Near field, beam spread and dead zone: how they differ in UT

One sentence each

• Near field: the zone near the element where on-axis pressure swings up and down. It ends at distance N.
• Beam spread: the widening of the beam past N, given as a half angle from the axis.
• Dead zone: the depth right under the surface where echoes drown in the initial pulse and ring-down.

Near field and beam spread are physics of the beam. Dead zone is a limit of the transducer and the electronics. A better pulser does not shorten the near field, and a bigger element does not remove the dead zone.

Near field: N = D² f / 4c

For a round element of diameter D, frequency f and velocity c in the material, the near field length is N = D² f / 4c. With D in mm, f in MHz and c in mm/µs, N comes out in mm. The near field guide shows the equivalent form with wavelength.

Probe A: 12 mm element, 4 MHz, 6.00 mm/µs. Wavelength = 6.00 / 4 = 1.50 mm. N = 144 / (4 × 1.50) = 24.0 mm.

Probe B: 10 mm element, 5 MHz, 5.90 mm/µs. Wavelength = 5.90 / 5 = 1.18 mm. N = 100 / (4 × 1.18) = 21.2 mm.

Probe C: 20 mm element, 2.25 MHz, 5.90 mm/µs. Wavelength = 5.90 / 2.25 = 2.62 mm. N = 400 / (4 × 2.62) = 38.1 mm.

Probe C has the lowest frequency and the longest near field, because the diameter is squared. Doubling D quadruples N. Doubling f doubles it. Some sources add a small correction term, so if your answer is a hair off a practice key, check which form it uses.

Beam spread: sin α = 0.514 c / (f D)

Past N the beam widens. Evident's published form for the half angle to the 6 dB edge is sin α = 0.514 c / (f D). The constant depends on where you call the edge of the beam, so use the one your question or procedure names. The beam spread guide covers the other conventions.

• Probe A: sin α = 0.514 × 6.00 / (4 × 12) = 0.0643, so α = 3.7°.
• Probe B: sin α = 0.514 × 5.90 / (5 × 10) = 0.0607, so α = 3.5°.
• Probe C: sin α = 0.514 × 5.90 / (2.25 × 20) = 0.0674, so α = 3.9°.

All three spread about the same here. Probe C pairs a big element with a low frequency, and the two effects nearly cancel in the ratio. The point to carry away: spread depends on wavelength divided by diameter. Make that ratio smaller and the beam is narrower. The angle is a half angle, so the full beam is about twice as wide.

Dead zone: what it looks like

At the start of the A-scan sits a tall initial pulse with a decaying tail. An echo that arrives while the tail is still large cannot be picked out. Shallow reflectors return soonest, so they are the ones lost. Heavy damping, higher frequency and lower gain shorten the zone, and a delay line, wedge or dual element probe moves or avoids it. There is no clean formula for it. It is read from the screen and from the probe's specification. See the dead zone guide.

Which one is the question asking about?

Mistakes that cost points

• Using the beam spread formula inside the near field. It describes the far field.
• Reversing the frequency rule. Higher frequency means shorter wavelength, a longer near field and less spread.
• Reading N as the first peak. It is the last on-axis maximum, the one farthest from the element.
• Blaming the near field for a missed shallow echo. That is the dead zone.
• Quoting a full angle as a half angle. The formula gives the half angle.
• Ignoring the wedge. On an angle beam probe, some of the near field is used up inside the wedge.

Practice it, then check it

Work Probes A to C by hand, then change one value at a time in the UT beam calculator and predict the direction before you press go. The UT formulas guide lists these equations beside Snell's law, skip distance and decibels. For the angle side, see Snell's law worked examples, and for the screen side see what ultrasonic testing is.

These terms sit under techniques and general evaluation in the UT General outline for exams before 5 February 2027, and the later outline regroups them.

See where you stand

Take the free 10-question UT check. It needs no signup, explains every answer and shows which topics to drill first. The course adds timed practice exams, a lesson for each published topic and a study plan built from your exam date. See the plans.

FAQ

What is the difference between near field and dead zone?

Near field is a property of the beam: pressure along the axis rises and falls near the element. Dead zone is a property of the transducer and instrument: the initial pulse and ring-down hide shallow echoes. They can cover the same depth but have different causes and different fixes.

Does a higher frequency make the near field longer or shorter?

Longer. N = D² f / 4c grows with frequency and with the square of the diameter, and shrinks as velocity rises.

Does a bigger element make the beam spread more or less?

Less. Spread depends on wavelength divided by diameter, so a larger element at the same frequency gives a narrower beam.

Sources

Original arithmetic and probe values, written for this article. The formulas and the 0.514 constant come from Evident's published material, read on 26 October 2026.

• Evident, ultrasonic transducer technical notes: near field and beam spread equations.
• Evident, ultrasonic flaw detection tutorial: beam and ring-down background.
• ASNT NDT Level II Examinations, ASNT Certification Services: UT General topic headings.