Section 2: Physics of Ultrasound

Of course, there are more than two points of origin along the face of a
transducer. The image below shows five points of sound origination. It can be
seen that near the face of the transducer, there are extensive fluctuations or
nodes and the sound field is very uneven. In ultrasonic testing, this in known
as the near field (near zone) or Fresnel zone. The sound field is more
uniform away from the transducer in the far field, or Fraunhofer zone, where
the beam spreads out in a pattern originating from the center of the
transducer. It should be noted that even in the far field, it is not a uniform
wave front. However, at some distance from the face of the transducer and
central to the face of the transducer, a uniform and intense wave field
develops.