Distance Amplitude correction (DAC) in ultrasonic inspection.

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 cure 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.
A distance Amplitude correction curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same material. A-scan echoes are displayed at their non - electronically compensated height and peak amplitude of each signal is marked on the flaw detector screen or, preferably, on a transparent plastic sheet attached to the screen. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH),or notches whereby the reflectors are located at varying Depths are commonly used.