Ultrasonic nozzles are a type of spray nozzle that uses high (20 kHz to 180 kHz) frequency vibration to produce very narrow drop size distribution and low velocity spray from a low viscosity liquid (such as water). The vibration of a piezoelectric crystal causes capillary waves on the nozzle surface liquid film. The primary factors influencing the drop size produced are frequency of vibration, surface tension, and density of the liquid.