Why do capacitors sound different?

In the past capacitors were just capacitors and sound quality was determined by the dialectric material with polypropylene considered by many as "the best" seeing as this type of capacitor had the lowest losses. But technology of the 21st century has brought us new measuring techniques and insights and there seems to be more to it. We can now measure things that were not possible a few years ago. In a nut-shell: microphony is the keyword - the mechanical resonance, a key feature of audio capacitors. This is a physical deformation of the capacitor walls which occurs as a result of the audio signal passing through the component - much like an electrostatic speaker. This resonance is dependent on the size, shape, materials and manufacturing parameters of the capacitors. This effect has been known about for years as it plays a part in the impulse strength of capacitors. However, the effect has never been considered read the white paper on mechanical resonances inside capacitors. Another interesting article was written by Martin Colloms back in 1985 in which he tested several capacitors on their sonic differences. Also look at the equivalent circuit diagram of a capacitor, this also explains a lot!