a wave barrier is produced when a wave source moves?

Question

a)nearly as fast as the waves it produce
b)as fast as the waves it produces
c)faster than the waves it produces

Answer 1

B. The barrier is a region of very high air pressure due to the summation of the compression portions of the waves emitted along the path of the source. These portions pile up at the front of the source, requiring a large additional force to break through. Speeds less or more than the speed of sound don't produce this in-phase summation. The ref. illustrates this well.