DAC – Distance Amplitude Curve is a method of compensating for the fact that the pulse-echo response of a reflector will decrease as the distance of the reflector from the ultrasonic probe increases. This occurs because the transmitted ultrasonic beam spreads out as it travels from the probe to the reflector and so the further the reflector is from the probe the lower the energy of the sound that actually hits the reflector. Similarly, the further the reflected pulse has to travel the lower the energy that is received back at the probe.
The DAC is generated by plotting the amplitude of a known calibration reflector at different distances from the probe. Generally, the gain of the flaw detector is set so that the amplitude of the nearest reference reflector (the one at the shallowest depth) is at 80% FSH. The amplitudes of the same calibration reflector at further depths are then plotted on the flaw detector screen whilst the gain setting is maintained constant. The DAC is generated by plotting a curve through these amplitudes.
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.
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 the 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. It is important to recognize that regardless of the type of reflector used, the size and shape of the reflector must be constant. Commercially available reference standards for constructing DAC include ASTM Distance/Area Amplitude and ASTM E1158 Distance Amplitude blocks, NAVSHIPS Test block, and ASME Basic Calibration Blocks.