Beam spread: what it means and why it matters

The formula

The half angle α follows sin α = K × λ / D, where λ is the wavelength, D is the element diameter and K is a constant. The constant depends on where you decide the beam ends. Evident's published material writes it as sin α = 0.514 c / (f D) for the point where the beam is down 6 dB, which is the same thing with λ = c / f. TWI gives larger constants for the points where the beam is down much further, which gives a wider angle for the same probe. The rule is to use the constant the question or the procedure names and not mix conventions in one answer.

Worked example with made-up numbers: a 12 mm element at 4 MHz on a material at 6.00 mm/µs and the 6 dB edge. sin α = 0.514 × 6.00 / (4 × 12) = 0.0643, so α is about 3.7 degrees from the axis to the edge.

What it does to a test

• Wide beam. It covers more volume per position, but it makes echoes from surfaces, sidewalls and geometry more likely, and it gives poorer separation between two close reflectors.
• Narrow beam. It gives better lateral resolution and a cleaner picture, but it can miss an off-axis or tilted reflector if the probe position is not close.
• Distance. The beam is still the width of the element in the near field. Spread only shows up past the near field length.

Two different losses

Beam spread makes the energy cover a larger area, so the amplitude at any one point falls with distance. That is a geometric effect. Attenuation is a different loss, caused by scattering and absorption in the material. A question that mentions grain structure is about attenuation. A question that mentions element size is about spread.

Mix-ups to avoid

• Full angle versus half angle. The formula gives the half angle. The total width of the beam is about twice the angle.
• Frequency direction. Higher frequency means a shorter wavelength, so smaller spread. People often reverse this.
• Near field versus far field. The formula is for the far field.

How the exam uses it

Beam spread appears under techniques and general evaluation, as a calculation of a half angle or as a ranking question: which probe has the narrowest beam, or what happens to spread when frequency is raised. A quick rule that gets most of those right is that spread depends on the ratio of wavelength to diameter, and you make it smaller by making that ratio smaller.

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

The UT beam calculator reports the half angle with the constant it used. Compare it with the worked method in the UT formulas guide. Before the beam spreads it passes through the near field, and after it spreads it also loses energy to attenuation. Browse all terms 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.