Answer #25

The correct answer is D. Most techniques for focusing a beam make the focal point shallower. These include use of a lens, curving the surface of the transducer and using phasing, which is an electronic technique where electrical impulses are sent out at different times to create a beam.

See the figure. Recall the natural focal length (also called the near zone length) is D^2*frequency/6. Therefore at a given frequency, the only thing which will change the focus past it's natural focus point is to change the crystal diameter - in effect creating a new, longer natural focus point. This is not trivial and would require a new transducer set up.