http://ef.engr.utk.edu/ef152-2008-08...52-lec-4-2.pdf
"If an observer moves towards a stationary source with speed Vr, the observer sees the wave crests with a speed:

V' = V + Vr

Wavelength remains the same, so the modified frequency is:

f' = V'/(lambda) = (V + Vr)/(lambda) = fo(1 + Vr/V)"

Pentcho Valev