Attenuation: what it means and why it matters

The two causes

Scattering happens when the wave meets grain boundaries, inclusions, porosity or other changes in the material. Part of the energy goes off in other directions, so less of it comes back to the probe. It rises quickly as the grain size approaches the wavelength, which is why coarse-grained castings and some austenitic welds are hard to test with high-frequency probes.

Absorption is the conversion of the wave's energy into heat through internal friction. It happens in every material, and it also rises with frequency.

Why frequency matters

A higher frequency gives a shorter wavelength, which is good for resolution and for finding small reflectors. The cost is more attenuation, so a high-frequency beam carries less energy a given distance. In coarse-grained material a lower frequency is often chosen to get any usable signal through, even though resolution suffers.

Reading it in dB

The decibel scale compares amplitudes: a change of 6 dB is a factor of about two in amplitude, and 20 dB is a factor of ten. Suppose two successive backwall echoes in a thick flat plate read 80 percent and 40 percent of screen height. The drop is a factor of two, which is about 6 dB. If beam spread is small enough to ignore, that loss is a rough measure of attenuation over one round trip. A real measurement also accounts for reflection losses at the surfaces, which is why the procedure may call for a correction.

What it is not

• Not beam spread. Spread is a geometric loss from the beam widening. Attenuation is a material loss.
• Not an interface loss. Losses at the couplant layer or at a boundary come from impedance mismatch, not from travel through the material.
• Not the same as sensitivity. Sensitivity is a setup property. Attenuation is a material property that reduces the sensitivity you can get at depth.

How the exam uses it

Questions on attenuation usually ask for the cause, the effect of raising frequency, or the reason a certain material needs a different probe. The calculation questions tend to involve decibel and amplitude ratios. Practice converting both ways until it is automatic, and remember that attenuation in a coarse-grained part is why a DAC curve built on one material cannot be used on another.

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

Convert between dB and amplitude ratios with the decibel calculator, then see the worked method in the UT formulas guide. The echo-height correction that depends on it is the DAC curve, and the geometric loss it is often confused with is beam spread. The rest of the terms are 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.

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