Sound is the mechanical vibrations of particles of the material medium. The human ear is sensitised to hear the sound waves between 20 Hz to 20 kHz, which is known as the audible range. Sound waves with frequencies above 20 kHz (above the audible range) are known as ultrasonic waves. The energy of ultrasonic waves is large on account of high frequency; so they can propagate through larger distance. The propagation also depends on the elastic properties and the density of the medium. Due to their smaller wavelength as compared to audible sound, a large number of ultrasonic applications are possible in almost all branches of technology, engineering and medicine. There are three types of ultrasonic waves depending on the mode of vibration of the particles of the medium with the propagation direction of the wave. They are (i) longitudinal or compressional type of waves, (ii) transverse or shear waves and (iii) surface or Rayleigh waves.