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