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Ultrasound is produced using a quartz or piezo-electric crystal. Quartz has a complex structure made up of a large number of repeating tetrahedral silicate units.
When a crystal is unstressed, the centres of charge of the positive and negative ions in the lattice of the crystal coincide. If a constant voltage is then applied across the electrodes, the positive silicon ions are attracted towards the cathode and the negative ions towards the anode. This causes distortion of the silicate unit, and vibration is set within it.
If the frequency of vibration of the crystal is equal to the frequency of the applied voltage, resonance occurs and the crystal vibrates with maximum amplitude. Ultrasound is produced when the vibration is in the ultrasonic frequency range.
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