The Sonic Ranger software assumes the speed of sound in air is 340.0 m/s as a default value when you start the program. The actual value fluctuates slightly due to changes in temperature, and you should calibrate your equipment more precisely before you begin to make measurements.
From the Main Menu, select Calibrate, and within the submenu select Calibrate By Measurement. Follow the instructions on the screen: first set the rider on the air-track at some precise distance from the Sonic Ranger (at least 41 cm) and take data at this distance, then set the rider at a position exactly one meter further away and take data again. The computer will then calculate the speed of sound by using the extra time it took the sound to travel one extra meter and return. How much does this value differ from the default value of 340.0 m/s, and how much will that difference affect your results?
Press Return to accept this calibration.