The oscillation of a vibrating body can cause a periodic disturbance that travels through an elastic medium (air, ground, or water) and radiates outward in straight lines in the form of a pressure wave perceived as sound. Based on whether or not it can be heard by the human ear, sound can be divided into communication waves (audible) and ultrasonic waves (or ultrasound, inaudible). Ultrasound, having little or no effect on the ear even at high intensities, vibrates at frequencies greater than 20 kHz and is produced by a transducer, which contains a piezoelectric substance such as a quartz crystal oscillator and converts high-frequency electric current (an input of energy) into vibrating ultrasonic waves (an output of energy) with a fixed relationship.
Ultrasound was first developed in World War II to locate submerged objects. Additional uses have been developed for industrial applications in the field of nondestructive testing, cleaning, welding, and sonochemistry .
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