1. The problem statement, all variables and given/known data As you drive at a speed of 15.0 m/s an ambulance approaches you from behind with its siren blaring at a frequency of 1022 Hz. After the ambulance has passed you the signal is now 981 Hz. If both you and the ambulance travel at a constant speed how fast was the ambulance traveling? 2. Relevant equations fo=fs(1+- vo/v)/(1+-vs/v) 3. The attempt at a solution So I tried to use the equation above by plugging in the frequencies, but was trapped because the frequencies show only what the source was emitting, and not what the observer was hearing.