Often, the surrounding material has competing reflectons, for example microstructure grains in metals and the aggregate of concrete. Cracks and volumetric defects can reflect ultrasonic waves quite differently. The detectability of a defect is directly proportional to its size. In other words, flaw detectability is inversely proportional to the transducer beam width. In other words, flaw detectability is inversely proportional to the duration of the pulse produced by an ultrasonic transducer.
Shorter pulses correspond to broader bandwidth frequency response.
See the figure below showing the waveform of a transducer and its corresponding frequency spectrum.