Frequency (Ultrasonic) — Definition & NDT Use
The number of ultrasonic wave cycles per second, measured in megahertz (MHz). Higher frequencies (5-25 MHz) provide better resolution and sensitivity to small flaws but have limited penetration due to attenuation. Lower frequencies (0.5-2 MHz) penetrate thicker materials and coarse-grained materials better. Frequency selection is a critical inspection parameter determined by material thickness, grain size, and required resolution. The relationship between frequency and wavelength governs spatial resolution.
As a physical principle, Frequency dictates how energy interacts with the test piece — and that interaction is what an NDT instrument reads out as a signal, image, or measurement. Frequency selection is a deliberate trade-off: higher MHz buys resolution and small-flaw sensitivity but loses penetration in coarse-grained or attenuative material, while lower MHz reaches deeper at the cost of resolution. 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 Frequency every time they pick a frequency, gain, or probe — even when they are not consciously thinking of the underlying physics.
- Etymology / Origin
- Latin frequentia (crowdedness); the cycle-per-second meaning is from Hertz (1888) for electromagnetic waves.
- Formula
- λ = c/f — wavelength = velocity ÷ frequency.
- Units
- Hz, kHz, MHz; UT typically 0.5–25 MHz; ET 100 Hz–10 MHz.
- Typical Range
- UT thickness gauging 5 MHz; weld inspection 2.25–5 MHz; composites 5–15 MHz; coarse-grain austenitic 1–2 MHz.
- Measured / Produced By
- Set on the flaw detector; verified against a known-velocity block.
- Code References
- ASME Section V T-432.2 (transducer characteristics); ISO 22232-2 (probe characterisation)
- Worked Example
- 5 MHz in steel (c = 5920 m/s): λ = 5920 / 5e6 = 1.18 mm; smallest reliably resolvable flaw ≈ λ/2 = 0.6 mm.
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 frequency 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 "Frequency" mean in NDT?
The number of ultrasonic wave cycles per second, measured in megahertz (MHz). Higher frequencies (5-25 MHz) provide better resolution and sensitivity to small flaws but have limited penetration due to attenuation
Why does frequency 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 Frequency?
The most directly related entries in this glossary are "wavelength", "attenuation", "resolution"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.