Acoustic Cavitation
   TU Darmstadt (Germany)
   http://www.physik.tu-darmstadt.de/nlp/cavitation.html

Acoustical Standards
   Acoustical Society of America
   http://asa.aip.org/map_standards.html

How Stuff Works: How Ultrasound Works
   Illustrated narrative takes you inside the ultrasound machine and shows you how this technique is used to look inside you.
   http://www.howstuffworks.com/ultrasound.htm

NDT-net (Rolf Diederichs)
   UT-Encyclopedia
   http://www.ndt.net/article/ut_az/ut_u/ut_u.htm

Nondestructive Material Testing with Ultrasonics
   Introduction to the Basic Principles (Krautkrämer, NDt-net)
   http://www.ndt.net/article/v05n09/berke/berke1.htm

Super High Resolution Ultrasonic Measurement System
   5-micro-meter resolution measurement by non contact system in the air; RS-232 interface; Thickness, Deviation, Distance, Level of any kind of material.
   http://www.hedc.co.jp/

Ultrasonics
   Dedicated to the art and science of ultrasonic nondestructive testing and allied fields.
   http://www.xdcr.com/ut.html

Vibrationdata.com Tutorials
   The tutorials and course materials require a username and password which can be obtained with an annual subscription fee.
   http://www.vibrationdata.com/tutorials.htm