To generate sound, it is necessary to have a vibrating source, such as the tuning fork shown here. When the source vibrates, it displaces adjacent particles and molecules in the medium, causing them to vibrate back and forth as well. Their vibrations cause more distant particles to vibrate, and so on. The audible sound that we hear is made up of tiny vibrations of air molecules, which are transmitted to our ears.  This  transmission of vibrations, starting from the source and continuing from one molecule to the next, is how sound travels through a medium. 


In medical ultrasound the vibrating sources are "piezoelectric  elements in an ultrasonic transducer. The elements vibrate in response to applied electrical signals.  The vibrating motion of the transducer elements cause particles in adjacent tissues to vibrate, and the ultrasonic vibrations travel through the tissue.

If the source vibrate continuously, a continuous sound is produced. In most cases in ultrasound, the source vibrates briefly, producing a pulse of sound, which travels through the tissue. After echoes are picked up, another pulse is sent, and so on. 

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