The term ultrasonic is used to describe a vibratory wave of a frequency above that of the upper frequency limit of the human ear; it generally embraces all frequencies above about 16Kc/s. The upper frequency range of ultrasonics is largely dependent on the generator, practical limitations imposing a maximum frequency in the region of 500 Mc/s. The range of wavelengths in varying media is very wide. For example, when propagated in a solid medium, a wave with frequency of 500Mc/s will have a wavelength of the order of 0.08 mm.
There are two types of waves, namely, shear waves and longitudinal waves. Longitudinal waves are mostly used in ultrasonic applications since waves. Longitudinal waves used in ultrasonic applications since they are easily generated. They can be propagated in solids, liquids and gases and can travel at a high velocity so that their wavelength is short in most media.
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