The experiment P1.7.4.2 utilizes the principle of an echo sounder to determine the velocity of sound in the air, as well as to determine distances. An echo sounder emits pulsed ultrasonic signals and measures the time at which the signal reflected at the boundary surface is received. For the sake of simplicity, the transmitter and receiver are set up as nearly as possible in the same place. When the velocity of sound c is known, the time difference t between transmission and reception can be used in the relationship
to determine the distance s to the reflector or, when the distance is known, the velocity of sound.