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