Near-Field

The near-field is the region close to an ultrasonic transducer where the beam has complex pressure variations and a converging shape; precise amplitude measurements are unreliable until beyond the near-field length N.

Definition

The near-field (Fresnel zone) extends from the transducer face to a distance N ≈ D²/(4λ), where D is element diameter and λ is wavelength. Within this zone the beam undergoes complex interference and pressure fluctuates rapidly.

Technical Context

Reliable amplitude-based sizing requires reflectors beyond the near-field, in the far field. Probe selection always considers the expected flaw depth versus N.

Related terms

Far-Field — The far-field is the region beyond the near-field distance N where the ultrasonic beam diverges in a predictable manner and amplitude decays inversely with distance, suitable for quantitative sizing.
Beam Spread — Beam spread is the angular divergence of an ultrasonic beam beyond the near-field distance, governed by transducer diameter and frequency; it sets the lateral resolution of the inspection.
Transducer (Ultrasonic) — An ultrasonic transducer is a device — typically containing a piezoelectric element — that converts electrical pulses to mechanical vibrations and vice versa, used to launch and receive ultrasonic waves in the test piece.

More physics terms

Back-Wall Echo · Dead Zone · Shear Wave · Longitudinal Wave · Surface Wave · Lamb Wave · Mode Conversion · Attenuation · Geometric Unsharpness · Radiographic Density

Where this comes up in practice

Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.

Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.