ULTRASONIC INSPECTION- THE PRINCIPLES OF SOUND, WAVE PROPAGATION, Longitudinal or Compression Waves, Transverse or Shear Waves, Surface (or Raleigh) Waves


In ultrasonic testing use is made of the basic physical property that sound waves travel at known constant velocities through any sympathetic medium. By measuring the time for a   sound wave to travel through a material it can be determined how far
that wave has travelled.  In this way sound waves can be used to measure distances. Use can also be made of the fact that sound waves are reflected at an interface between two materials such as steel and air to detect defects.


In order to develop and make the best of these principles the basic physical properties of sound should be understood.




Sound is a series of mechanical vibrations or pressure waves which bring about a state of alternative compression and rarefaction, (or dilation) of the molecules or particles in the material in which they are propagated.


To convey sound such as speech or a ringing bell, medium is required which possesses elastic properties, i.e. all solids, liquids and gases. The pulses of sound can be described as discrete events within the material following a waveform.


1. The higher the density, the lower the velocity.

2. The higher the strength or elasticity, the higher the velocity.

3.  The velocity of sound is constant for any given medium although it changes from medium to medium-



There are several different types of sound waves. The three principle types are as follows:


   Longitudinal or Compression Waves


Longitudinal waves like speech consist of alternate compression or dilation of pressure waves and these vibrations move in the same direction as the energy of propagation, The particle mechanism depends on the elastic interconnections, thus as each particle moves from equilibrium, it pushes or pulls the adjacent particle and transmits at the velocity of sound.

Longitudinal or Compression Waves
  Longitudinal or Compression Waves



   Transverse or Shear Waves


The particle vibrations are at right angles to the propagating source and can only exist in materials possessing shear elasticity”. By virtual of its path, the velocity of shear wave is approximately half of longitudinal velocity thus the wave length is also half. The shear wave motion can be compared to the whip-like action of a rope attached to a wall.

Transverse or Shear Waves
Transverse or Shear Waves

(or Raleigh) Waves

These are similar to transverse waves but differ in that they do not penetrate below the surface by more than one wavelength, and have a velocity of 2% less than transverse waves. The particle motions of surface waves are a combination of longitudinal and transverse waves and thus follow a rolling or orbital particle motion.

Surface (or Raleigh) Waves
Surface (or Raleigh) Waves

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