Ultrasounds are mechanical vibrations that are generated in and travel through any type of material (solid, liquid or gas) with a certain level of elasticity.
As a continuation of the range of sound, ultrasounds correspond to oscillating frequencies beyond the limit audible to humans and ranging from 15 kilohertz (cleaning) to more than 100 MHz (acoustic microscopy, electronics applications).
The 1 to 10 MHz range covers the vast majority of ultrasound applications for industrial non-destructive inspection. This is not a coincidence as these frequencies correspond, for common materials, to ultrasonic wavelengths measuring about one millimetre. This size provides a good compromise when it comes to directivity, absorption, detectability of small defects, ease of execution for electronic devices and reliable and affordable transducers.
The ultrasonic waves or elastic vibrations of matter have properties related to the elastic characteristics of the material. Therefore, in liquids and gases, which are media that offer no shearing resistance, ultrasonic waves are essentially longitudinal vibrations. The particles of matter move in relation to their equilibrium position, parallel to the direction of wave propagation. This type of wave is found in solids and is the most used, in practice.
However, another mode that does use shearing may exist: transverse wave mode where the particles vibrate perpendicularly to the direction of wave propagation.