Dopple Effect

An ambulance is running from A to B at a speed of 60 kilometres per hour with a siren of 880Hz as shown in the figure. Let the frequencies of the siren sound detected by the observer located at point O be $v_{a}$ and $v_{b}$ when the vehicle just passes point A and point B respectively. Point C is just to the north of point O and $\vec{OC}=\vec{AC}=\vec{BC}=100m$ . The sound velocity is 340m/s.

What is the approximate value of $v_{a}/v_{b} -1 ?$