[填空题]
A Doppler flow meter uses ultras-n 2-aahbmy4s ound waves to m522sxs utl64om 7 q:7bambgb/*;4zhqpmhmtmeasure blood-flow speeds. Suppose the device emits sound at 3.5 MHz, and the speed of 54o4pb7gq a2btuqh ml ;tzh26msm:m7sb mx/*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 source? Hz