You are sometimes asked to differentiate between "longitudinal"
and "transverse" waves. When particles in the medium vibrate
parallel to the direction the sound wave is traveling, we have a longitudinal
wave. This is the type of sound wave produced by our transducers and used
for medical diagnosis.
In some materials, it is possible to transmit what are known as transverse
waves, characterized by having the vibrations moving perpendicular
(transverse) to the direction of wave travel. Transverse waves are not
used in diagnostic ultrasound.