Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

4.2: The Calibrations
4.2.1: Distance Amplitude Correction (DAC)
Distance Amplitude Correction (DAC): Acoustic signals from the same
reflecting surface will have different amplitudes at different distances from the
transducer. Distance amplitude correction (DAC) provides a means of
establishing a graphic ‘reference level sensitivity’ as a function of sweep
distance on the A-scan display. The use of DAC allows signals reflected from
similar discontinuities to be evaluated where signal attenuation as a function
of depth has been correlated. Most often DAC will allow for loss in amplitude
over material depth (time), graphically on the A-scan display but can also be
done electronically by certain instruments. Because near field length and
beam spread vary according to transducer size and frequency, and materials
vary in attenuation and velocity, a DAC curve must be established for each
different situation. DAC may be employed in both longitudinal and shear
modes of operation as well as either contact or immersion inspection
techniques.