The sound that emanates from a piezoelectric transducer does not originate from a point but instead originates from most of the surface of the piezoelectric element, Round transducers are often referred to as pista source transducer because the sound field resembles a cylindrical mass in front of the transducer. The sound field from a typical piezoelectric transducer is shown below. The intensity of the sound in indicated by color, with tighter indicating higher indicating higher intensity. Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference. These are sometimes also referred to as diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variation with in a near field, it can be extremely difficult to accurately evaluate flaws in materials when the are positioned with in this area.The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. In the far field, the beam spreads out in a pattern originating from the centre of the transducer the transition between the near field and the far field occurs at a distance. N.and is sometimes referred to as the natural focus of a flat (or un focused)transducer. The near/far field distance. N is significant because amplitude variations that characterize the near field change to a smoothly declining amplitude at this point. The area just beyond the near field is where the sound waves is well behaved and at its maximum strength. Therefore optimal detection results will be obtained when flaws occurs in this area.