UT Decibel (dB) Amplitude Calculator
Ultrasonic evaluation lives in decibels: transfer correction, DAC offsets, the 6 dB and 20 dB drop sizing rules, and reference-level reporting all hinge on converting an amplitude ratio to dB. This tool does the conversion both ways — give it two screen heights and it returns the dB difference, plus the exact gain to drive a signal to the 80% full-screen-height reference.
How it works
Decibels express a ratio of two amplitudes logarithmically: ΔdB = 20 × log₁₀(A₁/A₂). A factor-of-2 amplitude change is 6.02 dB (the basis of the "6 dB drop" rule); a factor of 10 is 20 dB. Because the relationship is logarithmic, dB values add and subtract — that is why instrument gain in dB can be tallied directly against DAC/TCG offsets and transfer-correction values.
Formula
ΔdB = 20 · log₁₀(A₁ / A₂)
ΔdB = 20 · log₁₀(A₁ / A₂)Worked example
A signal at 40% FSH compared with an 80% reference: ΔdB = 20·log₁₀(80/40) = 20·log₁₀(2) = 6.0 dB. The measured signal is 6 dB below reference. To raise the 40% signal to the 80% reference you add 6 dB of gain — exactly the 6 dB drop relationship inverted.
| Variable | Value |
|---|---|
| input: amp1 | 80 |
| input: amp2 | 40 |
| output: db_diff | 6.0 |
| output: gain_to_80 | 6.0 |
When to use this tool
Use for transfer correction between calibration block and part, applying DAC/DGS offsets, executing 6 dB or 20 dB drop flaw sizing, normalising a signal to a reference level, or reporting indication amplitude relative to reference.
Limitations
Where this calculator stops being accurate:
- Assumes a linear, calibrated amplifier across the range used — verify instrument linearity (ASME V Article 4 requires vertical/horizontal linearity checks).
- Screen height is only proportional to acoustic pressure within the calibrated linear region (typically 20–80% FSH).
- Does not include attenuation or transfer correction — those are added separately as further dB terms.
- Saturated (>100% FSH) or very low (<20% FSH) signals fall outside the reliable linear band.
Frequently Asked Questions
Why is a doubling of amplitude 6 dB?
Because dB for amplitude uses 20·log₁₀(ratio). log₁₀(2) = 0.301, and 20 × 0.301 = 6.02 dB. This is the foundation of the 6 dB drop technique: when the echo from the edge of a large reflector falls to half (−6 dB) of its peak, the beam centre is taken to be at the reflector edge, allowing through-wall sizing.
What is the difference between 20·log and 10·log?
Use 20·log₁₀ for amplitude/voltage/screen-height ratios (what UT instruments display) and 10·log₁₀ for power/intensity ratios. UT evaluation is amplitude-based, so 20·log is correct. Mixing them up doubles or halves every dB value — a common rookie error in transfer-correction math.
How does this relate to setting a reference level?
Calibration sets a reference reflector to a fixed screen height (commonly 80% FSH) at a known gain — the reference level. Indications are then reported as "x dB above/below reference". This calculator gives that x directly from the two screen heights, and the "gain to 80%" output tells you how much instrument gain to add to re-peak a signal to the reference height.
References & Standards Cited
- ASME BPVC Section V, Article 4 — Ultrasonic Examination (amplitude, linearity, reference level).
- ISO 16811:2014 Non-destructive testing — Ultrasonic testing — Sensitivity and range setting.
- ASNT Ultrasonic Testing (Level II) — decibel system and amplitude evaluation.
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