Question

An ultrasound machine can measure blood flow speeds. Assume the machine emits acoustic energy at a frequency of 3.70 MHz , and the speed of the wave in human tissue is taken to be 1570.0 m/s . What is the beat frequency ?beat between the transmitted and reflected waves if blood is flowing in large arteries at 0.0160 m/s directly away from the sound source?

?beat= Hz

Answer 1

¦' = (1 - v0/v)¦ moving observer (We choose - to make the frequency lower)

¦' = (1 - (.0160 m/s)/(1570 m/s))(3700000 Hz)=3699962.293 HZ

And then there is also a moving source shift from the object:

¦' = ¦/(1 + vs/v) moving source (We choose the + again, to make the frequency lower, as the object is going away from us)

¦' = (3699962.293 Hz)/(1 ± (.0160 m/s)/(1570 m/s)) = 3699924.58 Hz

And finally, the beat frequency is:

fbeat = |f1 - f2| = |3700000 Hz -3699925 Hz| = 75 Hz