a) A stationary transducer sends an ultrasound signal of 3.500 MHz that is refle
ID: 1553377 • Letter: A
Question
a) A stationary transducer sends an ultrasound signal of 3.500 MHz that is reflected from a moving object that is located in a fluid (velocity of sound in fluid is 1520 m/s). The detected frequency (using the same transducer) is 3.493 MHz. How fast is the object moving with respect to the stationary transducer AND in what direction?
b) If the incident frequency is 1 MHz, the propagation speed is 1600 m/s, and the reflector speed is 16 m/s toward the source, the Doppler shift is ________ and the reflected frequency is _______.
c) If a 2 MHz ultrasound signal is reflected from a soft tissue boundary moving at 10 m/s away from the source, the Doppler shift is ______.
Explanation / Answer
Since the frequency is decreasing then the moving object moves away from the stationary transducer
then
f' = (V-VL)*f/V = (1520-VL)*3.5*10^6/(1520) = 3.493*10^6
VL = 3.04 m/sec away from the stationary tranducer
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b) f = 1*10^6 Hz
v = 1600 m/sec
VL = 16 m/sec
f' = (V+VL)*f/V = (1600+16)*1*10^6/1600 = 1.01*10^6 Hz
Doppler shift is 0.01*10^6 Hz = 1*10^4 Hz
reflected frequency is f' = 1.01*10^6 Hz
c) f' = 2*10^6 Hz
v = 1600 m/sec
VL = 10 m/sec
f' = (V-VL)*f/V = (1600-10)*f/1600 = 2*10^6 Hz
f = 2.012*10^6 Hz
Doppler shift is 0.012*10^6 Hz = 1.2*10^4 Hz
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