The Doppler effect/shift is the change in frequency of a wave moving relative to its source. the received frequency is higher during the approach.
New Frequency of Receding Receiver: f' = f0(1-ur/v)
where ur = Receiver Velocity, f0 = Source Frequency, f' = New Frequency of Receding Receiver, v = Wave Velocity.
For example, when Source Frequency(f0) = 2 Hz, Receiver Velocity(ur) = 10 m/s, Wave Velocity(v) = 12 m/s, New Frequency of Receding Receiver(f') = 0.33Hz.