Editing Doppler effect (section)

===Sirens===
[[File:Juli 2016 - Spoedtransport, Huisarts, Brandweer, Politie en Ambulances met spoed in Rotterdam -451.webm|start=7:14|end=8:30|thumb|Sirens on passing emergency vehicles.]]
A [[siren (alarm)|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 (astronomer)|John Dobson]] explained the effect thus:

{{Blockquote|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:
<math display="block">v_\text{radial} = v_\text{s}  \cos(\theta)</math>
where <math>\theta</math> is the angle between the object's forward velocity and the line of sight from the object to the observer.