The echo from an identical reflector gets weaker the further away it is, for two independent reasons: the beam spreads, and the material absorbs and scatters. A DAC curve records that decay from a row of identical reference reflectors at different sound paths so that any indication can be judged against a like-for-like reference rather than against a fixed screen height.

Beam spread alone gives a fixed law in the far field, set by how the reflector gathers sound. A plane back wall returns amplitude proportional to 1/z — 6 dB per doubling of sound path. A side-drilled hole behaves as a cylinder and falls as 1/z^1.5, 9 dB per doubling. A flat-bottomed hole or any small disc behaves as a point-like area reflector and falls as 1/z², 12 dB per doubling. Attenuation adds a further loss proportional to the distance travelled, 2αz for the round trip, and unlike beam spread it depends on the material, the frequency and the wave mode — shear waves in the same steel typically attenuate two to three times faster than compression waves.

Comparing an indication with the curve is bookkeeping in decibels. Take the indication’s screen height relative to the reference height, remove any gain added since the curve was drawn, and subtract the curve’s own drop at that sound path. What remains is the indication’s level relative to DAC — the number acceptance criteria are written in, whether that is 100 % DAC, the 50 % (−6 dB) evaluation line or the 20 % (−14 dB) recording line.

A drawn DAC is only valid over the range where reference points were actually recorded, on the same surface condition, with the same probe, cable and instrument settings. Extrapolating a curve past the last reference reflector, or transferring it to a rougher component without a transfer correction, is the usual way a real flaw ends up under-called.

Compression or shear

Worked example

Wave mode	compression
Shear velocity	3240 m/s
Compression velocity	5900 m/s
Reference reflector type	sdh
Sound path of the reference reflector	25 mm
Gain when the curve was set	40 dB
Reference screen height	80 %
Attenuation coefficient	0.02 dB/mm
Sound path of the indication	100 mm
Indication screen height	60 %
Gain at the indication	46 dB
Transfer correction	0 dB
DAC drop at the indication	-21.06 dB
DAC height at the indication	7.08 %
Indication relative to DAC	12.56 dB
Percentage of DAC	424.8 %
Round-trip time at the indication	33.9 µs

Side-drilled holes anchored at 25 mm, indication at 100 mm — two doublings of sound path. Beam spread costs 30 log₁₀(4) = 18.06 dB and attenuation costs 2 × 0.02 × 75 = 3.00 dB, so DAC has dropped 21.06 dB and sits at 80 × 10^(−21.06/20) = 7.08 % FSH. The indication is at 60 % but with 6 dB more gain: 20 log₁₀(60/80) = −2.50 dB, so −2.50 + 21.06 − 6 = +12.56 dB above DAC, which is 425 % of DAC.

Use at your own risk — verify before you act

These calculators support, and never replace, the judgement of qualified NDT and engineering personnel. Results are provided as is, without warranty of any kind, express or implied, and must be independently verified against the governing code edition named in your contract before being used in any inspection, acceptance, rejection, radiation-safety or fitness-for-service decision. By using them you accept full responsibility for how the results are applied; NDT Inspect, its owners and contributors accept no liability for any loss, damage, injury or death arising from their use or from reliance on them. If a result matters to safety, check it by hand and have it reviewed by a competent person.