Ultrasound reflected from an oncoming bloodstream that is moving at 31 cm/s is mixed with the original frequency of 1.3 MHz to produce beats. What is the beat frequency? (Velocity of sound in blood = 1540 m/s.) I know that there are two doppler shifts; one where the source is fixed and the other where the observer is fixed. So i'm supposed to use the doppler shift equation to solve for this: f1=f((V + Vo) / (V - Vs)) but I dont know what numbers to plug into which variables. After I find the frequency of the doppler shifted echo all I have to do is subtract it from the original frequency of 1.3 MHz, right?