F' = (1 = V1/U) F where U is speed of the sound
wave relative to air.
F' = F / (1 - V2/U)
This is equivalent to: Both objects are moving relative to
the air. The person would feel the unchanged frequency.
F' = F (1 + V/U)/(1 + V/U) = F
F' = F (1 + V1/U) / (1 - V2/U)
