Describe how an ultrasound signal is generated.

A sound wave is produced by sending an oscillatory voltage signal to a transducer (e.g., piezoelectric crystal).
The transducer vibrates with the frequency of the voltage signal (usually selected to be the resonant frequency of the crystal), produciing ultrasound waves.
The waves propagate through the tissue and are attenuated, reflected, or transmitted to different degrees by different tissues.
The reflected sound wave is received by the transducer and converted into an electronic signal.