Near Zone Calculator & Beam Visualiser

Understanding the Near Zone

The near zone, also known as the near field or Fresnel zone, is the section of the ultrasonic beam immediately in front of the probe. Within this area, sound waves generated across the transducer surface interact with one another, creating alternating regions of higher and lower sound pressure due to wave interference.

These pressure variations make signal amplitude inconsistent throughout the near zone. As a result, echo height alone cannot be relied upon to accurately assess reflector size or defect characteristics. Once the beam reaches the end of the near zone, it enters the far field, where the sound beam becomes more stable, spreads at a predictable angle, and its intensity decreases in a consistent manner. Most ultrasonic inspections that require accurate sizing are therefore performed in the far field.

Key Definitions

For a circular ultrasonic transducer operating in a uniform material, the near field length is calculated using:

Where:

• D = Active element diameter
• f = Probe frequency
• v = Sound velocity in the test material
• λ = Wavelength

The beam divergence half angle is calculated as:

Additional relationships include:

A more precise near field equation is: N = (D² − λ²) / (4λ)
However, because ultrasonic probe diameters are typically much larger than the wavelength, the simplified equation is accurate for almost all industrial NDT applications.

Example Calculation

Improved Sizing Accuracy Many amplitude based sizing techniques, including DAC, DGS and AVG, are intended for use in the far field where beam behaviour is predictable. Indications detected within the near field may produce inconsistent amplitudes, reducing sizing accuracy.

Selecting the Right Probe Probe diameter and operating frequency both affect near field length. Larger probes and lower frequencies increase the near field distance, while smaller probes and higher frequencies shorten it. Selecting the correct combination helps position the inspection area within the most suitable part of the beam.

Reliable Calibration Reference reflectors such as side drilled holes and flat bottom holes are generally positioned beyond the near field whenever practical. This allows calibration curves to be established under stable beam conditions, improving inspection consistency.

Beam Coverage Assessment The far field divergence angle determines how the beam spreads with increasing depth. Understanding this behaviour helps inspectors confirm adequate scan coverage, probe spacing and inspection overlap during examinations.