The picture is missing one important little detail ... something to show
that the ambulance is moving to the right, not standing still.
With that detail added, the picture kind of suggests the Doppler Shift,
which is the change in wave frequency due to the motion of the wave source.
The result has no effect on the loudness or softness of the sound.
The result is that sounds from a moving source sound higher-pitch if the
source approaches you, and lower-pitch if the object moves away from you.
