Decibel (dB)
A decibel is a logarithmic unit of relative amplitude ratio used throughout ultrasonic testing; 6 dB equals a factor of 2 in amplitude, and 20 dB equals a factor of 10.
Definition
The decibel (dB) is 20 × log₁₀(amplitude ratio). It is the standard way to express signal levels, gain settings, and reference offsets in ultrasonic testing. Each 6 dB change doubles or halves amplitude; each 20 dB change is a factor of 10.
What it represents
The logarithmic ratio unit ultrasonics uses for amplitude: 6 dB is a factor of two in signal amplitude, 20 dB a factor of ten. Gain, attenuation and sizing methods are all expressed in it.
How it is derived
The 6 dB drop sizing method takes its name directly: moving the probe until amplitude halves nominally places the beam edge at the reflector edge. Transfer corrections and DAC curves are likewise dB constructs.
Where it is defined
Every ultrasonic procedure under ASME Section V Article 4 and ISO 17640 states sensitivities and corrections in dB.
How it is misread
Adding percentages instead of dB. Screen-height percentages do not add; decibels do. Corrections mishandled as percentages produce sensitivity errors that compound silently.
Where Decibel fits in an inspection programme
A term is only useful when it connects to a decision. Decibel appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Distance-Amplitude Correction (DAC) — DAC is an ultrasonic calibration curve plotted on the A-scan display that shows expected echo amplitude from a reference reflector at varying depths, used to evaluate flaw size while compensating for beam spread and attenuation.
- Attenuation (Ultrasonic) — Attenuation is the loss of ultrasonic energy as it propagates through a material due to absorption, scattering, and beam spread, expressed in decibels per unit path length and material-dependent.
More data terms
A-Scan · B-Scan · C-Scan · D-Scan · S-Scan · L-Scan · Distance-Gain-Size · Time-Corrected Gain · Gating · Indication
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.