Ultrasonic amplitudes are compared in decibels: ΔdB = 20 · log₁₀(A₁ / A₂). The factor is 20 and not 10 because the A-scan displays pressure amplitude, not intensity. Doubling the screen height is +6.02 dB, halving it is −6.02 dB, and a factor of ten is exactly 20 dB. Those three numbers cover most of the arithmetic done at the flaw detector.

The comparison only means anything if both readings are taken on the same instrument settings, the same probe, the same couplant and the same sound path, and if both signals lie inside the linear (vertical linearity) range of the display. Signals above roughly 100 % full screen height are clipped and signals below about 20 % FSH sit too close to the noise, which is why reference levels are set at 80 % FSH: it leaves 2 dB of headroom above and about 12 dB of usable range below.

Every sensitivity setting in a UT procedure is this calculation. Recording level, evaluation level and rejection level are stated as dB relative to a reference reflector; transfer correction, DAC and DGS curve offsets, and scanning gain are all added and subtracted in dB. Getting the sign wrong reverses the meaning of the result, so state whether the figure is a gain to be added or a loss to be compensated.

Worked example

Amplitude 1	80 %
Amplitude 2	40 %
Gain change to apply to Amplitude 1	6 dB
Amplitude ratio A₁/A₂	2
Difference A₁ relative to A₂	6.02 dB
Amplitude 1 after the gain change	159.62 %

80 % FSH compared with 40 % FSH is a ratio of 2, so ΔdB = 20·log₁₀(2) = 6.02 dB. Adding a further 6 dB to the 80 % signal would put it at 80 × 10^(6/20) = 80 × 1.9953 = 159.62 % FSH — off screen, so the gain would have to be reduced or the reference re-set.