A Doppler blood flow meter emits ultrasound at a frequency of 5.0 MHz. What is the beat frequency between the emitted waves and the waves reflected from blood cells moving away from the emitter at 0.15m/s? Assume that v_sound=1540m/s in blood.
The Doppler shift, delta f, of the blood cells moving away is 2 V/v_sound * 5.0 *10^6 Hz.
V = 0.15 m/s = the blood cell speed and v_sound = 1540 m/s.
The factor of 2 is there because the frequency is shifted twice (when received and when sent back).
delta f = 2*5*10^6*0.15/1540 = 974 Hz
The beat frequency (of the transmitted beam combined with the reflected beam)equals delta f