Definition of ultrasonic waves:

Ultrasonic waves are acoustic waves whose frequency is more than 20kHz .They travel with the speed of sound. Hence their wave length is smaller than 333200cms-1/ 20000Hz = 1.66 cm

 (ג = v/υ) . These waves possess a number of properties of sound waves and exhibit some new phenomena also.

Types or modes of ultrasonics waves:

Ultrasonic waves can propagate through a medium as stress or strain waves depending upon the elastic properties of medium. Based on particle displacement of the media, ultrasonic waves are are classified into four types or modes:

(i)                 Longitudinal or Compressional or Pressure ultrasonic Waves. In the longitudinal waves particles of medium vibrate back and forth parallel to the direction of propagation of wave. These waves propagate through the medium as a series of alternate compression and rarefaction. These waves are most widely used in the ultrasonic inspection of materials. This mode is exhibited when medium of propagation has no boundaries i.e. it has infinite span. Due to propagation of these waves both pressure and density of medium fluctuate periodically.

(ii)               Transverse or Shear ultrasonic Waves. In the transeverse waves particles of the medium vibrate perpendicular to the direction of waves propagation. In this case the medium undergoes shear deformations periodically. These waves can propagate through this rods.

(iii)             Surface or Rayleigh Waves. The surface waves travel along the flat or curved surface of thick solids without influencing the bulk of medium below the surface. The depth to which these waves propagate below the surface with considerable intensity is approximately equal to wavelength of the wave.Practically all of its energy is attenuated within this depth. These waves are used to detect cracks or flaws on or near the surface of test objects. During the propagation of surface waves, the particles of medium describe elliptical orbits.

(iv)             Lamb or Flexural or Plate Waves. The lamb waves are produced in thin metal, whose thickness  is comparable to the wavelength of ultrasonic wave.

These are the four types or modes of ultrasonic waves.