The ultrasound field from a circular ultrasonic transducer is shown below. The pressure amplitude is indicated by red color, with heavier colors indicating higher amplitude.

Since the ultrasound originates from a number of points among the transducer front face, the pressure amplitude along the beam is affected by constructive and destructive wave interference, which leads to significant fluctuations near the source. The ultrasound Near Field is defines as the distance immediately in front of the transducer in which the ultrasonic beam exhibits complex and changing wavefronts. The Near Field is also called Fresnel Zone.

The pressure waves combine to form a relatively uniform front at the end of the Near Field, and the pressure amplitude decreases monotonically in proportion to distance. The zone beyond the near field is called the Far Field, or Fraunhofer zone. The pressure reaches its maximum amplitude at the boundary between the Near Field and Far Field, and this point is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer.

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