Near-field and far-field

Transducers have both near-field and far-field regions. Within the near-field, wave fronts produced by the transducer are not parallel and the intensity of the wave oscillates with range. For that reason, echo levels from targets within the near-field region can vary greatly with small changes in location. Once in the far-field, wave fronts are nearly parallel and intensity varies with range squared under the inverse-squared rule.  Within the far-field, the beam is properly formed and echo levels are predictable from standard equations. (See Near-field distance example.)