{HEAD ~ The first directivity} is different from {TAIL ~ a second directivity} of the ultrasonic wave transmitted from single transmission element 
{ HEAD ~ The first directivity } is different from { TAIL ~ a second directivity } of the ultrasonic wave transmitted from single transmission element
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH