Ultrasound is an acoustic wave with a frequency higher than the audible range of the human ear, which is 20 kHz. Ultrasound can be within the audible range for some animals, like dogs, bats, or dolphins. Around 1883, Sir Francis Galton performed the Ÿrst known experiments with whistles generating ultrasound. Many decades later, people started to Ÿnd ultrasound applications in engineering, medicine, and daily life. œe basic principle for the use of ultrasound as a measurement tool is the time-of-¦ight technique. œe pulse-echo method is one example. In the pulse-echo method, a pulse of ultrasound is transmitted in a medium. When the pulse reaches another medium, it is totally or partially režected, and the elapsed time from emission todetection of the režected pulse is measured. œis time depends on the distance and the velocity of the sound. When sound travels with aknown velocity c,the time t elapsed between the outgoing signal and its incoming echo is a measure of the distance d to the object causing the echo:
Figure 29.1 shows a simple pulse-echo system. œe transmitter and the receiver could be the same device, but they are separated for clarity in this Ÿgure.