckeye"?  Have you no shame!

+1 onhwy61 I didn't catch that. Get rid of the buckeye part and will get along fine.

The capacitors, resistors, inductors, internal wire, solder joints and binding posts. Capacitors alone take up to 500 hours to sound their best.  

In addition to what Bill stated, IMHO, the mechanical side, (spiders, surrounds, voice-coils, etc.)  is just as important to break-in.

Nothing changes *physically*. What changes is your ears and perception. Try the same thing with second hand speakers. You'll swear they too are breaking in. Balderdash.  If the compliance of all mechanical parts "broke in" over several hundred hours, that *degradation* in materials would not magically stop at the precise point at which they magically reach their best sound.  It's not magic, it's accommodation <erects flameguard>.