(youtube) The laser beam makes sound when it strikes the plate. By turning the laser on and off hundreds of times per second it acts almost like a speaker cone moving back and forth hundreds of times per second - making various pitches. Some pitches are off key and will always be due to the laser "only" being accurate to .001". Due to how close different note frequencies are together it would need to be accurate to .0002" or better (totally useless for industrial marking) to always be on key. The rounding up/down to .001" gets the notes "close enough" but still off by a few cents.