The lateral wave is the compression wave that travels just beneath the scanning surface directly from the transmitter to the receiver. Its path is simply the probe centre separation, so it arrives at t = PCS/c plus the delay through the two wedges. In sound material it is always the first signal on a ToFD A-scan, and it is the reference from which near-surface depths are measured.

Because its arrival depends only on PCS, velocity and wedge delay, the lateral wave is the practical check on all three. If it arrives earlier or later than predicted, either the PCS is not what it was believed to be, the assumed material velocity is wrong, or the delay has changed through wedge wear, a temperature shift or a loose probe holder. Re-establish the setup before evaluating any indication.

The lateral wave also defines the near-surface dead zone. Any diffracted signal arriving while the lateral wave pulse is still ringing cannot be separated from it, so a shorter pulse and a smaller PCS both shrink that zone. Loss or weakening of the lateral wave over part of a scan is itself an indication: it usually means a near-surface flaw is interrupting the surface-following path, or the probes have lifted.

Compression wave

Worked example

Probe centre separation	70 mm
Compression velocity	5900 m/s
Total probe delay	2.5 µs
Lateral wave transit time in material	11.86 µs
Lateral wave arrival on the A-scan	14.36 µs

c = 5900 m/s = 5.9 mm/us. Lateral wave path equals the PCS, 70 mm, so the time in material is 70 / 5.9 = 11.8644 us, that is 11.86 us to 2 dp. Adding the 2.50 us combined wedge delay gives 14.3644 us, so 14.36 us on the A-scan.

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