Sound pulse traveling through the sample

Ultrasonic waves are generated by applying a voltage pulse to a piezoelectric element called a vibrator. By pressing the transducer against the sample, the ultrasonic pulse wave generated at the time t0 travels from the left to the right of the sample, is reflected at the right end, and returns to the piezoelectric element at the time t4. At the moment of returning, a force is applied to the piezoelectric element and a voltage is generated in the element. In the figure, it is expressed as a wave with one peak, but when the peak is expanded, it becomes a wave with many oscillations. This device is a generator that generates voltage pulses of ultrasonic frequency.

Time evolution of voltage pulse

The ultrasonic pulse wave repeatedly travels back and forth in the sample. At the time t4, t8 and t12, a voltage pulse is generated at the piezoelectric element when the pulse wave returns to the end. A device called ossilloscope observes this voltage as a function of time. Currently, digital oscilloscopes are usually used, and if you position the cursors on the screen, the time interval of the cursors will be displayed on the screen.