Decibel (dB) — Definition & NDT Use
A logarithmic unit for expressing signal amplitude ratios commonly used in ultrasonic testing to measure gain settings. A 20 dB increase represents a 10-fold increase in voltage amplitude. Ultrasonic instruments typically display gain in decibels from 0 to 100 dB or higher. Decibel notation allows convenient expression of wide amplitude ranges and makes it easier to set consistent sensitivity levels across different test objects and operators.
As a physical principle, Decibel dictates how energy interacts with the test piece — and that interaction is what an NDT instrument reads out as a signal, image, or measurement. Gain is set in decibels referenced to a known reflector — a side-drilled hole, flat-bottom hole, or notch on a reference block — so two operators on two instruments can produce comparable amplitudes from the same indication. Every parameter on the instrument front panel — frequency, gain, range, gate — is ultimately a physical lever on the same underlying interaction, which is why understanding the physics is what turns a button-pusher into a Level II.
Inspectors apply the principle of Decibel every time they pick a frequency, gain, or probe — even when they are not consciously thinking of the underlying physics.
- Etymology / Origin
- 1/10th of a Bel, named after Alexander Graham Bell; introduced by Bell Labs (1928) for telephone-circuit signal loss.
- Formula
- dB = 20 × log10(A1/A2) for amplitude ratios; dB = 10 × log10(P1/P2) for power ratios.
- Units
- Dimensionless logarithmic ratio.
- Typical Range
- UT flaw-detector gain range 0–110 dB; +6 dB doubles amplitude, +20 dB is 10×, +40 dB is 100×.
- Measured / Produced By
- Read directly from instrument gain control; verified against a reference block during calibration.
- Code References
- ASME Section V T-471.6 (gain settings); ASTM E317 (instrument linearity ±1 dB over 30 dB range)
- Worked Example
- Indication at 40% FSH with 50 dB gain; reference at 80% FSH with 56 dB gain — indication is 6 dB lower amplitude AND 6 dB less gain, net rating −12 dB to reference.
ASME Section V Article 4
Ultrasonic examination methods for welds and components.
ASTM E114 / E164 / E2375
ASTM straight-beam, contact, and wrought-product UT practices.
ISO 16810 / ISO 16811
General principles and sensitivity setting for industrial UT.
Operators sometimes treat decibel as a black-box instrument behaviour rather than a physical lever; the knock-on effect is that they do not realise when a knob change has invalidated the calibration.
What does "Decibel" mean in NDT?
A logarithmic unit for expressing signal amplitude ratios commonly used in ultrasonic testing to measure gain settings. A 20 dB increase represents a 10-fold increase in voltage amplitude
Why does decibel matter to an inspector?
It directly influences the inspection parameters — frequency, probe choice, gain, scanning pattern — that decide whether a small flaw is caught or missed. Inspectors who treat the underlying physics as background detail tend to misset their instruments under unusual conditions.
What other NDT concepts should I read alongside Decibel?
The most directly related entries in this glossary are "amplitude", "gain", "logarithm"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.