There are eight Doppler Effect Formulas for frequency depending on cases:

(i) When the source is moving towards a observer at rest
                                     
(ii) When the source is moving away from the observer at rest
                                  
(iii) When observer is moving towards the stationary source
                                  
(iv) When observer moving away from a stationary source
                                  
(v) When both Source and observer moves towards each other