Sound propagates through vibration. Sound waves are mechanical waves generated by vibrating objects, which travel via a medium (such as air, water, or solids) and cause the molecules of the medium to vibrate in a dense-rarefied pattern. This is how sound transmits from the source to distant locations.
The human ear can detect sounds within a frequency range of 20 Hz to 20,000 Hz. When the frequency of a sound wave exceeds 20,000 Hz, it becomes inaudible to humans; such high-frequency sound waves are defined as ultrasonic waves. In simple terms, ultrasonic waves are high-frequency sound waves that we cannot hear.
So how are they generated? Like ordinary sound waves, ultrasonic waves are produced by the vibration of objects. Common methods for generating ultrasonic waves include the piezoelectric method, electromagnetic method, and laser method, among which the piezoelectric method is the most widely used.
Scientists discovered many years ago that certain materials generate vibrations when an electric current is applied. These vibrations produce ultrasonic waves—a phenomenon known as the piezoelectric effect. Materials that exhibit this characteristic are called piezoelectric materials.
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