What could cause a screen pattern with a large number of low-level indications, often referred to as "hash"?

A screen pattern that shows a large number of low-level indications, often termed "hash," can indeed be attributed to coarse-grained materials. When the grain size of the material is larger, it can lead to multiple scatterings of the ultrasonic waves. As the sound waves interact with these larger grains, they can produce numerous small reflections, resulting in a noisy pattern on the screen. This scattering prevents clear identification of significant reflectors or true indications, manifesting as a background of low-level noise or hash.

Coarse-grained materials typically diminish the resolution of ultrasonic testing. Therefore, when assessing such materials, the backscatter from the grain boundaries contributes to the appearance of low-level signals, overwhelming any potential true defects that may exist. This characteristic response is crucial in interpreting ultrasonic testing results and understanding the limitations posed by the material structure.