I have been looking through the physics.se and all over the internet for weeks now honestly, and I still don't understand how harmonics are formatted on the particle level. Yes, I know that only certain wavelengths fit an integer number of times into the length of the system and I understand how and why they would resonate if we drive the system with the frequency matching this wavelength. I see how the incident and reflected waves would have constructive interference and create nodes and antinodes.
However what I fail to understand is how a single pulse or a driving force at the fundamental frequency can possibly lead to formation of harmonics? The compressions at the fundamental frequency travel from one end to the other and back, only to be met with newly introduced disturbance wave. At what point and how do the particles of the wavefront separate into distinct wavefront of the harmonics?
EDIT: Are the harmonic frequencies introduced by the forcing vibration in the first place? For example by non-linearities in vibrations of lip-buzzing, reeds, mouthpieces and oral cavities - or are there other phenomena that are causing harmonics? Would a sine wave driving a trumpet at its natural frequency (lip buzzing at only one resonant frequency of the trumpet) create harmonics, and if yes how?
And again just to be clear, I am NOT asking how and why they resonate once they are created and what frequencies fit the Fourier series etc - but the principle of their creation in the first place.