{HEAD ~ A longer characteristic length} of {TAIL ~ the transmission surface} causes the ultrasonic wave to less spread 
of
{ HEAD ~ A longer characteristic length } of { TAIL ~ the transmission surface } causes the ultrasonic wave to less spread
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH