Near field: what it means and why it matters
The near field is the region closest to a transducer where the sound pressure along the beam axis swings up and down through several maxima and minima. It ends at the last on-axis maximum, a distance N from the element. Beyond that point the beam is in the far field and spreads smoothly.
The formula
For a round element of diameter D, the near field length is N = D² / 4λ, where λ is the wavelength in the test material. Because λ = c / f, the same thing can be written N = D² f / 4c. Larger elements, higher frequencies and slower materials all make the near field longer.
Worked example with made-up numbers: a 12 mm element at 4 MHz on a material with a longitudinal velocity of 6.00 mm/µs. The wavelength is 6.00 / 4 = 1.50 mm, so N = 144 / (4 × 1.50) = 24.0 mm. Some sources add a small correction term, which gives about 23.6 mm here. If your answer is slightly off from a practice key, check which form it uses.
Try one question
Question 1 of the free 10-question check on UT L2. No signup.
Discontinuity Detection
A planar reflector gives its highest echo amplitude when the beam strikes it
Choose an answer to begin
Why amplitude is unreliable inside it
In the near field the pressure on the beam axis has several peaks and dips. A small reflector can give a strong echo at one distance and a weak one at another, even though its size has not changed. That makes size evaluation by echo height unreliable there. The far field, beyond N, gives a steady decline with distance, which is why distance amplitude comparisons are normally made out there.
Ask Echo before you buy AI your Angle Beam study buddy
See what's on the UT L2 exam, test yourself with 5 quick questions, or find the plan that fits you.
The UT L2 course
Available now · instant accessLearn every General exam topic, work the calculations step by step, then prove it on timed exams.
- 7modules of lessonsEvery task on the outline
- 5full-length timed exams50 questions in 120 minutes each
- 340practice questionsEach one explained
- 5worked calculationsStep by step
- 1week-by-week study planSet to your exam date
Turn a practice answer into a study step. In untimed practice, ask Echo AI about any question you answer, follow up, then see your weakest topics and what to drill next.
Instant access. Terms and refunds · Secure checkout by Stripe
- All three courses bundle$299$108 less than buying the 3 courses separatelyIncludes the UT L2, MT L2 and PT L2 courses
Terms and refunds · Secure checkout by Stripe
- Free sample lessonFree
- Question of the dayFree
Every answer comes with a full explanation.
What changes it
- Diameter: doubling D quadruples N, because D is squared.
- Frequency: doubling f doubles N.
- Velocity: a faster material makes the wavelength longer and N shorter.
- Wedge or delay: on an angle beam probe part of the near field is used up inside the wedge, so the useful near field in the test material is shorter than the number from the formula.
Mix-ups to avoid
- Near field versus dead zone. The near field is about beam physics and exists even with perfect electronics. The dead zone is about the initial pulse and ring-down masking echoes. They can overlap in depth, but they are different causes.
- Near field versus beam spread. Spread describes how fast the beam widens in the far field. The near field has almost no spread, which is why the beam looks parallel there.
- Last maximum versus first. N is the distance of the last peak, the one farthest from the element.
How the exam uses it
Near field comes up under general evaluation and techniques, usually as a calculation from a diameter, a frequency and a velocity, or as a conceptual question about why a reflector's echo height cannot be trusted close to the probe. Know the formula, the units that make it work (mm, MHz and mm/µs) and the direction each variable pushes N.
Topic names here follow the UT General outline for exams before 5 February 2027; the UT outline change guide shows how the later list lines up.
Practice it
Try your own diameters and frequencies in the UT beam calculator, and read the other beam formulas in the UT formulas guide. What happens past N is covered in beam spread, and the masking problem close to the surface is the dead zone. The full term list is in the NDT terms glossary.
Know the term? Check the rest of the UT General topics
Take the free 10-question check. It covers the UT General exam, needs no signup and explains every answer, so you see which topic to study next. The course then adds timed practice exams and a study plan built from your exam date.
Sources
Show the list
Background reading: Evident's published UT material and ASNT's published topic headings. Outline checked 25 October 2026.
- Evident, ultrasonic flaw detection tutorial
- Evident, ultrasonic transducer technical notes
- ASNT NDT Level II examinations, published topic headings.
Not affiliated with or endorsed by ASNT.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
Your next step
Ready to see where you stand on UT L2?
Free, untimed and scored by domain. No signup.
Start the free 10-question checkNDT UT Level II General course. Lessons, timed practice exams and a plan around your exam date. Get NDT UT Level II General course · $149 See the plans or read the free sample lesson.Terms and refunds