During startup of this rig I noticed as the electric pulses of music pass through the capacitors they vibrate. That is you can hear the capacitors playing music.
I've never seen vibrating capacitors and never heard them play music too.
microphonic caps? new one on me. I have seen i on guitar amp tubes and some pedals but not on discrete components. that aside it seems risky to assume the nature of sound from a cap is linked to the sound of the system they are installed in. Sounds like your burn in rig is driving the crap out of those caps and likely just burning them up.
Capacitor microphonics are easily detected at the surface with a sensitive instrument microphone. Try placing a phono cartridge stylus on the cap's surface and listen to it rumble with the music. We use a laser vibrometer to quantify it during the design process. Our ABX focus groups confirmed that microphonic behavior is a key determinant of sonic performance.
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its my understanding that most film capacitors are dielectric- 2 sheets placed side by side, then rolled tightly.Each sheet is one side of the electrical contact. During initial break in the voltage applied finds uneven gaps and a settling in process occurs where the use over time "irons out" the gaps because of the force of the voltage and the cap finally settles into steady state performance. Also possible that some chemical changes occur but chemistry isn't my cup of tea.