The following is a chart of the four most common situations in which only one
object (either the detector or source) moves.
This logical approach also works when both the detector and source are moving. If
they’re going in the same direction, but the source is moving faster than the detector,
then we would expect the frequency heard by the detector to decrease. However, it
would decrease by a factor far less than if the detector were stationary and the
source were moving away from it at the same speed.