When a moving object reflects sound back to a stationary source of frequency, f, two Doppler shifts of this frequency occur - when the object receives the sound and when it re-emits it. If the object's speed, u, is much smaller than the sound speed, v, the net Doppler frequency shift, Af, simplifies to: Calculate this shift for a blood flow of 12.6 cm/s interrogated by a 7.4 MHz signal. Report your answer in units of Hz. U A~ 2f- V Note: Use 1540 m/s for the speed of sound in blood.