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Microphonics describes the phenomenon where certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise). The term is derived by analogy to real microphones where that behavior is intended rather than inadvertent.
When electronic equipment was built using vacuum tubes, microphonics used to be a very serious design problem. The charged elements in the vacuum tubes would vibrate and the motion would change the distance between the elements, producing charge flows in and out of the tube in a manner identical to a capacitor microphone. A system sufficiently susceptible to microphonics could experience feedback.
With the advent of solid state electronics (transistors), this major source of microphonics was eliminated but smaller sources still remain. The ceramic dielectrics used in high-K capacitors ("Z5U" and "X7R") are piezoelectric and will directly transform mechanical vibration into a voltage in exactly the same fashion as a ceramic microphone. Wiring and cables can also exhibit microphonics as charged conductors move around and various materials can develop triboelectric ("static") charges that couple to the electronic circuits.
The sound of guitar amplifiers, since they usually incorporate the electronic chassis into the same cabinet as the speaker, can be highly affected by microphonics. This is usually part of the special sound of a guitar amplifier, though a faulty vacuum tube or other component can cause out of control feedback.