⁵¹If you would like to prove this to yourself, you may do so by substituting path length (L), fluid velocity (v), and sound velocity (c) for the times in the flow formula. Use t_up = L _ c−v and t_down = L _ c+v as your substitutions, then algebraically reduce the flow formula until you find that all the c terms cancel. Your final result should be Q = 2kv L .