<P> The above formula assumes that the source is either directly approaching or receding from the observer . If the source approaches the observer at an angle (but still with a constant velocity), the observed frequency that is first heard is higher than the object's emitted frequency . Thereafter, there is a monotonic decrease in the observed frequency as it gets closer to the observer, through equality when it is coming from a direction perpendicular to the relative motion (and was emitted at the point of closest approach; but when the wave is received, the source and observer will no longer be at their closest), and a continued monotonic decrease as it recedes from the observer . When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt . When the observer is far from the path of the object, the transition from high to low frequency is gradual . </P> <P> If the speeds v s (\ displaystyle v_ (\ text (s)) \,) and v r (\ displaystyle v_ (\ text (r)) \,) are small compared to the speed of the wave, the relationship between observed frequency f (\ displaystyle f) and emitted frequency f 0 (\ displaystyle f_ (\ text (0))) is approximately </P> <Table> <Tr> <Th> Observed frequency </Th> <Th> Change in frequency </Th> </Tr> <Tr> <Td> f = (1 + Δ v c) f 0 (\ displaystyle f = \ left (1 + (\ frac (\ Delta v) (c)) \ right) f_ (0)) </Td> <Td> Δ f = Δ v c f 0 (\ displaystyle \ Delta f = (\ frac (\ Delta v) (c)) f_ (0)) </Td> </Tr> </Table> <Tr> <Th> Observed frequency </Th> <Th> Change in frequency </Th> </Tr>

What type of wave is the doppler effect