Ultrasonic wavelength

Wavelength is the physical length of one cycle of the sound wave inside the material: λ = c / f. Velocity is fixed by the material and by the wave mode; frequency is fixed by the probe. Wavelength is therefore the one number that links probe selection to what the inspection can actually resolve.

Wave mode changes the answer by nearly a factor of two. Compression (longitudinal) waves in ferritic steel travel at about 5900 m/s, shear (transverse) waves at about 3240 m/s. A 5 MHz probe has a 1.18 mm wavelength working in compression but only 0.65 mm working in shear — which is why a 5 MHz angle-beam shear probe resolves finer detail than a 5 MHz straight-beam compression probe on the same material.

Most of the practical beam rules are expressed in wavelengths: near-field length is D²/4λ, beam divergence is proportional to λ/D, and the theoretical axial resolution of a pulse is half its spatial pulse length — one wavelength for a clean two-cycle pulse. A reflector much smaller than λ/2 will not return a signal that can be separated from the noise, and grain scattering rises steeply once grain size approaches about λ/10, which is why coarse austenitic material is inspected at low frequency.

Enter a measured velocity whenever you are not on plain carbon steel. Velocity varies by a few per cent between steel grades, falls as temperature rises, and is strongly direction-dependent in austenitic weld metal, cladding and nickel alloys.

λ = c / f
λ [mm] = c [m/s] ÷ ( f [MHz] × 1000 )
T = 1 / f