The crossword solution for the term 'PREFIX WITH SONIC' is ULTRA. The prefix 'ULTRA-' is derived from Latin and means 'BEYOND' or 'on the far side'. It is often used to describe something that is extreme or BEYOND normal limits. In this case, it is being used to describe something that is BEYOND the speed of sound, which is what 'SONIC' refers to. Therefore, the answer to the crossword clue is ULTRA, which when combined with 'SONIC' creates the phrase 'ultrasonic'.