<P> The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer . Astronomer John Dobson explained the effect thus: </P> <Dl> <Dd> "The reason the siren slides is because it doesn't hit you ." </Dd> </Dl> <Dd> "The reason the siren slides is because it doesn't hit you ." </Dd> <P> In other words, if the siren approached the observer directly, the pitch would remain constant, at a higher than stationary pitch, until the vehicle hit him, and then immediately jump to a new lower pitch . Because the vehicle passes by the observer, the radial velocity does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity: </P>

In the doppler effect for sound waves factors