A wave composed of alternate surfaced of compression and rarefication traveling perpendicular to these surfaces. A wave composed of alternate surfaced of compression and rarefication traveling perpendicular to these surfaces. The particle displacement at each point in a material is perpendicular to the direction of wave propagation.
Any mechanical wave is composed of discrete particles of material.
Provided a material is not stressed by compression or tension beyond its elastic limit, its individual particles perform elastic oscillations. The following figure illustrates the basic nature of longitudinal and shear wave propagations, the two most widely used modes of ultrasound in nondestructive testing. The figure also illustrates the relationships between ultrasonic wavelength, frequency, and propagation velocity. The top wave is call a longitudinal wave because oscillations occur in the longitudinal direction, the direction of propagation. Since compressional and dilational forces are active in it, it is also called a pressure or compressional wave.