Techshore explains an Ultra Sound Beam

An Ultrasonic Beam produced by piexo electric crystal. Electricity is used to excite the crystal hence forth making it vibrate , the vibrational(mechanical) energy

is converted to waves which are used in the testing process.

An ultrasound beam consist of three zones

1. Dead Zone
   Its is referred distance travelled by wave during as crystal ringing time / crystal vibration time. Every probe has independent crystal ringing time .  No defects will be captured during this time.
2. Near Zone
   Near zone is adjacent to Dead zone. In Near Zone the intensity of sound is varying and does not have any consistence. Hence the defects detected in this zone    is either magnified or diminished once when compared with the actual defect due to varying sound intensity
3. Far Zone
   Far zone is the ideal zone for observing the defects. Here the intensity of sound decreases gradually / periodically, which means there exist a consistency for sound waves in the Far Zone.

Video :

Additional references :

1. https://en.wikipedia.org/wiki/Ultrasonic_transducer
2. https://books.google.co.in/books?id=KUd6bLOtrxoC&pg=PA6&lpg=PA6&dq=crystal+ringing+time+of+a+probe&source=bl&ots=7uE2o4icY7&sig=deGFnPsjM5zvPE_GrhpraDHmu5Q&hl=en&sa=X&ved=0ahUKEwi2kO-t48zPAhWIQo8KHe5BCtIQ6AEIIDAB#v=onepage&q=crystal%20ringing%20time%20of%20a%20probe&f=false
3. http://traktoria.org/files/sonar/transducer/simple_transducer_intro.pdf