A phased array instrument fires one law per pulse. The rate at which complete frames arrive is therefore PRF / (laws × averaging). Every law added to the sweep, every extra focal depth and every additional averaging pass divides the frame rate by the same factor.

The PRF itself is capped by the sound path. The next pulse must not be fired before the echoes from the last one have died away, otherwise wrap-around signals from the previous shot appear in the display at false ranges. The absolute ceiling is c / (2 × range); in practice instruments are run well below it to let multiple back wall reflections decay.

Frame rate is the number that limits productivity. Combined with the required scan axis resolution it fixes the maximum scan speed, and exceeding that speed means skipped acquisition points – missing lines in the C-scan that an encoder will happily interpolate over unless data completeness checks are switched on.

Compression or shear

Worked example

Laws per acquisition point	101
Averaging factor	1
PRF set on the instrument	2000 Hz
Displayed sound path range	100 mm
Wave mode	shear
Material velocity	3240 m/s
Maximum PRF for this range	16200 Hz
Effective PRF	2000 Hz
Acquisition (frame) rate	19.8 Hz
Time per acquisition point	50.5 ms

A 100 mm shear path takes 2 × 100/3240 s = 61.73 µs, so the PRF ceiling is 16 200 Hz and the 2000 Hz setting governs. With 101 laws and no averaging the frame rate is 2000/101 = 19.80 Hz, i.e. 50.50 ms per acquisition point.

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