Why does my motion detector measure a distance that is too small?

A possible cause for this phenomenon is a measurement frequency that is too high. The sampling frequency used in experiments with a motion detector is limited by the speed of sound in air (about 340 m/s): if a distance of 10 m is measured, the sound pulse takes about 59 ms to travel from the motion detector to an object and back. This means that if a sample frequency of more than 17 Hz is used, a new sound pulse is emitted before the previous one is received, leading to erratic readings. The maximal frequency is also limited by the conditions of the experiment. Typical sampling frequencies are: 1 m range – 40 Hz. 2 m to 6 m range – 25 Hz and 6 m to 10 m range – 10 Hz.