Crystal oscillators, filters and microphones depend upon which principle?

Crystals, crystal filters, and crystal microphones all operate based on the piezoelectric effect. This is a scientific phenomenon where certain materials, such as quartz, generate an electrical voltage when mechanical pressure is applied to them. Conversely, when an electrical voltage is applied to a piezoelectric material, it physically deforms. This two-way electromechanical interaction allows a crystal to vibrate at a very stable resonant frequency when placed in an alternating electric field, making it perfect for oscillators and filters. In a crystal microphone, sound waves provide the physical pressure on the crystal element, which the piezoelectric effect then translates directly into an audio electrical signal.