Experiment 9.2 says that the two people should move 250 meters apart. This is over 2 1⁄2 football fields (273 yards), and most people do not have access to a straight piece of property that allows line of sight for that distance. This experiment can certainly be done over a shorter distance. The experimenters should aim for at least 100 meters but make full use of the area available. The difficulty with using smaller spaces is that the accuracy of the time measurement becomes less when measuring a smaller change in time, so the calculations will be less precise. Estimate the area available (100-200 m), and use that number to replace the 250 meters for the experiment in both measuring the distance between and for the calculation for the speed of sound. It is recommended not to go below 100 meters since the time recorded will be much more difficult to measure accurately.

Another point to recognize is that the person hitting the two rocks together needs to make an exaggerated arm movement (like big crocodile arms), so that the person far away can see the motion. They don't have to be able to see exactly when the rocks touch, but they will know more approximately by the big arm movement.