Using the immersion method, a distance amplitude curve (DAC) for a 3/4 inch diameter, 5 MHz search unit shows the high point of the DAC at the 2 inch block. One day later, the high point of the DAC for the same search unit is at the 4 inch block. Assuming that calibration has not changed, this would indicate that the search unit:

Using the immersion method, a distance amplitude curve (DAC) for a 3/4 inch diameter, 5 MHz search unit shows the high point of the DAC at the 2 inch block. One day later, the high point of the DAC for the same search unit is at the 4 inch block. Assuming that calibration has not changed, this would indicate that the search unit:

• A. Is improving in resolution
• B. Is becoming defective
• C. Has the beam of a smaller search unit
• D. Both B and C

Answer: (D)

When the impedance and energy distribution of the transducer change, the depth at which the energy is most efficiently sent and received—the peak of the distance amplitude curve—can shift. If calibration is unchanged but the DAC high point moves from 2 inches to 4 inches, that means the probe’s beam characteristics or sensitivity have altered, not the display scale or gain. A defect in the transducer (such as a damaged element or degraded matching layer) would distort the energy pattern and typically shift or weaken the DAC peak. Likewise, using a probe with a smaller effective aperture (a smaller search unit) changes the beam geometry and focusing, which can also move the peak to a different depth. Because either change in the transducer or its beam can produce the observed shift, both possibilities are consistent explanations.

When the impedance and energy distribution of the transducer change, the depth at which the energy is most efficiently sent and received—the peak of the distance amplitude curve—can shift. If calibration is unchanged but the DAC high point moves from 2 inches to 4 inches, that means the probe’s beam characteristics or sensitivity have altered, not the display scale or gain.

A defect in the transducer (such as a damaged element or degraded matching layer) would distort the energy pattern and typically shift or weaken the DAC peak. Likewise, using a probe with a smaller effective aperture (a smaller search unit) changes the beam geometry and focusing, which can also move the peak to a different depth. Because either change in the transducer or its beam can produce the observed shift, both possibilities are consistent explanations.