Our last paper in Ultrasonics, just published!
It is about passive focusing systems for air-coupled ultrasonic transducers.
A novel system inspired in the Newtonian Cassegrain telecope, http://authors.elsevier.com/a/1SUmB,yaZKxmK
Access through this link is free (no sign up or registration is needed) until March 25, 2016:
Ultrasounds for Biological Applications and Materials Science 