Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Although beam spread must be considered when performing an ultrasonic
inspection, it is important to note that in the far field, or Fraunhofer zone, the
maximum sound pressure is always found along the acoustic axis (centerline)
of the transducer. Therefore, the strongest reflections are likely to come from
the area directly in front of the transducer.
Beam spread occurs because the vibrating particle of the material (through
which the wave is traveling) do not always transfer all of their energy in the
direction of wave propagation. Recall that waves propagate through the
transfer of energy from one particle to another in the medium. If the particles
are not directly aligned in the direction of wave propagation, some of the
energy will get transferred off at an angle. (Picture what happens when one
ball hits another ball slightly off center). In the near field, constructive and
destructive wave interference fill the sound field with fluctuation. At the start of
the far field, however, the beam strength is always greatest at the center of
the beam and diminishes as it spreads outward.