When an ambulance approaches an observer, the sound of the siren perceived
during the approach is more acute (greater frequency) than the sound perceived
when the ambulance moves away, which is a more bass sound (decrease in
frequency). In these situations, the apparent frequency \(f'\), which is
perceived by the observer, does not coincide with the frequency \(f\) of the
source. This phenomenon is known as Doppler effect .
Doppler effect . The figure illustrates two sources (speakers)
emitting waves (sound). When the source is stopped, figure on the left, the
sound propagates equally in all directions. When the source moves, figure
on the right, the wave events arrive faster and faster in one direction and
slower and slower in another. Therefore, depending on the position, the
frequency will be different.