[填空题]
A Doppler flow meter uses ultrasound+tc *s:a llla-7oy9oo waves to measure blood-flow speeds. Suppovng:,: psp nn0;k9wtt se the device emits sound at 3.5 MHz, and the speed of sound in human tissue is taken to be 1540 m/s What is the expected beat frequency if blood is flowing in large leg arteries at 2 cm/s directly away from the sound 9nk tn:p;stg: w,p0vn source? Hz
