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DAC curve: what it means and why it matters

Angle Beam editorial teamSources checked 3 min read

A distance amplitude correction curve, or DAC curve, is a line drawn on the screen through the echo heights from identical reference reflectors at different depths. Because the same reflector gives a lower echo the deeper it is, the curve lets an inspector judge a flaw echo against what a reference reflector would give at that same depth.

Three equal reference echoes at increasing depth joined by a falling curve, with a flaw echo compared against the curve at its own depth
Identical reflectors give lower echoes at depth, so the curve falls. A flaw echo is read against the curve at its own depth.

How it is built

The setup uses a reference block with identical reflectors, such as side-drilled holes, at several depths. With the probe and instrument settings fixed, the inspector moves the probe to peak the echo from each reflector in turn and marks its height on the screen. Joining the marks gives the curve. Depth runs along the screen and echo height up it, so the curve slopes down, because beam spread and attenuation reduce echo height with distance.

The curve belongs to the exact combination used to build it: probe, wedge, frequency, beam angle, instrument settings and material. Change any of those and the curve is no longer valid.

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How a flaw echo is read against it

Once the curve exists, a flaw echo is compared with the curve height at the flaw's depth. Reports typically give the flaw echo as a percentage of the curve or as a number of decibels above or below it. If the curve is at 50 percent of screen height at that depth and the flaw echo is 25 percent, the echo is half the curve height, or 6 dB below it. The acceptance limit in terms of the curve comes from the governing procedure or code, not from this page.

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Every answer comes with a full explanation.

DAC compared with similar ideas

  • Time corrected gain (TCG). The instrument raises gain with time so the reference echoes line up at one height. The curve becomes a flat line. Both ideas correct for distance loss; TCG does it electronically.
  • Reference level. A single reference reflector at a single depth gives one amplitude, not a curve across depths.
  • DGS or AVG diagrams. These estimate reflector size from calculated curves for a probe, not from reference reflectors in a block.

Where it goes wrong

  • Different material. If the test part has a different attenuation or surface condition than the block, the curve under- or over-states the flaw. A transfer correction may be needed.
  • Near field. Echoes from inside the near field are irregular, so the curve often starts beyond it or is built from points past it.
  • Changed couplant or gain. Any setting change after the curve is built means rebuilding or verifying it.

How the exam uses it

DAC appears under general evaluation. Expect a sketch of a curve with a flaw echo, and a question asking for its percentage of DAC or its dB difference from the curve, or a question on what makes the curve invalid. Practice the dB conversion until a half or a double is automatic.

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

Use the decibel calculator to turn an echo ratio into dB, and read the calibration topics in the UT formulas guide. Two of the reasons the curve falls are covered in attenuation and the near field. All terms are listed in the NDT terms glossary.

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Sources

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Background reading: Evident's published UT material and ASNT's published topic headings. Outline checked 25 October 2026.

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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.

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