A Ultrasonic transducer is a device that converts other types of energy into ultrasonic vibrations. There are many basic types, classified by energy source and wave-forming medium. Mechanical devices include gas-driven or pneumatic sensors, such as whistles and their liquid-driven sensors, such as hydrodynamic oscillators and vibrating blades. This equipment is limited to low ultrasonic frequencies and has many industrial applications including drying, ultrasonic cleaning and fuel injection into burners. Electromechanical transducers are widely used, including piezoelectric and magnetostrictive devices. A magnetostrictive transducer uses a magnetic material in which an oscillating magnetic field is added to squeeze the atoms of the material together to produce periodic changes in the length of the material, resulting in high-frequency mechanical vibrations. Magnetostrictive transducers are mainly used in low frequency range, and are commonly used in ultrasonic cleaning machines and ultrasonic processing applications.