1. Introduction

Ultrasonic as defined by Wikitonary is the science and technology of ultrasound. Ultrasound is sound with a frequency greater than the upper limit of human hearing.

Sound is the propagation of mechanical energy in the form of vibrations through solid, liquid and gaseous materials.

We can classify sounds using the frequency of the movement of particles over a given time period. In order for the sound to be audible to the human ear the frequency can't be too low or too high. Sound below approximately 10Hz and above 20,000Hz is inaudible to the human ear. Hertz(Hz) is cycles per second

In the same way that light waves that have a higher frequency than the human eye can process are called ultra-violet, sound waves that are above the human frequency range are called ultra-sound.

For centuries sound has been used to test materials, just imagine a worker hitting a casting and then listening to the sound. If there was a significant defect in the material the sound would be noticeably different compared to the sound from the same casting with no defect. This was probably the first application of NDT.

At frequencies above 100Khz sound can be formed into beams similar to the light beam that emanates from a flashlight. To continue the analogy these sound beams follow similar rules of optics in that they can be reflected, refracted, diffracted and absorbed.

There exists several different sound wave types