The Ejs Simultaneity model displays the effect of relative motion on the relative ordering of the detection of events. The wave source and two equidistant detectors are at rest in reference frame S', which moves with constant velocity, v, in frame S. The initial relative velocity as well as whether the wave source emits sound waves or light can be changed. You can modify this simulation if you have Ejs installed by right-clicking within the plot and selecting "Open Ejs Model" from the pop-up menu item.
Quelle: Open Source Physics