Doppler effect
Sirens

A 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:

The reason the siren slides is because it doesn't hit you.

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 speed does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity: v radial = v s cos (θ) {\displaystyle v_{\text{radial}}=v_{\text{s}}\cos(\theta)} where θ {\displaystyle \theta } is the angle between the object's forward velocity and the line of sight from the object to the observer.
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