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.

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