EMCrit.org

http://www.upei.ca/~vca341/usphysics/img0.html

http://courses.washington.edu/radxphys/Lectures05-06/Ultrasound%20-%20Chapter%2016%20-%20Lecture%202%20-%20060302_files/frame.htm

http://courses.washington.edu/radxphys/PhysicsCourse.html