How is ultrasound generated?

Answer 1

Ultrasonic signals (or even sonic ones - ones we can hear) are generated by a device called a transducer. The transducer has a crystal driver that converts electrical energy into mechanical energy (and sound is mechanical energy) and vice versa because of piezoelectric effect. A crystal can be made to change shape (get a tiny bit longer or shorter) by electrical stimulation. And changing the shape of a crystal, squeezing it a bit, can cause it to generate an electrical signal. That's piezoelectric effect. It can be called a bi-directional process. Send an electrical signal to a transducer and it will convert the electrical energy (signal) into mechanical energy (sound). Because the process works in reverse, the transducer is used to send out a signal and then be used to listen for the return echo. All in ultrasound. That's sound that is above the range of human hearing. Human hearing is roughly 20 cycles per second (cps or Hertz) to 20,000 cps. Medical ultrasonic diagnostic transducers work at well in excess of 1,000,000 cps. Neither your dog nor any bats in the neighborhood can hear that.
One of the many ways is piezoelectric generation, a high frequency current is put across a certain type of crystal and it causes the crystal to oscillate.