Why parabolas are used for sound

Mike Day
Why do they use parabolic shapes at football games to capture sound? The blue ray is sound coming into a parabola and reflecting on the tangent line at that point. The reflection is the pink ray. Move the point along the parabola and watch what happens to the pink ray. This is where the sound travels. To remove the trace, click the refresh or undo button.
It seems the pink rays all have something in common. What is it? Do you remember how to find that point? Find it from the equation.